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Mercury (planet)

January 26, 2023

"First planet" redirects here. For other systems of numbering planets, see Planet § History.

Mercury is the smallest planet in the Solar System and the closest to the Sun. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman god Mercurius (Mercury), god of commerce, messenger of the gods, and mediator between gods and mortals, corresponding to the Greek god Hermes (Ἑρμῆς). Like Venus, Mercury orbits the Sun within Earth's orbit as an inferior planet; its apparent distance from the Sun as viewed from Earth never exceeds 28°. This proximity to the Sun means the planet can only be seen near the western horizon after sunset or the eastern horizon before sunrise, usually in twilight. At this time, it may appear as a bright star-like object, but is more difficult to observe than Venus. From Earth, the planet telescopically displays the complete range of phases, similar to Venus and the Moon, which recurs over its synodic period of approximately 116 days. Due to its synodic proximity to Earth, Mercury is most often the closest planet to Earth, with Venus periodically taking this role.[18][19]

Mercury
Mercury in true color (by MESSENGER in 2008)
Designations
Pronunciation/ˈmɜːrkjʊri/ (listen)
AdjectivesMercurian /mərˈkjʊəriən/,[1]
Mercurial /mərˈkjʊəriəl/[2]
Orbital characteristics[5]
Epoch J2000
Aphelion
  • 0.466697 AU
  • 69,816,900 km
Perihelion
  • 0.307499 AU
  • 46,001,200 km
  • 0.387098 AU
  • 57,909,050 km
Eccentricity0.205630[3]
115.88 d[3]
47.36 km/s[3]
174.796°
Inclination
48.331°
29.124°
SatellitesNone
Physical characteristics
Mean diameter
4880 km
Mean radius
  • 2,439.7±1.0 km[6][7]
  • 0.3829 Earths
Flattening0.0009[3]
  • 7.48×107 km2[6]
  • 0.147 Earths
Volume
  • 6.083×1010 km3[6]
  • 0.056 Earths
Mass
  • 3.3011×1023 kg[8]
  • 0.055 Earths
Mean density
5.427 g/cm3[6]
0.346±0.014[9]
4.25 km/s[6]
176 d[10]
  • 58.646 d
  • 1407.5 h[6]
Equatorial rotation velocity
10.892 km/h (3.026 m/s)
2.04′ ± 0.08′ (to orbit)[9]
(0.034°)[3]
North pole right ascension
  • 18h 44m 2s
  • 281.01°[3]
North pole declination
61.45°[3]
Albedo
Temperature437 K (164 °C) (blackbody temperature)[13]
Surface temp. min mean max
0°N, 0°W [14] -173 °C 67 °C 427 °C
85°N, 0°W[14] -193 °C -73 °C 106.85 °C
−2.48 to +7.25[15]
4.5–13″[3]
Atmosphere[3][16][17]
Surface pressure
trace (≲ 0.5 nPa)
Composition by volume

Mercury rotates in a way that is unique in the Solar System. It is tidally locked with the Sun in a 3:2 spin–orbit resonance,[20] meaning that relative to the fixed stars, it rotates on its axis exactly three times for every two revolutions it makes around the Sun.[a][21] As seen from the Sun, in a frame of reference that rotates with the orbital motion, it appears to rotate only once every two Mercurian years. An observer on Mercury would therefore see only one day every two Mercurian years.

Mercury's axis has the smallest tilt of any of the Solar System's planets (about 1⁄30 degree). Its orbital eccentricity is the largest of all known planets in the Solar System;[b] at perihelion, Mercury's distance from the Sun is only about two-thirds (or 66%) of its distance at aphelion. Mercury's surface appears heavily cratered and is similar in appearance to the Moon's, indicating that it has been geologically inactive for billions of years. Having almost no atmosphere to retain heat, it has surface temperatures that vary diurnally more than on any other planet in the Solar System, ranging from 100 K (−173 °C; −280 °F) at night to 700 K (427 °C; 800 °F) during the day across the equatorial regions.[22] The polar regions are constantly below 180 K (−93 °C; −136 °F). The planet has no natural satellites.

Two spacecraft have visited Mercury: Mariner 10 flew by in 1974 and 1975; and MESSENGER, launched in 2004, orbited Mercury over 4,000 times in four years before exhausting its fuel and crashing into the planet's surface on April 30, 2015.[23][24][25] The BepiColombo spacecraft is planned to arrive at Mercury in 2025.

Nomenclature

The ancients knew Mercury by different names depending on whether it was an evening star or a morning star. By about 350 BC, the ancient Greeks had realized the two stars were one.[26] They knew the planet as Στίλβων Stilbōn, meaning "twinkling", and Ἑρμής Hermēs, for its fleeting motion,[27] a name that is retained in modern Greek (Ερμής Ermis).[28] The Romans named the planet after the swift-footed Roman messenger god, Mercury (Latin Mercurius), which they equated with the Greek Hermes, because it moves across the sky faster than any other planet.[26][29] The astronomical symbol for Mercury is a stylized version of Hermes' caduceus; a Christian cross was added in the 16th century:  .[30][31]

Physical characteristics

Mercury is one of four terrestrial planets in the Solar System, and is a rocky body like Earth. It is the smallest planet in the Solar System, with an equatorial radius of 2,439.7 kilometres (1,516.0 mi).[3] Mercury is also smaller—albeit more massive—than the largest natural satellites in the Solar System, Ganymede and Titan. Mercury consists of approximately 70% metallic and 30% silicate material.[32]

Internal structure

 
Mercury's internal structure and magnetic field

Mercury appears to have a solid silicate crust and mantle overlying a solid, iron sulfide outer core layer, a deeper liquid core layer, and a solid inner core.[33][34] The planet's density is the second highest in the Solar System at 5.427 g/cm3, only slightly less than Earth's density of 5.515 g/cm3.[3] If the effect of gravitational compression were to be factored out from both planets, the materials of which Mercury is made would be denser than those of Earth, with an uncompressed density of 5.3 g/cm3 versus Earth's 4.4 g/cm3.[35] Mercury's density can be used to infer details of its inner structure. Although Earth's high density results appreciably from gravitational compression, particularly at the core, Mercury is much smaller and its inner regions are not as compressed. Therefore, for it to have such a high density, its core must be large and rich in iron.[36]

The radius of Mercury's core is estimated to be 2,020 ± 30 km (1,255 ± 19 mi), based on interior models constrained to be consistent with the value of the moment of inertia factor given in the infobox.[9][37] Hence, Mercury's core occupies about 57% of its volume; for Earth this proportion is 17%. Research published in 2007 suggests that Mercury has a molten core.[38][39] Surrounding the core is a 500–700 km (310–430 mi) mantle consisting of silicates.[40][41] Based on data from the Mariner 10 and MESSENGER missions, in addition to Earth-based observation, Mercury's crust is estimated to be 35 km (22 mi) thick.[42][43] However, this model may be an overestimate and the crust could be 26 ± 11 km (16.2 ± 6.8 mi) thick based on an Airy isostacy model.[44] One distinctive feature of Mercury's surface is the presence of numerous narrow ridges, extending up to several hundred kilometers in length. It is thought that these were formed as Mercury's core and mantle cooled and contracted at a time when the crust had already solidified.[45][46][47]

Mercury's core has a higher iron content than that of any other major planet in the Solar System, and several theories have been proposed to explain this. The most widely accepted theory is that Mercury originally had a metal–silicate ratio similar to common chondrite meteorites, thought to be typical of the Solar System's rocky matter, and a mass approximately 2.25 times its current mass.[48] Early in the Solar System's history, Mercury may have been struck by a planetesimal of approximately 1/6 Mercury's mass and several thousand kilometers across.[48] The impact would have stripped away much of the original crust and mantle, leaving the core behind as a relatively major component.[48] A similar process, known as the giant impact hypothesis, has been proposed to explain the formation of the Moon.[48]

Alternatively, Mercury may have formed from the solar nebula before the Sun's energy output had stabilized. It would initially have had twice its present mass, but as the protosun contracted, temperatures near Mercury could have been between 2,500 and 3,500 K and possibly even as high as 10,000 K.[49] Much of Mercury's surface rock could have been vaporized at such temperatures, forming an atmosphere of "rock vapor" that could have been carried away by the solar wind.[49] A third hypothesis proposes that the solar nebula caused drag on the particles from which Mercury was accreting, which meant that lighter particles were lost from the accreting material and not gathered by Mercury.[50]

Each hypothesis predicts a different surface composition, and two space missions have been tasked with making observations of this composition. The first MESSENGER, which ended in 2015, found higher-than-expected potassium and sulfur levels on the surface, suggesting that the giant impact hypothesis and vaporization of the crust and mantle did not occur because said potassium and sulfur would have been driven off by the extreme heat of these events.[51] BepiColombo, which will arrive at Mercury in 2025, will make observations to test these hypotheses.[52] The findings so far would seem to favor the third hypothesis; however, further analysis of the data is needed.[53]

Surface geology

Main article: Geology of Mercury

Mercury's surface is similar in appearance to that of the Moon, showing extensive mare-like plains and heavy cratering, indicating that it has been geologically inactive for billions of years. It is more heterogeneous than the surface of Mars or the Moon, both of which contain significant stretches of similar geology, such as maria and plateaus.[54] Albedo features are areas of markedly different reflectivity, which include impact craters, the resulting ejecta, and ray systems. Larger albedo features correspond to higher reflectivity plains.[55] Mercury has dorsa (also called "wrinkle-ridges"), Moon-like highlands, montes (mountains), planitiae (plains), rupes (escarpments), and valles (valleys).[56][57]

 
MASCS spectrum scan of Mercury's surface by MESSENGER

The planet's mantle is chemically heterogeneous, suggesting the planet went through a magma ocean phase early in its history. Crystallization of minerals and convective overturn resulted in layered, chemically heterogeneous crust with large-scale variations in chemical composition observed on the surface. The crust is low in iron but high in sulfur, resulting from the stronger early chemically reducing conditions than is found in the other terrestrial planets. The surface is dominated by iron-poor pyroxene and olivine, as represented by enstatite and forsterite, respectively, along with sodium-rich plagioclase and minerals of mixed magnesium, calcium, and iron-sulfide. The less reflective regions of the crust are high in carbon, most likely in the form of graphite.[58]

Names for features on Mercury come from a variety of sources. Names coming from people are limited to the deceased. Craters are named for artists, musicians, painters, and authors who have made outstanding or fundamental contributions to their field. Ridges, or dorsa, are named for scientists who have contributed to the study of Mercury. Depressions or fossae are named for works of architecture. Montes are named for the word "hot" in a variety of languages. Plains or planitiae are named for Mercury in various languages. Escarpments or rupēs are named for ships of scientific expeditions. Valleys or valles are named for abandoned cities, towns, or settlements of antiquity.[59]

Impact basins and craters

 
Enhanced-color image of craters Munch (left), Sander (center) and Poe (right) amid volcanic plains (orange) near Caloris Basin

Mercury was heavily bombarded by comets and asteroids during and shortly following its formation 4.6 billion years ago, as well as during a possibly separate subsequent episode called the Late Heavy Bombardment that ended 3.8 billion years ago.[60] Mercury received impacts over its entire surface during this period of intense crater formation,[57] facilitated by the lack of any atmosphere to slow impactors down.[61] During this time Mercury was volcanically active; basins were filled by magma, producing smooth plains similar to the maria found on the Moon.[62][63] One of the most unusual craters is Apollodorus, or "the Spider", which hosts a serious of radiating troughs extending outwards from its impact site.[64]

Craters on Mercury range in diameter from small bowl-shaped cavities to multi-ringed impact basins hundreds of kilometers across. They appear in all states of degradation, from relatively fresh rayed craters to highly degraded crater remnants. Mercurian craters differ subtly from lunar craters in that the area blanketed by their ejecta is much smaller, a consequence of Mercury's stronger surface gravity.[65] According to International Astronomical Union rules, each new crater must be named after an artist who was famous for more than fifty years, and dead for more than three years, before the date the crater is named.[66]

 
Overhead view of Caloris Basin
 
Perspective view of Caloris Basin – high (red); low (blue)

The largest known crater is Caloris Planitia, or Caloris Basin, with a diameter of 1,550 km.[67] The impact that created the Caloris Basin was so powerful that it caused lava eruptions and left a concentric mountainous ring ~2 km tall surrounding the impact crater. The floor of the Caloris Basin is filled by a geologically distinct flat plain, broken up by ridges and fractures in a roughly polygonal pattern. It is not clear whether they are volcanic lava flows induced by the impact or a large sheet of impact melt.[65]

At the antipode of the Caloris Basin is a large region of unusual, hilly terrain known as the "Weird Terrain". One hypothesis for its origin is that shock waves generated during the Caloris impact traveled around Mercury, converging at the basin's antipode (180 degrees away). The resulting high stresses fractured the surface.[68] Alternatively, it has been suggested that this terrain formed as a result of the convergence of ejecta at this basin's antipode.[69]

 
Tolstoj basin is along the bottom of this image of Mercury's limb

Overall, 46 impact basins have been identified.[70] A notable basin is the 400 km wide, multi-ring Tolstoj Basin that has an ejecta blanket extending up to 500 km from its rim and a floor that has been filled by smooth plains materials. Beethoven Basin has a similar-sized ejecta blanket and a 625 km diameter rim.[65] Like the Moon, the surface of Mercury has likely incurred the effects of space weathering processes, including solar wind and micrometeorite impacts.[71]

Plains

There are two geologically distinct plains regions on Mercury.[65][72] Gently rolling, hilly plains in the regions between craters are Mercury's oldest visible surfaces,[65] predating the heavily cratered terrain. These inter-crater plains appear to have obliterated many earlier craters, and show a general paucity of smaller craters below about 30 km in diameter.[72]

Smooth plains are widespread flat areas that fill depressions of various sizes and bear a strong resemblance to the lunar maria. Unlike lunar maria, the smooth plains of Mercury have the same albedo as the older inter-crater plains. Despite a lack of unequivocally volcanic characteristics, the localisation and rounded, lobate shape of these plains strongly support volcanic origins.[65] All the smooth plains of Mercury formed significantly later than the Caloris basin, as evidenced by appreciably smaller crater densities than on the Caloris ejecta blanket.[65]

Compressional features

One unusual feature of Mercury's surface is the numerous compression folds, or rupes, that crisscross the plains. These also exist on the moon, but are much more prominent on Mercury.[73] As Mercury's interior cooled, it contracted and its surface began to deform, creating wrinkle ridges and lobate scarps associated with thrust faults. The scarps can reach lengths of 1000 km and heights of 3 km.[74] These compressional features can be seen on top of other features, such as craters and smooth plains, indicating they are more recent.[75] Mapping of the features has suggested a total shrinkage of Mercury's radius in the range of ~1 to 7 km.[76] Most activity along the major thrust systems probably ended about 3.6–3.7 billion years ago.[77] Small-scale thrust fault scarps have been found, tens of meters in height and with lengths in the range of a few km, that appear to be less than 50 million years old, indicating that compression of the interior and consequent surface geological activity continue to the present.[74][76]

Volcanism

 
Picasso crater — the large arc-shaped pit located on the eastern side of its floor are postulated to have formed when subsurface magma subsided or drained, causing the surface to collapse into the resulting void.

There is evidence for pyroclastic flows on Mercury from low-profile shield volcanoes.[78][79][80] 51 pyroclastic deposits have been identified,[81] where 90% of them are found within impact craters.[81] A study of the degradation state of the impact craters that host pyroclastic deposits suggests that pyroclastic activity occurred on Mercury over a prolonged interval.[81]

A "rimless depression" inside the southwest rim of the Caloris Basin consists of at least nine overlapping volcanic vents, each individually up to 8 km in diameter. It is thus a "compound volcano".[82] The vent floors are at least 1 km below their brinks and they bear a closer resemblance to volcanic craters sculpted by explosive eruptions or modified by collapse into void spaces created by magma withdrawal back down into a conduit.[82] Scientists could not quantify the age of the volcanic complex system but reported that it could be on the order of a billion years.[82]

Surface conditions and exosphere

Main article: Atmosphere of Mercury
 
Composite of the north pole of Mercury, where NASA confirmed the discovery of a large volume of water ice, in permanently dark craters that are found there.[83]

The surface temperature of Mercury ranges from 100 to 700 K (−173 to 427 °C; −280 to 800 °F)[22] at the most extreme places: 0°N, 0°W, or 180°W. It never rises above 180 K at the poles,[14] due to the absence of an atmosphere and a steep temperature gradient between the equator and the poles. The subsolar point reaches about 700 K during perihelion (0°W or 180°W), but only 550 K at aphelion (90° or 270°W).[84] On the dark side of the planet, temperatures average 110 K.[14][85] The intensity of sunlight on Mercury's surface ranges between 4.59 and 10.61 times the solar constant (1,370 W·m−2).[86]

Although the daylight temperature at the surface of Mercury is generally extremely high, observations strongly suggest that ice (frozen water) exists on Mercury. The floors of deep craters at the poles are never exposed to direct sunlight, and temperatures there remain below 102 K, far lower than the global average.[87] This creates a cold trap where ice can accumulate. Water ice strongly reflects radar, and observations by the 70-meter Goldstone Solar System Radar and the VLA in the early 1990s revealed that there are patches of high radar reflection near the poles.[88] Although ice was not the only possible cause of these reflective regions, astronomers think it was the most likely.[89]

The icy regions are estimated to contain about 1014–1015 kg of ice,[90] and may be covered by a layer of regolith that inhibits sublimation.[91] By comparison, the Antarctic ice sheet on Earth has a mass of about 4×1018 kg, and Mars's south polar cap contains about 1016 kg of water.[90] The origin of the ice on Mercury is not yet known, but the two most likely sources are from outgassing of water from the planet's interior or deposition by impacts of comets.[90]

Mercury is too small and hot for its gravity to retain any significant atmosphere over long periods of time; it does have a tenuous surface-bounded exosphere[92] containing hydrogen, helium, oxygen, sodium, calcium, potassium and others[16][17] at a surface pressure of less than approximately 0.5 nPa (0.005 picobars).[3] This exosphere is not stable—atoms are continuously lost and replenished from a variety of sources. Hydrogen atoms and helium atoms probably come from the solar wind, diffusing into Mercury's magnetosphere before later escaping back into space. Radioactive decay of elements within Mercury's crust is another source of helium, as well as sodium and potassium. MESSENGER found high proportions of calcium, helium, hydroxide, magnesium, oxygen, potassium, silicon and sodium. Water vapor is present, released by a combination of processes such as: comets striking its surface, sputtering creating water out of hydrogen from the solar wind and oxygen from rock, and sublimation from reservoirs of water ice in the permanently shadowed polar craters. The detection of high amounts of water-related ions like O+, OH−, and H3O+ was a surprise.[93][94] Because of the quantities of these ions that were detected in Mercury's space environment, scientists surmise that these molecules were blasted from the surface or exosphere by the solar wind.[95][96]

Sodium, potassium and calcium were discovered in the atmosphere during the 1980–1990s, and are thought to result primarily from the vaporization of surface rock struck by micrometeorite impacts[97] including presently from Comet Encke.[98] In 2008, magnesium was discovered by MESSENGER.[99] Studies indicate that, at times, sodium emissions are localized at points that correspond to the planet's magnetic poles. This would indicate an interaction between the magnetosphere and the planet's surface.[100]

On November 29, 2012, NASA confirmed that images from MESSENGER had detected that craters at the north pole contained water ice. MESSENGER's principal investigator Sean Solomon is quoted in The New York Times estimating the volume of the ice to be large enough to "encase Washington, D.C., in a frozen block two and a half miles deep".[83]

According to NASA, Mercury is not a suitable planet for Earth-like life. It has a surface boundary exosphere instead of a layered atmosphere, extreme temperatures, and high solar radiation. It is unlikely that any living beings can withstand those conditions.[101] Some parts of the subsurface of Mercury may have been habitable, and perhaps life forms, albeit likely primitive microorganisms, may have existed on the planet.[102][103][104]

Magnetic field and magnetosphere

Main article: Mercury's magnetic field
 
Graph showing relative strength of Mercury's magnetic field

Despite its small size and slow 59-day-long rotation, Mercury has a significant, and apparently global, magnetic field. According to measurements taken by Mariner 10, it is about 1.1% the strength of Earth's. The magnetic-field strength at Mercury's equator is about 300 nT.[105][106] Like that of Earth, Mercury's magnetic field is dipolar.[100] Unlike Earth's, Mercury's poles are nearly aligned with the planet's spin axis.[107] Measurements from both the Mariner 10 and MESSENGER space probes have indicated that the strength and shape of the magnetic field are stable.[107]

It is likely that this magnetic field is generated by a dynamo effect, in a manner similar to the magnetic field of Earth.[108][109] This dynamo effect would result from the circulation of the planet's iron-rich liquid core. Particularly strong tidal heating effects caused by the planet's high orbital eccentricity would serve to keep part of the core in the liquid state necessary for this dynamo effect.[40][110]

Mercury's magnetic field is strong enough to deflect the solar wind around the planet, creating a magnetosphere. The planet's magnetosphere, though small enough to fit within Earth,[100] is strong enough to trap solar wind plasma. This contributes to the space weathering of the planet's surface.[107] Observations taken by the Mariner 10 spacecraft detected this low energy plasma in the magnetosphere of the planet's nightside. Bursts of energetic particles in the planet's magnetotail indicate a dynamic quality to the planet's magnetosphere.[100]

During its second flyby of the planet on October 6, 2008, MESSENGER discovered that Mercury's magnetic field can be extremely "leaky". The spacecraft encountered magnetic "tornadoes" – twisted bundles of magnetic fields connecting the planetary magnetic field to interplanetary space – that were up to 800 km wide or a third of the radius of the planet. These twisted magnetic flux tubes, technically known as flux transfer events, form open windows in the planet's magnetic shield through which the solar wind may enter and directly impact Mercury's surface via magnetic reconnection[111] This also occurs in Earth's magnetic field. The MESSENGER observations showed the reconnection rate is ten times higher at Mercury, but its proximity to the Sun only accounts for about a third of the reconnection rate observed by MESSENGER.[111]

Orbit, rotation, and longitude

 
Orbit of Mercury (2006)
 
Animation of Mercury's and Earth's revolution around the Sun

Mercury has the most eccentric orbit of all the planets in the Solar System; its eccentricity is 0.21 with its distance from the Sun ranging from 46,000,000 to 70,000,000 km (29,000,000 to 43,000,000 mi). It takes 87.969 Earth days to complete an orbit. The diagram illustrates the effects of the eccentricity, showing Mercury's orbit overlaid with a circular orbit having the same semi-major axis. Mercury's higher velocity when it is near perihelion is clear from the greater distance it covers in each 5-day interval. In the diagram, the varying distance of Mercury to the Sun is represented by the size of the planet, which is inversely proportional to Mercury's distance from the Sun. This varying distance to the Sun leads to Mercury's surface being flexed by tidal bulges raised by the Sun that are about 17 times stronger than the Moon's on Earth.[112] Combined with a 3:2 spin–orbit resonance of the planet's rotation around its axis, it also results in complex variations of the surface temperature.[32] The resonance makes a single solar day (the length between two meridian transits of the Sun) on Mercury last exactly two Mercury years, or about 176 Earth days.[113]

Mercury's orbit is inclined by 7 degrees to the plane of Earth's orbit (the ecliptic), the largest of all eight known solar planets.[114] As a result, transits of Mercury across the face of the Sun can only occur when the planet is crossing the plane of the ecliptic at the time it lies between Earth and the Sun, which is in May or November. This occurs about every seven years on average.[115]

Mercury's axial tilt is almost zero,[116] with the best measured value as low as 0.027 degrees.[117] This is significantly smaller than that of Jupiter, which has the second smallest axial tilt of all planets at 3.1 degrees. This means that to an observer at Mercury's poles, the center of the Sun never rises more than 2.1 arcminutes above the horizon.[117]

At certain points on Mercury's surface, an observer would be able to see the Sun peek up a little more than two-thirds of the way over the horizon, then reverse and set before rising again, all within the same Mercurian day.[c] This is because approximately four Earth days before perihelion, Mercury's angular orbital velocity equals its angular rotational velocity so that the Sun's apparent motion ceases; closer to perihelion, Mercury's angular orbital velocity then exceeds the angular rotational velocity. Thus, to a hypothetical observer on Mercury, the Sun appears to move in a retrograde direction. Four Earth days after perihelion, the Sun's normal apparent motion resumes.[32] A similar effect would have occurred if Mercury had been in synchronous rotation: the alternating gain and loss of rotation over revolution would have caused a libration of 23.65° in longitude.[118]

For the same reason, there are two points on Mercury's equator, 180 degrees apart in longitude, at either of which, around perihelion in alternate Mercurian years (once a Mercurian day), the Sun passes overhead, then reverses its apparent motion and passes overhead again, then reverses a second time and passes overhead a third time, taking a total of about 16 Earth-days for this entire process. In the other alternate Mercurian years, the same thing happens at the other of these two points. The amplitude of the retrograde motion is small, so the overall effect is that, for two or three weeks, the Sun is almost stationary overhead, and is at its most brilliant because Mercury is at perihelion, its closest to the Sun. This prolonged exposure to the Sun at its brightest makes these two points the hottest places on Mercury. Maximum temperature occurs when the Sun is at an angle of about 25 degrees past noon due to diurnal temperature lag, at 0.4 Mercury days and 0.8 Mercury years past sunrise.[119] Conversely, there are two other points on the equator, 90 degrees of longitude apart from the first ones, where the Sun passes overhead only when the planet is at aphelion in alternate years, when the apparent motion of the Sun in Mercury's sky is relatively rapid. These points, which are the ones on the equator where the apparent retrograde motion of the Sun happens when it is crossing the horizon as described in the preceding paragraph, receive much less solar heat than the first ones described above.[120]

Mercury attains inferior conjunction (nearest approach to Earth) every 116 Earth days on average,[3] but this interval can range from 105 days to 129 days due to the planet's eccentric orbit. Mercury can come as near as 82,200,000 km (0.549 astronomical units; 51.1 million miles) to Earth, and that is slowly declining: The next approach to within 82,100,000 km (51 million mi) is in 2679, and to within 82,000,000 km (51 million mi) in 4487, but it will not be closer to Earth than 80,000,000 km (50 million mi) until 28,622.[121] Its period of retrograde motion as seen from Earth can vary from 8 to 15 days on either side of inferior conjunction. This large range arises from the planet's high orbital eccentricity.[32] Essentially because Mercury is closest to the Sun, when taking an average over time, Mercury is most often the closest planet to the Earth,[18][19] and—in that measure—it is the closest planet to each of the other planets in the Solar System.[122][123][124][d]

Longitude convention

The longitude convention for Mercury puts the zero of longitude at one of the two hottest points on the surface, as described above. However, when this area was first visited, by Mariner 10, this zero meridian was in darkness, so it was impossible to select a feature on the surface to define the exact position of the meridian. Therefore, a small crater further west was chosen, called Hun Kal, which provides the exact reference point for measuring longitude.[125][126] The center of Hun Kal defines the 20° west meridian. A 1970 International Astronomical Union resolution suggests that longitudes be measured positively in the westerly direction on Mercury.[127] The two hottest places on the equator are therefore at longitudes 0° W and 180° W, and the coolest points on the equator are at longitudes 90° W and 270° W. However, the MESSENGER project uses an east-positive convention.[128]

Spin-orbit resonance

 
After one orbit, Mercury has rotated 1.5 times, so after two complete orbits the same hemisphere is again illuminated.

For many years it was thought that Mercury was synchronously tidally locked with the Sun, rotating once for each orbit and always keeping the same face directed towards the Sun, in the same way that the same side of the Moon always faces Earth. Radar observations in 1965 proved that the planet has a 3:2 spin-orbit resonance, rotating three times for every two revolutions around the Sun. The eccentricity of Mercury's orbit makes this resonance stable—at perihelion, when the solar tide is strongest, the Sun is nearly still in Mercury's sky.[129]

The rare 3:2 resonant tidal locking is stabilized by the variance of the tidal force along Mercury's eccentric orbit, acting on a permanent dipole component of Mercury's mass distribution.[130] In a circular orbit there is no such variance, so the only resonance stabilized in such an orbit is at 1:1 (e.g., Earth–Moon), when the tidal force, stretching a body along the "center-body" line, exerts a torque that aligns the body's axis of least inertia (the "longest" axis, and the axis of the aforementioned dipole) to point always at the center. However, with noticeable eccentricity, like that of Mercury's orbit, the tidal force has a maximum at perihelion and therefore stabilizes resonances, like 3:2, ensuring that the planet points its axis of least inertia roughly at the Sun when passing through perihelion.[130]

The original reason astronomers thought it was synchronously locked was that, whenever Mercury was best placed for observation, it was always nearly at the same point in its 3:2 resonance, hence showing the same face. This is because, coincidentally, Mercury's rotation period is almost exactly half of its synodic period with respect to Earth. Due to Mercury's 3:2 spin-orbit resonance, a solar day lasts about 176 Earth days.[32] A sidereal day (the period of rotation) lasts about 58.7 Earth days.[32]

Simulations indicate that the orbital eccentricity of Mercury varies chaotically from nearly zero (circular) to more than 0.45 over millions of years due to perturbations from the other planets.[32][131] This was thought to explain Mercury's 3:2 spin-orbit resonance (rather than the more usual 1:1), because this state is more likely to arise during a period of high eccentricity.[132] However, accurate modeling based on a realistic model of tidal response has demonstrated that Mercury was captured into the 3:2 spin-orbit state at a very early stage of its history, within 20 (more likely, 10) million years after its formation.[133]

Numerical simulations show that a future secular orbital resonant perihelion interaction with Jupiter may cause the eccentricity of Mercury's orbit to increase to the point where there is a 1% chance that the orbit will be destabilized in the next five billion years. If this happens, Mercury may fall into the Sun, collide with Venus, be ejected from the Solar System, or even disrupt the rest of the inner Solar System.[134][135]

Advance of perihelion

Main article: Perihelion precession of Mercury

In 1859, the French mathematician and astronomer Urbain Le Verrier reported that the slow precession of Mercury's orbit around the Sun could not be completely explained by Newtonian mechanics and perturbations by the known planets. He suggested, among possible explanations, that another planet (or perhaps instead a series of smaller "corpuscules") might exist in an orbit even closer to the Sun than that of Mercury, to account for this perturbation.[136] (Other explanations considered included a slight oblateness of the Sun.) The success of the search for Neptune based on its perturbations of the orbit of Uranus led astronomers to place faith in this possible explanation, and the hypothetical planet was named Vulcan, but no such planet was ever found.[137]

The observed perihelion precession of Mercury is 5,600 arcseconds (1.5556°) per century relative to Earth, or 574.10±0.65 arcseconds per century[138] relative to the inertial ICRF. Newtonian mechanics, taking into account all the effects from the other planets (included 0.0254 arcsecond per century due to the "flatteness" of the Sun), predicts a precession of 5,557 arcseconds (1.5436°) per century relative to Earth, or 531.63±0.69 arcseconds per century relative to ICRF.[138] In the early 20th century, Albert Einstein's general theory of relativity provided the explanation for the observed precession, by formalizing gravitation as being mediated by the curvature of spacetime. The effect is small: just 42.98 arcseconds per century (or 0.43 arcsecond per year, or 0.1038 arcsecond per orbital period) for Mercury; it therefore requires a little over twelve million orbits, or 2.8 million years, for a full excess turn. Similar, but much smaller, effects exist for other Solar System bodies: 8.62 arcseconds per century for Venus, 3.84 for Earth, 1.35 for Mars, and 10.05 for 1566 Icarus.[139][140]

Observation

 
Image mosaic by Mariner 10, 1974

Mercury's apparent magnitude is calculated to vary between −2.48 (brighter than Sirius) around superior conjunction and +7.25 (below the limit of naked-eye visibility) around inferior conjunction.[15] The mean apparent magnitude is 0.23 while the standard deviation of 1.78 is the largest of any planet. The mean apparent magnitude at superior conjunction is −1.89 while that at inferior conjunction is +5.93.[15] Observation of Mercury is complicated by its proximity to the Sun, as it is lost in the Sun's glare for much of the time. Mercury can be observed for only a brief period during either morning or evening twilight.[141]

But in some cases Mercury can better be observed in daylight with a telescope when the position is known because it is higher in the sky and less atmospheric effects affect the view of the planet. When proper safety precautions are taken to prevent inadvertently pointing the telescope at the Sun (and thus blinding the user), Mercury can be viewed as close as 4° to the Sun when near superior conjunction when it is almost at its brightest.

Mercury can, like several other planets and the brightest stars, be seen during a total solar eclipse.[142]

Like the Moon and Venus, Mercury exhibits phases as seen from Earth. It is "new" at inferior conjunction and "full" at superior conjunction. The planet is rendered invisible from Earth on both of these occasions because of its being obscured by the Sun,[141] except its new phase during a transit.

Mercury is technically brightest as seen from Earth when it is at a full phase. Although Mercury is farthest from Earth when it is full, the greater illuminated area that is visible and the opposition brightness surge more than compensates for the distance.[143] The opposite is true for Venus, which appears brightest when it is a crescent, because it is much closer to Earth than when gibbous.[143][144]

 
False-color map showing the maximum temperatures of the north polar region

Nonetheless, the brightest (full phase) appearance of Mercury is an essentially impossible time for practical observation, because of the extreme proximity of the Sun. Mercury is best observed at the first and last quarter, although they are phases of lesser brightness. The first and last quarter phases occur at greatest elongation east and west of the Sun, respectively. At both of these times Mercury's separation from the Sun ranges anywhere from 17.9° at perihelion to 27.8° at aphelion.[145][146] At greatest western elongation, Mercury rises at its earliest before sunrise, and at greatest eastern elongation, it sets at its latest after sunset.[147]

 
False-color image of Carnegie Rupes, a tectonic landform—high terrain (red); low (blue).

Mercury is more often and easily visible from the Southern Hemisphere than from the Northern. This is because Mercury's maximum western elongation occurs only during early autumn in the Southern Hemisphere, whereas its greatest eastern elongation happens only during late winter in the Southern Hemisphere.[147] In both of these cases, the angle at which the planet's orbit intersects the horizon is maximized, allowing it to rise several hours before sunrise in the former instance and not set until several hours after sundown in the latter from southern mid-latitudes, such as Argentina and South Africa.[147]

An alternate method for viewing Mercury involves observing the planet during daylight hours when conditions are clear, ideally when it is at its greatest elongation. This allows the planet to be found easily, even when using telescopes with 8 cm (3.1 in) apertures. However, great care must be taken to obstruct the Sun from sight because of the extreme risk for eye damage.[148] This method bypasses the limitation of twilight observing when the ecliptic is located at a low elevation (e.g. on autumn evenings).

Ground-based telescope observations of Mercury reveal only an illuminated partial disk with limited detail. The first of two spacecraft to visit the planet was Mariner 10, which mapped about 45% of its surface from 1974 to 1975. The second is the MESSENGER spacecraft, which after three Mercury flybys between 2008 and 2009, attained orbit around Mercury on March 17, 2011,[149] to study and map the rest of the planet.[150]

The Hubble Space Telescope cannot observe Mercury at all, due to safety procedures that prevent its pointing too close to the Sun.[151]

Because the shift of 0.15 revolutions in a year makes up a seven-year cycle (0.15 × 7 ≈ 1.0), in the seventh year Mercury follows almost exactly (earlier by 7 days) the sequence of phenomena it showed seven years before.[145]

Observation history

Ancient astronomers

 
Mercury, from Liber astronomiae, 1550

The earliest known recorded observations of Mercury are from the MUL.APIN tablets. These observations were most likely made by an Assyrian astronomer around the 14th century BC.[152] The cuneiform name used to designate Mercury on the MUL.APIN tablets is transcribed as UDU.IDIM.GU\U4.UD ("the jumping planet").[e][153] Babylonian records of Mercury date back to the 1st millennium BC. The Babylonians called the planet Nabu after the messenger to the gods in their mythology.[154]

The Greco-Egyptian[155] astronomer Ptolemy wrote about the possibility of planetary transits across the face of the Sun in his work Planetary Hypotheses. He suggested that no transits had been observed either because planets such as Mercury were too small to see, or because the transits were too infrequent.[156]

 
Ibn al-Shatir's model for the appearances of Mercury, showing the multiplication of epicycles using the Tusi couple, thus eliminating the Ptolemaic eccentrics and equant.

In ancient China, Mercury was known as "the Hour Star" (Chen-xing 辰星). It was associated with the direction north and the phase of water in the Five Phases system of metaphysics.[157] Modern Chinese, Korean, Japanese and Vietnamese cultures refer to the planet literally as the "water star" (水星), based on the Five elements.[158][159][160] Hindu mythology used the name Budha for Mercury, and this god was thought to preside over Wednesday.[161] The god Odin (or Woden) of Germanic paganism was associated with the planet Mercury and Wednesday.[162] The Maya may have represented Mercury as an owl (or possibly four owls; two for the morning aspect and two for the evening) that served as a messenger to the underworld.[163]

In medieval Islamic astronomy, the Andalusian astronomer Abū Ishāq Ibrāhīm al-Zarqālī in the 11th century described the deferent of Mercury's geocentric orbit as being oval, like an egg or a pignon, although this insight did not influence his astronomical theory or his astronomical calculations.[164][165] In the 12th century, Ibn Bajjah observed "two planets as black spots on the face of the Sun", which was later suggested as the transit of Mercury and/or Venus by the Maragha astronomer Qotb al-Din Shirazi in the 13th century.[166] (Note that most such medieval reports of transits were later taken as observations of sunspots.[167])

In India, the Kerala school astronomer Nilakantha Somayaji in the 15th century developed a partially heliocentric planetary model in which Mercury orbits the Sun, which in turn orbits Earth, similar to the Tychonic system later proposed by Tycho Brahe in the late 16th century.[168]

Ground-based telescopic research

 
Transit of Mercury. Mercury is visible as a black dot below and to the left of center. The dark area above the center of the solar disk is a sunspot.
 
Elongation is the angle between the Sun and the planet, with Earth as the reference point. Mercury appears close to the Sun.

The first telescopic observations of Mercury were made by Thomas Harriot and Galileo from 1610. In 1612, Simon Marius observed the brightness of Mercury varied with the planet's orbital position and concluded it had phases "in the same way as Venus and the Moon".[169] In 1631, Pierre Gassendi made the first telescopic observations of the transit of a planet across the Sun when he saw a transit of Mercury predicted by Johannes Kepler. In 1639, Giovanni Zupi used a telescope to discover that the planet had orbital phases similar to Venus and the Moon. The observation demonstrated conclusively that Mercury orbited around the Sun.[32]

A rare event in astronomy is the passage of one planet in front of another (occultation), as seen from Earth. Mercury and Venus occult each other every few centuries, and the event of May 28, 1737 is the only one historically observed, having been seen by John Bevis at the Royal Greenwich Observatory.[170] The next occultation of Mercury by Venus will be on December 3, 2133.[171]

The difficulties inherent in observing Mercury mean that it was far less studied than the other planets. In 1800, Johann Schröter made observations of surface features, claiming to have observed 20-kilometre-high (12 mi) mountains. Friedrich Bessel used Schröter's drawings to erroneously estimate the rotation period as 24 hours and an axial tilt of 70°.[172] In the 1880s, Giovanni Schiaparelli mapped the planet more accurately, and suggested that Mercury's rotational period was 88 days, the same as its orbital period due to tidal locking.[173] This phenomenon is known as synchronous rotation. The effort to map the surface of Mercury was continued by Eugenios Antoniadi, who published a book in 1934 that included both maps and his own observations.[100] Many of the planet's surface features, particularly the albedo features, take their names from Antoniadi's map.[174]

In June 1962, Soviet scientists at the Institute of Radio-engineering and Electronics of the USSR Academy of Sciences, led by Vladimir Kotelnikov, became the first to bounce a radar signal off Mercury and receive it, starting radar observations of the planet.[175][176][177] Three years later, radar observations by Americans Gordon H. Pettengill and Rolf B. Dyce, using the 300-meter Arecibo radio telescope in Puerto Rico, showed conclusively that the planet's rotational period was about 59 days.[178][179] The theory that Mercury's rotation was synchronous had become widely held, and it was a surprise to astronomers when these radio observations were announced. If Mercury were tidally locked, its dark face would be extremely cold, but measurements of radio emission revealed that it was much hotter than expected. Astronomers were reluctant to drop the synchronous rotation theory and proposed alternative mechanisms such as powerful heat-distributing winds to explain the observations.[180]

 
Water ice (yellow) at Mercury's north polar region

Italian astronomer Giuseppe Colombo noted that the rotation value was about two-thirds of Mercury's orbital period, and proposed that the planet's orbital and rotational periods were locked into a 3:2 rather than a 1:1 resonance.[181] Data from Mariner 10 subsequently confirmed this view.[182] This means that Schiaparelli's and Antoniadi's maps were not "wrong". Instead, the astronomers saw the same features during every second orbit and recorded them, but disregarded those seen in the meantime, when Mercury's other face was toward the Sun, because the orbital geometry meant that these observations were made under poor viewing conditions.[172]

Ground-based optical observations did not shed much further light on Mercury, but radio astronomers using interferometry at microwave wavelengths, a technique that enables removal of the solar radiation, were able to discern physical and chemical characteristics of the subsurface layers to a depth of several meters.[183][184] Not until the first space probe flew past Mercury did many of its most fundamental morphological properties become known. Moreover, recent technological advances have led to improved ground-based observations. In 2000, high-resolution lucky imaging observations were conducted by the Mount Wilson Observatory 1.5 meter Hale telescope. They provided the first views that resolved surface features on the parts of Mercury that were not imaged in the Mariner 10 mission.[185] Most of the planet has been mapped by the Arecibo radar telescope, with 5 km (3.1 mi) resolution, including polar deposits in shadowed craters of what may be water ice.[186]

Research with space probes

Main article: Exploration of Mercury
 
MESSENGER being prepared for launch
 
Mercury transiting the Sun as viewed by the Mars rover Curiosity (June 3, 2014).[187]

Reaching Mercury from Earth poses significant technical challenges, because it orbits so much closer to the Sun than Earth. A Mercury-bound spacecraft launched from Earth must travel over 91 million kilometres (57 million miles) into the Sun's gravitational potential well. Mercury has an orbital speed of 47.4 km/s (29.5 mi/s), whereas Earth's orbital speed is 29.8 km/s (18.5 mi/s).[114] Therefore, the spacecraft must make a large change in velocity (delta-v) to get to Mercury and then enter orbit,[188] as compared to the delta-v required for, say, Mars planetary missions.

The potential energy liberated by moving down the Sun's potential well becomes kinetic energy, requiring a delta-v change to do anything other than pass by Mercury. Some portion of this delta-v budget can be provided from a gravity assist during one or more fly-bys of Venus.[189] To land safely or enter a stable orbit the spacecraft would rely entirely on rocket motors. Aerobraking is ruled out because Mercury has a negligible atmosphere. A trip to Mercury requires more rocket fuel than that required to escape the Solar System completely. As a result, only three space probes have visited it so far.[190] A proposed alternative approach would use a solar sail to attain a Mercury-synchronous orbit around the Sun.[191]

Mariner 10

Main article: Mariner 10
 
Mariner 10, the first probe to visit Mercury

The first spacecraft to visit Mercury was NASA's Mariner 10 (1974–1975).[26] The spacecraft used the gravity of Venus to adjust its orbital velocity so that it could approach Mercury, making it both the first spacecraft to use this gravitational "slingshot" effect and the first NASA mission to visit multiple planets.[192] Mariner 10 provided the first close-up images of Mercury's surface, which immediately showed its heavily cratered nature, and revealed many other types of geological features, such as the giant scarps that were later ascribed to the effect of the planet shrinking slightly as its iron core cools.[193] Unfortunately, the same face of the planet was lit at each of Mariner 10's close approaches. This made close observation of both sides of the planet impossible,[194] and resulted in the mapping of less than 45% of the planet's surface.[195]

The spacecraft made three close approaches to Mercury, the closest of which took it to within 327 km (203 mi) of the surface.[196] At the first close approach, instruments detected a magnetic field, to the great surprise of planetary geologists—Mercury's rotation was expected to be much too slow to generate a significant dynamo effect. The second close approach was primarily used for imaging, but at the third approach, extensive magnetic data were obtained. The data revealed that the planet's magnetic field is much like Earth's, which deflects the solar wind around the planet. For many years after the Mariner 10 encounters, the origin of Mercury's magnetic field remained the subject of several competing theories.[197][198]

On March 24, 1975, just eight days after its final close approach, Mariner 10 ran out of fuel. Because its orbit could no longer be accurately controlled, mission controllers instructed the probe to shut down.[199] Mariner 10 is thought to be still orbiting the Sun, passing close to Mercury every few months.[200]

MESSENGER

Main article: MESSENGER
 
Estimated details of the impact of MESSENGER on April 30, 2015

A second NASA mission to Mercury, named MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging), was launched on August 3, 2004. It made a fly-by of Earth in August 2005, and of Venus in October 2006 and June 2007 to place it onto the correct trajectory to reach an orbit around Mercury.[201] A first fly-by of Mercury occurred on January 14, 2008, a second on October 6, 2008,[202] and a third on September 29, 2009.[203] Most of the hemisphere not imaged by Mariner 10 was mapped during these fly-bys. The probe successfully entered an elliptical orbit around the planet on March 18, 2011. The first orbital image of Mercury was obtained on March 29, 2011. The probe finished a one-year mapping mission,[202] and then entered a one-year extended mission into 2013. In addition to continued observations and mapping of Mercury, MESSENGER observed the 2012 solar maximum.[204]

The mission was designed to clear up six key issues: Mercury's high density, its geological history, the nature of its magnetic field, the structure of its core, whether it has ice at its poles, and where its tenuous atmosphere comes from. To this end, the probe carried imaging devices that gathered much-higher-resolution images of much more of Mercury than Mariner 10, assorted spectrometers to determine abundances of elements in the crust, and magnetometers and devices to measure velocities of charged particles. Measurements of changes in the probe's orbital velocity were expected to be used to infer details of the planet's interior structure.[205] MESSENGER's final maneuver was on April 24, 2015, and it crashed into Mercury's surface on April 30, 2015.[206][207][208] The spacecraft's impact with Mercury occurred near 3:26 pm EDT on April 30, 2015, leaving a crater estimated to be 16 m (52 ft) in diameter.[209]

BepiColombo

Main article: BepiColombo

The European Space Agency and the Japanese Space Agency developed and launched a joint mission called BepiColombo, which will orbit Mercury with two probes: one to map the planet and the other to study its magnetosphere.[210] Launched on October 20, 2018, BepiColombo is expected to reach Mercury in 2025.[211] It will release a magnetometer probe into an elliptical orbit, then chemical rockets will fire to deposit the mapper probe into a circular orbit. Both probes will operate for one terrestrial year.[210] The mapper probe carries an array of spectrometers similar to those on MESSENGER, and will study the planet at many different wavelengths including infrared, ultraviolet, X-ray and gamma ray.[212] BepiColombo conducted the first of its six planned Mercury flybys on October 1, 2021.[213]

See also

Notes

  1. ^ In astronomy, the words "rotation" and "revolution" have different meanings. "Rotation" is the turning of a body about an axis that passes through the body, as in "Earth rotates once a day." "Revolution" is motion around a centre that is external to the body, usually in orbit, as in "Earth takes a year for each revolution around the Sun." The verbs "rotate" and "revolve" mean doing rotation and revolution, respectively.
  2. ^ Pluto was considered a planet from its discovery in 1930 to 2006, but after that it has been reclassified as a dwarf planet. Pluto's orbital eccentricity is greater than Mercury's. Pluto is also smaller than Mercury, but was thought to be larger until 1976.
  3. ^ The Sun's total angular displacement during its apparent retrograde motion as seen from the surface of Mercury is ~1.23°, while the Sun's angular diameter when the apparent retrograde motion begins and ends is ~1.71°, increasing to ~1.73° at perihelion (midway through the retrograde motion).
  4. ^ It is important to be clear about the meaning of "closeness". In the astronomical literature, the term "closest planets" often means "the two planets that approach each other most closely". In other words, the orbits of the two planets approach each other most closely. However, this does not mean that the two planets are closest over time. For example, essentially because Mercury is closer to the Sun than Venus, Mercury spends more time in proximity to Earth; it could, therefore, be said that Mercury is the planet that is "closest to Earth when averaged over time". However, using this time-average definition of 'closeness'—as noted above—it turns out that Mercury is the closest planet to all other planets in the solar system. For that reason, arguably, the proximity-definition is not particularly helpful. An episode of the BBC Radio 4 programme "More or Less" explains the different notions of proximity well.[19]
  5. ^ Some sources precede the cuneiform transcription with "MUL". "MUL" is a cuneiform sign that was used in the Sumerian language to designate a star or planet, but it is not considered part of the actual name. The "4" is a reference number in the Sumero–Akkadian transliteration system to designate which of several syllables a certain cuneiform sign is most likely designating.

References

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  • Atlas of Mercury. NASA. 1978. SP-423.
  • Mercury nomenclature and from the USGS/IAU Gazetteer of Planetary Nomenclature
  • Equirectangular map of Mercury by Applied Coherent Technology Corp
  • 3D globe of Mercury by Google
  • Mercury at Solarviews.com
  • Mercury by Astronomy Cast
  • MESSENGER mission web site
  • BepiColombo mission web site

January 26, 2023
mercury, planet, first, planet, redirects, here, other, systems, numbering, planets, planet, history, mercury, smallest, planet, solar, system, closest, orbit, around, takes, earth, days, shortest, planets, named, after, roman, mercurius, code, promoted, code,. First planet redirects here For other systems of numbering planets see Planet History Mercury is the smallest planet in the Solar System and the closest to the Sun Its orbit around the Sun takes 87 97 Earth days the shortest of all the Sun s planets It is named after the Roman god Mercurius code lat promoted to code la Mercury god of commerce messenger of the gods and mediator between gods and mortals corresponding to the Greek god Hermes Ἑrmῆs code ell promoted to code el Like Venus Mercury orbits the Sun within Earth s orbit as an inferior planet its apparent distance from the Sun as viewed from Earth never exceeds 28 This proximity to the Sun means the planet can only be seen near the western horizon after sunset or the eastern horizon before sunrise usually in twilight At this time it may appear as a bright star like object but is more difficult to observe than Venus From Earth the planet telescopically displays the complete range of phases similar to Venus and the Moon which recurs over its synodic period of approximately 116 days Due to its synodic proximity to Earth Mercury is most often the closest planet to Earth with Venus periodically taking this role 18 19 MercuryMercury in true color by MESSENGER in 2008 DesignationsPronunciation ˈ m ɜːr k j ʊr i listen AdjectivesMercurian m er ˈ k jʊer i e n 1 Mercurial m er ˈ k jʊer i e l 2 Orbital characteristics 5 Epoch J2000Aphelion0 466697 AU 69 816 900 kmPerihelion0 307499 AU 46 001 200 kmSemi major axis0 387098 AU 57 909 050 kmEccentricity0 205630 3 Orbital period sidereal 87 9691 d 0 240846 yr 0 5 Mercury synodic dayOrbital period synodic 115 88 d 3 Average orbital speed47 36 km s 3 Mean anomaly174 796 Inclination7 005 to ecliptic 3 38 to Sun s equator 6 35 to invariable plane 4 Longitude of ascending node48 331 Argument of perihelion29 124 SatellitesNonePhysical characteristicsMean diameter4880 kmMean radius2 439 7 1 0 km 6 7 0 3829 EarthsFlattening0 0009 3 Surface area7 48 107 km2 6 0 147 EarthsVolume6 083 1010 km3 6 0 056 EarthsMass3 3011 1023 kg 8 0 055 EarthsMean density5 427 g cm3 6 Surface gravity3 7 m s2 0 38 g 6 Moment of inertia factor0 346 0 014 9 Escape velocity4 25 km s 6 Synodic rotation period176 d 10 Sidereal rotation period58 646 d 1407 5 h 6 Equatorial rotation velocity10 892 km h 3 026 m s Axial tilt2 04 0 08 to orbit 9 0 034 3 North pole right ascension18h 44m 2s 281 01 3 North pole declination61 45 3 Albedo0 088 Bond 11 0 142 geom 12 Temperature437 K 164 C blackbody temperature 13 Surface temp min mean max0 N 0 W 14 173 C 67 C 427 C85 N 0 W 14 193 C 73 C 106 85 CApparent magnitude 2 48 to 7 25 15 Angular diameter4 5 13 3 Atmosphere 3 16 17 Surface pressuretrace 0 5 nPa Composition by volumeatomic oxygen sodium magnesium atomic hydrogen potassium calcium helium Trace amounts of iron aluminium argon dinitrogen dioxygen carbon dioxide water vapor xenon krypton and neonMercury rotates in a way that is unique in the Solar System It is tidally locked with the Sun in a 3 2 spin orbit resonance 20 meaning that relative to the fixed stars it rotates on its axis exactly three times for every two revolutions it makes around the Sun a 21 As seen from the Sun in a frame of reference that rotates with the orbital motion it appears to rotate only once every two Mercurian years An observer on Mercury would therefore see only one day every two Mercurian years Mercury s axis has the smallest tilt of any of the Solar System s planets about 1 30 degree Its orbital eccentricity is the largest of all known planets in the Solar System b at perihelion Mercury s distance from the Sun is only about two thirds or 66 of its distance at aphelion Mercury s surface appears heavily cratered and is similar in appearance to the Moon s indicating that it has been geologically inactive for billions of years Having almost no atmosphere to retain heat it has surface temperatures that vary diurnally more than on any other planet in the Solar System ranging from 100 K 173 C 280 F at night to 700 K 427 C 800 F during the day across the equatorial regions 22 The polar regions are constantly below 180 K 93 C 136 F The planet has no natural satellites Two spacecraft have visited Mercury Mariner 10 flew by in 1974 and 1975 and MESSENGER launched in 2004 orbited Mercury over 4 000 times in four years before exhausting its fuel and crashing into the planet s surface on April 30 2015 23 24 25 The BepiColombo spacecraft is planned to arrive at Mercury in 2025 Contents 1 Nomenclature 2 Physical characteristics 2 1 Internal structure 2 2 Surface geology 2 2 1 Impact basins and craters 2 2 2 Plains 2 2 3 Compressional features 2 2 4 Volcanism 2 3 Surface conditions and exosphere 2 4 Magnetic field and magnetosphere 3 Orbit rotation and longitude 3 1 Longitude convention 3 2 Spin orbit resonance 3 3 Advance of perihelion 4 Observation 5 Observation history 5 1 Ancient astronomers 5 2 Ground based telescopic research 5 3 Research with space probes 5 3 1 Mariner 10 5 3 2 MESSENGER 5 3 3 BepiColombo 6 See also 7 Notes 8 References 9 External linksNomenclatureThe ancients knew Mercury by different names depending on whether it was an evening star or a morning star By about 350 BC the ancient Greeks had realized the two stars were one 26 They knew the planet as Stilbwn Stilbōn meaning twinkling and Ἑrmhs Hermes for its fleeting motion 27 a name that is retained in modern Greek Ermhs Ermis 28 The Romans named the planet after the swift footed Roman messenger god Mercury Latin Mercurius which they equated with the Greek Hermes because it moves across the sky faster than any other planet 26 29 The astronomical symbol for Mercury is a stylized version of Hermes caduceus a Christian cross was added in the 16th century 30 31 Physical characteristicsMercury is one of four terrestrial planets in the Solar System and is a rocky body like Earth It is the smallest planet in the Solar System with an equatorial radius of 2 439 7 kilometres 1 516 0 mi 3 Mercury is also smaller albeit more massive than the largest natural satellites in the Solar System Ganymede and Titan Mercury consists of approximately 70 metallic and 30 silicate material 32 Internal structure Mercury s internal structure and magnetic field Mercury appears to have a solid silicate crust and mantle overlying a solid iron sulfide outer core layer a deeper liquid core layer and a solid inner core 33 34 The planet s density is the second highest in the Solar System at 5 427 g cm3 only slightly less than Earth s density of 5 515 g cm3 3 If the effect of gravitational compression were to be factored out from both planets the materials of which Mercury is made would be denser than those of Earth with an uncompressed density of 5 3 g cm3 versus Earth s 4 4 g cm3 35 Mercury s density can be used to infer details of its inner structure Although Earth s high density results appreciably from gravitational compression particularly at the core Mercury is much smaller and its inner regions are not as compressed Therefore for it to have such a high density its core must be large and rich in iron 36 The radius of Mercury s core is estimated to be 2 020 30 km 1 255 19 mi based on interior models constrained to be consistent with the value of the moment of inertia factor given in the infobox 9 37 Hence Mercury s core occupies about 57 of its volume for Earth this proportion is 17 Research published in 2007 suggests that Mercury has a molten core 38 39 Surrounding the core is a 500 700 km 310 430 mi mantle consisting of silicates 40 41 Based on data from the Mariner 10 and MESSENGER missions in addition to Earth based observation Mercury s crust is estimated to be 35 km 22 mi thick 42 43 However this model may be an overestimate and the crust could be 26 11 km 16 2 6 8 mi thick based on an Airy isostacy model 44 One distinctive feature of Mercury s surface is the presence of numerous narrow ridges extending up to several hundred kilometers in length It is thought that these were formed as Mercury s core and mantle cooled and contracted at a time when the crust had already solidified 45 46 47 Mercury s core has a higher iron content than that of any other major planet in the Solar System and several theories have been proposed to explain this The most widely accepted theory is that Mercury originally had a metal silicate ratio similar to common chondrite meteorites thought to be typical of the Solar System s rocky matter and a mass approximately 2 25 times its current mass 48 Early in the Solar System s history Mercury may have been struck by a planetesimal of approximately 1 6 Mercury s mass and several thousand kilometers across 48 The impact would have stripped away much of the original crust and mantle leaving the core behind as a relatively major component 48 A similar process known as the giant impact hypothesis has been proposed to explain the formation of the Moon 48 Alternatively Mercury may have formed from the solar nebula before the Sun s energy output had stabilized It would initially have had twice its present mass but as the protosun contracted temperatures near Mercury could have been between 2 500 and 3 500 K and possibly even as high as 10 000 K 49 Much of Mercury s surface rock could have been vaporized at such temperatures forming an atmosphere of rock vapor that could have been carried away by the solar wind 49 A third hypothesis proposes that the solar nebula caused drag on the particles from which Mercury was accreting which meant that lighter particles were lost from the accreting material and not gathered by Mercury 50 Each hypothesis predicts a different surface composition and two space missions have been tasked with making observations of this composition The first MESSENGER which ended in 2015 found higher than expected potassium and sulfur levels on the surface suggesting that the giant impact hypothesis and vaporization of the crust and mantle did not occur because said potassium and sulfur would have been driven off by the extreme heat of these events 51 BepiColombo which will arrive at Mercury in 2025 will make observations to test these hypotheses 52 The findings so far would seem to favor the third hypothesis however further analysis of the data is needed 53 Surface geology Main article Geology of Mercury Mercury s surface is similar in appearance to that of the Moon showing extensive mare like plains and heavy cratering indicating that it has been geologically inactive for billions of years It is more heterogeneous than the surface of Mars or the Moon both of which contain significant stretches of similar geology such as maria and plateaus 54 Albedo features are areas of markedly different reflectivity which include impact craters the resulting ejecta and ray systems Larger albedo features correspond to higher reflectivity plains 55 Mercury has dorsa also called wrinkle ridges Moon like highlands montes mountains planitiae plains rupes escarpments and valles valleys 56 57 MASCS spectrum scan of Mercury s surface by MESSENGER The planet s mantle is chemically heterogeneous suggesting the planet went through a magma ocean phase early in its history Crystallization of minerals and convective overturn resulted in layered chemically heterogeneous crust with large scale variations in chemical composition observed on the surface The crust is low in iron but high in sulfur resulting from the stronger early chemically reducing conditions than is found in the other terrestrial planets The surface is dominated by iron poor pyroxene and olivine as represented by enstatite and forsterite respectively along with sodium rich plagioclase and minerals of mixed magnesium calcium and iron sulfide The less reflective regions of the crust are high in carbon most likely in the form of graphite 58 Names for features on Mercury come from a variety of sources Names coming from people are limited to the deceased Craters are named for artists musicians painters and authors who have made outstanding or fundamental contributions to their field Ridges or dorsa are named for scientists who have contributed to the study of Mercury Depressions or fossae are named for works of architecture Montes are named for the word hot in a variety of languages Plains or planitiae are named for Mercury in various languages Escarpments or rupes are named for ships of scientific expeditions Valleys or valles are named for abandoned cities towns or settlements of antiquity 59 Impact basins and craters Enhanced color image of craters Munch left Sander center and Poe right amid volcanic plains orange near Caloris Basin Mercury was heavily bombarded by comets and asteroids during and shortly following its formation 4 6 billion years ago as well as during a possibly separate subsequent episode called the Late Heavy Bombardment that ended 3 8 billion years ago 60 Mercury received impacts over its entire surface during this period of intense crater formation 57 facilitated by the lack of any atmosphere to slow impactors down 61 During this time Mercury was volcanically active basins were filled by magma producing smooth plains similar to the maria found on the Moon 62 63 One of the most unusual craters is Apollodorus or the Spider which hosts a serious of radiating troughs extending outwards from its impact site 64 Craters on Mercury range in diameter from small bowl shaped cavities to multi ringed impact basins hundreds of kilometers across They appear in all states of degradation from relatively fresh rayed craters to highly degraded crater remnants Mercurian craters differ subtly from lunar craters in that the area blanketed by their ejecta is much smaller a consequence of Mercury s stronger surface gravity 65 According to International Astronomical Union rules each new crater must be named after an artist who was famous for more than fifty years and dead for more than three years before the date the crater is named 66 Overhead view of Caloris Basin Perspective view of Caloris Basin high red low blue The largest known crater is Caloris Planitia or Caloris Basin with a diameter of 1 550 km 67 The impact that created the Caloris Basin was so powerful that it caused lava eruptions and left a concentric mountainous ring 2 km tall surrounding the impact crater The floor of the Caloris Basin is filled by a geologically distinct flat plain broken up by ridges and fractures in a roughly polygonal pattern It is not clear whether they are volcanic lava flows induced by the impact or a large sheet of impact melt 65 At the antipode of the Caloris Basin is a large region of unusual hilly terrain known as the Weird Terrain One hypothesis for its origin is that shock waves generated during the Caloris impact traveled around Mercury converging at the basin s antipode 180 degrees away The resulting high stresses fractured the surface 68 Alternatively it has been suggested that this terrain formed as a result of the convergence of ejecta at this basin s antipode 69 Tolstoj basin is along the bottom of this image of Mercury s limb Overall 46 impact basins have been identified 70 A notable basin is the 400 km wide multi ring Tolstoj Basin that has an ejecta blanket extending up to 500 km from its rim and a floor that has been filled by smooth plains materials Beethoven Basin has a similar sized ejecta blanket and a 625 km diameter rim 65 Like the Moon the surface of Mercury has likely incurred the effects of space weathering processes including solar wind and micrometeorite impacts 71 Plains There are two geologically distinct plains regions on Mercury 65 72 Gently rolling hilly plains in the regions between craters are Mercury s oldest visible surfaces 65 predating the heavily cratered terrain These inter crater plains appear to have obliterated many earlier craters and show a general paucity of smaller craters below about 30 km in diameter 72 Smooth plains are widespread flat areas that fill depressions of various sizes and bear a strong resemblance to the lunar maria Unlike lunar maria the smooth plains of Mercury have the same albedo as the older inter crater plains Despite a lack of unequivocally volcanic characteristics the localisation and rounded lobate shape of these plains strongly support volcanic origins 65 All the smooth plains of Mercury formed significantly later than the Caloris basin as evidenced by appreciably smaller crater densities than on the Caloris ejecta blanket 65 Compressional features One unusual feature of Mercury s surface is the numerous compression folds or rupes that crisscross the plains These also exist on the moon but are much more prominent on Mercury 73 As Mercury s interior cooled it contracted and its surface began to deform creating wrinkle ridges and lobate scarps associated with thrust faults The scarps can reach lengths of 1000 km and heights of 3 km 74 These compressional features can be seen on top of other features such as craters and smooth plains indicating they are more recent 75 Mapping of the features has suggested a total shrinkage of Mercury s radius in the range of 1 to 7 km 76 Most activity along the major thrust systems probably ended about 3 6 3 7 billion years ago 77 Small scale thrust fault scarps have been found tens of meters in height and with lengths in the range of a few km that appear to be less than 50 million years old indicating that compression of the interior and consequent surface geological activity continue to the present 74 76 Volcanism Picasso crater the large arc shaped pit located on the eastern side of its floor are postulated to have formed when subsurface magma subsided or drained causing the surface to collapse into the resulting void There is evidence for pyroclastic flows on Mercury from low profile shield volcanoes 78 79 80 51 pyroclastic deposits have been identified 81 where 90 of them are found within impact craters 81 A study of the degradation state of the impact craters that host pyroclastic deposits suggests that pyroclastic activity occurred on Mercury over a prolonged interval 81 A rimless depression inside the southwest rim of the Caloris Basin consists of at least nine overlapping volcanic vents each individually up to 8 km in diameter It is thus a compound volcano 82 The vent floors are at least 1 km below their brinks and they bear a closer resemblance to volcanic craters sculpted by explosive eruptions or modified by collapse into void spaces created by magma withdrawal back down into a conduit 82 Scientists could not quantify the age of the volcanic complex system but reported that it could be on the order of a billion years 82 Surface conditions and exosphere Main article Atmosphere of Mercury Composite of the north pole of Mercury where NASA confirmed the discovery of a large volume of water ice in permanently dark craters that are found there 83 The surface temperature of Mercury ranges from 100 to 700 K 173 to 427 C 280 to 800 F 22 at the most extreme places 0 N 0 W or 180 W It never rises above 180 K at the poles 14 due to the absence of an atmosphere and a steep temperature gradient between the equator and the poles The subsolar point reaches about 700 K during perihelion 0 W or 180 W but only 550 K at aphelion 90 or 270 W 84 On the dark side of the planet temperatures average 110 K 14 85 The intensity of sunlight on Mercury s surface ranges between 4 59 and 10 61 times the solar constant 1 370 W m 2 86 Although the daylight temperature at the surface of Mercury is generally extremely high observations strongly suggest that ice frozen water exists on Mercury The floors of deep craters at the poles are never exposed to direct sunlight and temperatures there remain below 102 K far lower than the global average 87 This creates a cold trap where ice can accumulate Water ice strongly reflects radar and observations by the 70 meter Goldstone Solar System Radar and the VLA in the early 1990s revealed that there are patches of high radar reflection near the poles 88 Although ice was not the only possible cause of these reflective regions astronomers think it was the most likely 89 The icy regions are estimated to contain about 1014 1015 kg of ice 90 and may be covered by a layer of regolith that inhibits sublimation 91 By comparison the Antarctic ice sheet on Earth has a mass of about 4 1018 kg and Mars s south polar cap contains about 1016 kg of water 90 The origin of the ice on Mercury is not yet known but the two most likely sources are from outgassing of water from the planet s interior or deposition by impacts of comets 90 Mercury is too small and hot for its gravity to retain any significant atmosphere over long periods of time it does have a tenuous surface bounded exosphere 92 containing hydrogen helium oxygen sodium calcium potassium and others 16 17 at a surface pressure of less than approximately 0 5 nPa 0 005 picobars 3 This exosphere is not stable atoms are continuously lost and replenished from a variety of sources Hydrogen atoms and helium atoms probably come from the solar wind diffusing into Mercury s magnetosphere before later escaping back into space Radioactive decay of elements within Mercury s crust is another source of helium as well as sodium and potassium MESSENGER found high proportions of calcium helium hydroxide magnesium oxygen potassium silicon and sodium Water vapor is present released by a combination of processes such as comets striking its surface sputtering creating water out of hydrogen from the solar wind and oxygen from rock and sublimation from reservoirs of water ice in the permanently shadowed polar craters The detection of high amounts of water related ions like O OH and H3O was a surprise 93 94 Because of the quantities of these ions that were detected in Mercury s space environment scientists surmise that these molecules were blasted from the surface or exosphere by the solar wind 95 96 Sodium potassium and calcium were discovered in the atmosphere during the 1980 1990s and are thought to result primarily from the vaporization of surface rock struck by micrometeorite impacts 97 including presently from Comet Encke 98 In 2008 magnesium was discovered by MESSENGER 99 Studies indicate that at times sodium emissions are localized at points that correspond to the planet s magnetic poles This would indicate an interaction between the magnetosphere and the planet s surface 100 On November 29 2012 NASA confirmed that images from MESSENGER had detected that craters at the north pole contained water ice MESSENGER s principal investigator Sean Solomon is quoted in The New York Times estimating the volume of the ice to be large enough to encase Washington D C in a frozen block two and a half miles deep 83 According to NASA Mercury is not a suitable planet for Earth like life It has a surface boundary exosphere instead of a layered atmosphere extreme temperatures and high solar radiation It is unlikely that any living beings can withstand those conditions 101 Some parts of the subsurface of Mercury may have been habitable and perhaps life forms albeit likely primitive microorganisms may have existed on the planet 102 103 104 Magnetic field and magnetosphere Main article Mercury s magnetic field Graph showing relative strength of Mercury s magnetic field Despite its small size and slow 59 day long rotation Mercury has a significant and apparently global magnetic field According to measurements taken by Mariner 10 it is about 1 1 the strength of Earth s The magnetic field strength at Mercury s equator is about 300 nT 105 106 Like that of Earth Mercury s magnetic field is dipolar 100 Unlike Earth s Mercury s poles are nearly aligned with the planet s spin axis 107 Measurements from both the Mariner 10 and MESSENGER space probes have indicated that the strength and shape of the magnetic field are stable 107 It is likely that this magnetic field is generated by a dynamo effect in a manner similar to the magnetic field of Earth 108 109 This dynamo effect would result from the circulation of the planet s iron rich liquid core Particularly strong tidal heating effects caused by the planet s high orbital eccentricity would serve to keep part of the core in the liquid state necessary for this dynamo effect 40 110 Mercury s magnetic field is strong enough to deflect the solar wind around the planet creating a magnetosphere The planet s magnetosphere though small enough to fit within Earth 100 is strong enough to trap solar wind plasma This contributes to the space weathering of the planet s surface 107 Observations taken by the Mariner 10 spacecraft detected this low energy plasma in the magnetosphere of the planet s nightside Bursts of energetic particles in the planet s magnetotail indicate a dynamic quality to the planet s magnetosphere 100 During its second flyby of the planet on October 6 2008 MESSENGER discovered that Mercury s magnetic field can be extremely leaky The spacecraft encountered magnetic tornadoes twisted bundles of magnetic fields connecting the planetary magnetic field to interplanetary space that were up to 800 km wide or a third of the radius of the planet These twisted magnetic flux tubes technically known as flux transfer events form open windows in the planet s magnetic shield through which the solar wind may enter and directly impact Mercury s surface via magnetic reconnection 111 This also occurs in Earth s magnetic field The MESSENGER observations showed the reconnection rate is ten times higher at Mercury but its proximity to the Sun only accounts for about a third of the reconnection rate observed by MESSENGER 111 Orbit rotation and longitude Orbit of Mercury 2006 Animation of Mercury s and Earth s revolution around the Sun Mercury has the most eccentric orbit of all the planets in the Solar System its eccentricity is 0 21 with its distance from the Sun ranging from 46 000 000 to 70 000 000 km 29 000 000 to 43 000 000 mi It takes 87 969 Earth days to complete an orbit The diagram illustrates the effects of the eccentricity showing Mercury s orbit overlaid with a circular orbit having the same semi major axis Mercury s higher velocity when it is near perihelion is clear from the greater distance it covers in each 5 day interval In the diagram the varying distance of Mercury to the Sun is represented by the size of the planet which is inversely proportional to Mercury s distance from the Sun This varying distance to the Sun leads to Mercury s surface being flexed by tidal bulges raised by the Sun that are about 17 times stronger than the Moon s on Earth 112 Combined with a 3 2 spin orbit resonance of the planet s rotation around its axis it also results in complex variations of the surface temperature 32 The resonance makes a single solar day the length between two meridian transits of the Sun on Mercury last exactly two Mercury years or about 176 Earth days 113 Mercury s orbit is inclined by 7 degrees to the plane of Earth s orbit the ecliptic the largest of all eight known solar planets 114 As a result transits of Mercury across the face of the Sun can only occur when the planet is crossing the plane of the ecliptic at the time it lies between Earth and the Sun which is in May or November This occurs about every seven years on average 115 Mercury s axial tilt is almost zero 116 with the best measured value as low as 0 027 degrees 117 This is significantly smaller than that of Jupiter which has the second smallest axial tilt of all planets at 3 1 degrees This means that to an observer at Mercury s poles the center of the Sun never rises more than 2 1 arcminutes above the horizon 117 At certain points on Mercury s surface an observer would be able to see the Sun peek up a little more than two thirds of the way over the horizon then reverse and set before rising again all within the same Mercurian day c This is because approximately four Earth days before perihelion Mercury s angular orbital velocity equals its angular rotational velocity so that the Sun s apparent motion ceases closer to perihelion Mercury s angular orbital velocity then exceeds the angular rotational velocity Thus to a hypothetical observer on Mercury the Sun appears to move in a retrograde direction Four Earth days after perihelion the Sun s normal apparent motion resumes 32 A similar effect would have occurred if Mercury had been in synchronous rotation the alternating gain and loss of rotation over revolution would have caused a libration of 23 65 in longitude 118 For the same reason there are two points on Mercury s equator 180 degrees apart in longitude at either of which around perihelion in alternate Mercurian years once a Mercurian day the Sun passes overhead then reverses its apparent motion and passes overhead again then reverses a second time and passes overhead a third time taking a total of about 16 Earth days for this entire process In the other alternate Mercurian years the same thing happens at the other of these two points The amplitude of the retrograde motion is small so the overall effect is that for two or three weeks the Sun is almost stationary overhead and is at its most brilliant because Mercury is at perihelion its closest to the Sun This prolonged exposure to the Sun at its brightest makes these two points the hottest places on Mercury Maximum temperature occurs when the Sun is at an angle of about 25 degrees past noon due to diurnal temperature lag at 0 4 Mercury days and 0 8 Mercury years past sunrise 119 Conversely there are two other points on the equator 90 degrees of longitude apart from the first ones where the Sun passes overhead only when the planet is at aphelion in alternate years when the apparent motion of the Sun in Mercury s sky is relatively rapid These points which are the ones on the equator where the apparent retrograde motion of the Sun happens when it is crossing the horizon as described in the preceding paragraph receive much less solar heat than the first ones described above 120 Mercury attains inferior conjunction nearest approach to Earth every 116 Earth days on average 3 but this interval can range from 105 days to 129 days due to the planet s eccentric orbit Mercury can come as near as 82 200 000 km 0 549 astronomical units 51 1 million miles to Earth and that is slowly declining The next approach to within 82 100 000 km 51 million mi is in 2679 and to within 82 000 000 km 51 million mi in 4487 but it will not be closer to Earth than 80 000 000 km 50 million mi until 28 622 121 Its period of retrograde motion as seen from Earth can vary from 8 to 15 days on either side of inferior conjunction This large range arises from the planet s high orbital eccentricity 32 Essentially because Mercury is closest to the Sun when taking an average over time Mercury is most often the closest planet to the Earth 18 19 and in that measure it is the closest planet to each of the other planets in the Solar System 122 123 124 d Longitude convention The longitude convention for Mercury puts the zero of longitude at one of the two hottest points on the surface as described above However when this area was first visited by Mariner 10 this zero meridian was in darkness so it was impossible to select a feature on the surface to define the exact position of the meridian Therefore a small crater further west was chosen called Hun Kal which provides the exact reference point for measuring longitude 125 126 The center of Hun Kal defines the 20 west meridian A 1970 International Astronomical Union resolution suggests that longitudes be measured positively in the westerly direction on Mercury 127 The two hottest places on the equator are therefore at longitudes 0 W and 180 W and the coolest points on the equator are at longitudes 90 W and 270 W However the MESSENGER project uses an east positive convention 128 Spin orbit resonance After one orbit Mercury has rotated 1 5 times so after two complete orbits the same hemisphere is again illuminated For many years it was thought that Mercury was synchronously tidally locked with the Sun rotating once for each orbit and always keeping the same face directed towards the Sun in the same way that the same side of the Moon always faces Earth Radar observations in 1965 proved that the planet has a 3 2 spin orbit resonance rotating three times for every two revolutions around the Sun The eccentricity of Mercury s orbit makes this resonance stable at perihelion when the solar tide is strongest the Sun is nearly still in Mercury s sky 129 The rare 3 2 resonant tidal locking is stabilized by the variance of the tidal force along Mercury s eccentric orbit acting on a permanent dipole component of Mercury s mass distribution 130 In a circular orbit there is no such variance so the only resonance stabilized in such an orbit is at 1 1 e g Earth Moon when the tidal force stretching a body along the center body line exerts a torque that aligns the body s axis of least inertia the longest axis and the axis of the aforementioned dipole to point always at the center However with noticeable eccentricity like that of Mercury s orbit the tidal force has a maximum at perihelion and therefore stabilizes resonances like 3 2 ensuring that the planet points its axis of least inertia roughly at the Sun when passing through perihelion 130 The original reason astronomers thought it was synchronously locked was that whenever Mercury was best placed for observation it was always nearly at the same point in its 3 2 resonance hence showing the same face This is because coincidentally Mercury s rotation period is almost exactly half of its synodic period with respect to Earth Due to Mercury s 3 2 spin orbit resonance a solar day lasts about 176 Earth days 32 A sidereal day the period of rotation lasts about 58 7 Earth days 32 Simulations indicate that the orbital eccentricity of Mercury varies chaotically from nearly zero circular to more than 0 45 over millions of years due to perturbations from the other planets 32 131 This was thought to explain Mercury s 3 2 spin orbit resonance rather than the more usual 1 1 because this state is more likely to arise during a period of high eccentricity 132 However accurate modeling based on a realistic model of tidal response has demonstrated that Mercury was captured into the 3 2 spin orbit state at a very early stage of its history within 20 more likely 10 million years after its formation 133 Numerical simulations show that a future secular orbital resonant perihelion interaction with Jupiter may cause the eccentricity of Mercury s orbit to increase to the point where there is a 1 chance that the orbit will be destabilized in the next five billion years If this happens Mercury may fall into the Sun collide with Venus be ejected from the Solar System or even disrupt the rest of the inner Solar System 134 135 Advance of perihelion Main article Perihelion precession of Mercury In 1859 the French mathematician and astronomer Urbain Le Verrier reported that the slow precession of Mercury s orbit around the Sun could not be completely explained by Newtonian mechanics and perturbations by the known planets He suggested among possible explanations that another planet or perhaps instead a series of smaller corpuscules might exist in an orbit even closer to the Sun than that of Mercury to account for this perturbation 136 Other explanations considered included a slight oblateness of the Sun The success of the search for Neptune based on its perturbations of the orbit of Uranus led astronomers to place faith in this possible explanation and the hypothetical planet was named Vulcan but no such planet was ever found 137 The observed perihelion precession of Mercury is 5 600 arcseconds 1 5556 per century relative to Earth or 574 10 0 65 arcseconds per century 138 relative to the inertial ICRF Newtonian mechanics taking into account all the effects from the other planets included 0 0254 arcsecond per century due to the flatteness of the Sun predicts a precession of 5 557 arcseconds 1 5436 per century relative to Earth or 531 63 0 69 arcseconds per century relative to ICRF 138 In the early 20th century Albert Einstein s general theory of relativity provided the explanation for the observed precession by formalizing gravitation as being mediated by the curvature of spacetime The effect is small just 42 98 arcseconds per century or 0 43 arcsecond per year or 0 1038 arcsecond per orbital period for Mercury it therefore requires a little over twelve million orbits or 2 8 million years for a full excess turn Similar but much smaller effects exist for other Solar System bodies 8 62 arcseconds per century for Venus 3 84 for Earth 1 35 for Mars and 10 05 for 1566 Icarus 139 140 Observation Image mosaic by Mariner 10 1974 Mercury s apparent magnitude is calculated to vary between 2 48 brighter than Sirius around superior conjunction and 7 25 below the limit of naked eye visibility around inferior conjunction 15 The mean apparent magnitude is 0 23 while the standard deviation of 1 78 is the largest of any planet The mean apparent magnitude at superior conjunction is 1 89 while that at inferior conjunction is 5 93 15 Observation of Mercury is complicated by its proximity to the Sun as it is lost in the Sun s glare for much of the time Mercury can be observed for only a brief period during either morning or evening twilight 141 But in some cases Mercury can better be observed in daylight with a telescope when the position is known because it is higher in the sky and less atmospheric effects affect the view of the planet When proper safety precautions are taken to prevent inadvertently pointing the telescope at the Sun and thus blinding the user Mercury can be viewed as close as 4 to the Sun when near superior conjunction when it is almost at its brightest Mercury can like several other planets and the brightest stars be seen during a total solar eclipse 142 Like the Moon and Venus Mercury exhibits phases as seen from Earth It is new at inferior conjunction and full at superior conjunction The planet is rendered invisible from Earth on both of these occasions because of its being obscured by the Sun 141 except its new phase during a transit Mercury is technically brightest as seen from Earth when it is at a full phase Although Mercury is farthest from Earth when it is full the greater illuminated area that is visible and the opposition brightness surge more than compensates for the distance 143 The opposite is true for Venus which appears brightest when it is a crescent because it is much closer to Earth than when gibbous 143 144 False color map showing the maximum temperatures of the north polar region Nonetheless the brightest full phase appearance of Mercury is an essentially impossible time for practical observation because of the extreme proximity of the Sun Mercury is best observed at the first and last quarter although they are phases of lesser brightness The first and last quarter phases occur at greatest elongation east and west of the Sun respectively At both of these times Mercury s separation from the Sun ranges anywhere from 17 9 at perihelion to 27 8 at aphelion 145 146 At greatest western elongation Mercury rises at its earliest before sunrise and at greatest eastern elongation it sets at its latest after sunset 147 False color image of Carnegie Rupes a tectonic landform high terrain red low blue Mercury is more often and easily visible from the Southern Hemisphere than from the Northern This is because Mercury s maximum western elongation occurs only during early autumn in the Southern Hemisphere whereas its greatest eastern elongation happens only during late winter in the Southern Hemisphere 147 In both of these cases the angle at which the planet s orbit intersects the horizon is maximized allowing it to rise several hours before sunrise in the former instance and not set until several hours after sundown in the latter from southern mid latitudes such as Argentina and South Africa 147 An alternate method for viewing Mercury involves observing the planet during daylight hours when conditions are clear ideally when it is at its greatest elongation This allows the planet to be found easily even when using telescopes with 8 cm 3 1 in apertures However great care must be taken to obstruct the Sun from sight because of the extreme risk for eye damage 148 This method bypasses the limitation of twilight observing when the ecliptic is located at a low elevation e g on autumn evenings Ground based telescope observations of Mercury reveal only an illuminated partial disk with limited detail The first of two spacecraft to visit the planet was Mariner 10 which mapped about 45 of its surface from 1974 to 1975 The second is the MESSENGER spacecraft which after three Mercury flybys between 2008 and 2009 attained orbit around Mercury on March 17 2011 149 to study and map the rest of the planet 150 The Hubble Space Telescope cannot observe Mercury at all due to safety procedures that prevent its pointing too close to the Sun 151 Because the shift of 0 15 revolutions in a year makes up a seven year cycle 0 15 7 1 0 in the seventh year Mercury follows almost exactly earlier by 7 days the sequence of phenomena it showed seven years before 145 Observation historyAncient astronomers Mercury from Liber astronomiae 1550 The earliest known recorded observations of Mercury are from the MUL APIN tablets These observations were most likely made by an Assyrian astronomer around the 14th century BC 152 The cuneiform name used to designate Mercury on the MUL APIN tablets is transcribed as UDU IDIM GU U4 UD the jumping planet e 153 Babylonian records of Mercury date back to the 1st millennium BC The Babylonians called the planet Nabu after the messenger to the gods in their mythology 154 The Greco Egyptian 155 astronomer Ptolemy wrote about the possibility of planetary transits across the face of the Sun in his work Planetary Hypotheses He suggested that no transits had been observed either because planets such as Mercury were too small to see or because the transits were too infrequent 156 Ibn al Shatir s model for the appearances of Mercury showing the multiplication of epicycles using the Tusi couple thus eliminating the Ptolemaic eccentrics and equant In ancient China Mercury was known as the Hour Star Chen xing 辰星 It was associated with the direction north and the phase of water in the Five Phases system of metaphysics 157 Modern Chinese Korean Japanese and Vietnamese cultures refer to the planet literally as the water star 水星 based on the Five elements 158 159 160 Hindu mythology used the name Budha for Mercury and this god was thought to preside over Wednesday 161 The god Odin or Woden of Germanic paganism was associated with the planet Mercury and Wednesday 162 The Maya may have represented Mercury as an owl or possibly four owls two for the morning aspect and two for the evening that served as a messenger to the underworld 163 In medieval Islamic astronomy the Andalusian astronomer Abu Ishaq Ibrahim al Zarqali in the 11th century described the deferent of Mercury s geocentric orbit as being oval like an egg or a pignon although this insight did not influence his astronomical theory or his astronomical calculations 164 165 In the 12th century Ibn Bajjah observed two planets as black spots on the face of the Sun which was later suggested as the transit of Mercury and or Venus by the Maragha astronomer Qotb al Din Shirazi in the 13th century 166 Note that most such medieval reports of transits were later taken as observations of sunspots 167 In India the Kerala school astronomer Nilakantha Somayaji in the 15th century developed a partially heliocentric planetary model in which Mercury orbits the Sun which in turn orbits Earth similar to the Tychonic system later proposed by Tycho Brahe in the late 16th century 168 Ground based telescopic research Transit of Mercury Mercury is visible as a black dot below and to the left of center The dark area above the center of the solar disk is a sunspot Elongation is the angle between the Sun and the planet with Earth as the reference point Mercury appears close to the Sun The first telescopic observations of Mercury were made by Thomas Harriot and Galileo from 1610 In 1612 Simon Marius observed the brightness of Mercury varied with the planet s orbital position and concluded it had phases in the same way as Venus and the Moon 169 In 1631 Pierre Gassendi made the first telescopic observations of the transit of a planet across the Sun when he saw a transit of Mercury predicted by Johannes Kepler In 1639 Giovanni Zupi used a telescope to discover that the planet had orbital phases similar to Venus and the Moon The observation demonstrated conclusively that Mercury orbited around the Sun 32 A rare event in astronomy is the passage of one planet in front of another occultation as seen from Earth Mercury and Venus occult each other every few centuries and the event of May 28 1737 is the only one historically observed having been seen by John Bevis at the Royal Greenwich Observatory 170 The next occultation of Mercury by Venus will be on December 3 2133 171 The difficulties inherent in observing Mercury mean that it was far less studied than the other planets In 1800 Johann Schroter made observations of surface features claiming to have observed 20 kilometre high 12 mi mountains Friedrich Bessel used Schroter s drawings to erroneously estimate the rotation period as 24 hours and an axial tilt of 70 172 In the 1880s Giovanni Schiaparelli mapped the planet more accurately and suggested that Mercury s rotational period was 88 days the same as its orbital period due to tidal locking 173 This phenomenon is known as synchronous rotation The effort to map the surface of Mercury was continued by Eugenios Antoniadi who published a book in 1934 that included both maps and his own observations 100 Many of the planet s surface features particularly the albedo features take their names from Antoniadi s map 174 In June 1962 Soviet scientists at the Institute of Radio engineering and Electronics of the USSR Academy of Sciences led by Vladimir Kotelnikov became the first to bounce a radar signal off Mercury and receive it starting radar observations of the planet 175 176 177 Three years later radar observations by Americans Gordon H Pettengill and Rolf B Dyce using the 300 meter Arecibo radio telescope in Puerto Rico showed conclusively that the planet s rotational period was about 59 days 178 179 The theory that Mercury s rotation was synchronous had become widely held and it was a surprise to astronomers when these radio observations were announced If Mercury were tidally locked its dark face would be extremely cold but measurements of radio emission revealed that it was much hotter than expected Astronomers were reluctant to drop the synchronous rotation theory and proposed alternative mechanisms such as powerful heat distributing winds to explain the observations 180 Water ice yellow at Mercury s north polar region Italian astronomer Giuseppe Colombo noted that the rotation value was about two thirds of Mercury s orbital period and proposed that the planet s orbital and rotational periods were locked into a 3 2 rather than a 1 1 resonance 181 Data from Mariner 10 subsequently confirmed this view 182 This means that Schiaparelli s and Antoniadi s maps were not wrong Instead the astronomers saw the same features during every second orbit and recorded them but disregarded those seen in the meantime when Mercury s other face was toward the Sun because the orbital geometry meant that these observations were made under poor viewing conditions 172 Ground based optical observations did not shed much further light on Mercury but radio astronomers using interferometry at microwave wavelengths a technique that enables removal of the solar radiation were able to discern physical and chemical characteristics of the subsurface layers to a depth of several meters 183 184 Not until the first space probe flew past Mercury did many of its most fundamental morphological properties become known Moreover recent technological advances have led to improved ground based observations In 2000 high resolution lucky imaging observations were conducted by the Mount Wilson Observatory 1 5 meter Hale telescope They provided the first views that resolved surface features on the parts of Mercury that were not imaged in the Mariner 10 mission 185 Most of the planet has been mapped by the Arecibo radar telescope with 5 km 3 1 mi resolution including polar deposits in shadowed craters of what may be water ice 186 Research with space probes Main article Exploration of Mercury MESSENGER being prepared for launch Mercury transiting the Sun as viewed by the Mars rover Curiosity June 3 2014 187 Reaching Mercury from Earth poses significant technical challenges because it orbits so much closer to the Sun than Earth A Mercury bound spacecraft launched from Earth must travel over 91 million kilometres 57 million miles into the Sun s gravitational potential well Mercury has an orbital speed of 47 4 km s 29 5 mi s whereas Earth s orbital speed is 29 8 km s 18 5 mi s 114 Therefore the spacecraft must make a large change in velocity delta v to get to Mercury and then enter orbit 188 as compared to the delta v required for say Mars planetary missions The potential energy liberated by moving down the Sun s potential well becomes kinetic energy requiring a delta v change to do anything other than pass by Mercury Some portion of this delta v budget can be provided from a gravity assist during one or more fly bys of Venus 189 To land safely or enter a stable orbit the spacecraft would rely entirely on rocket motors Aerobraking is ruled out because Mercury has a negligible atmosphere A trip to Mercury requires more rocket fuel than that required to escape the Solar System completely As a result only three space probes have visited it so far 190 A proposed alternative approach would use a solar sail to attain a Mercury synchronous orbit around the Sun 191 Mariner 10 Main article Mariner 10 Mariner 10 the first probe to visit Mercury The first spacecraft to visit Mercury was NASA s Mariner 10 1974 1975 26 The spacecraft used the gravity of Venus to adjust its orbital velocity so that it could approach Mercury making it both the first spacecraft to use this gravitational slingshot effect and the first NASA mission to visit multiple planets 192 Mariner 10 provided the first close up images of Mercury s surface which immediately showed its heavily cratered nature and revealed many other types of geological features such as the giant scarps that were later ascribed to the effect of the planet shrinking slightly as its iron core cools 193 Unfortunately the same face of the planet was lit at each of Mariner 10 s close approaches This made close observation of both sides of the planet impossible 194 and resulted in the mapping of less than 45 of the planet s surface 195 The spacecraft made three close approaches to Mercury the closest of which took it to within 327 km 203 mi of the surface 196 At the first close approach instruments detected a magnetic field to the great surprise of planetary geologists Mercury s rotation was expected to be much too slow to generate a significant dynamo effect The second close approach was primarily used for imaging but at the third approach extensive magnetic data were obtained The data revealed that the planet s magnetic field is much like Earth s which deflects the solar wind around the planet For many years after the Mariner 10 encounters the origin of Mercury s magnetic field remained the subject of several competing theories 197 198 On March 24 1975 just eight days after its final close approach Mariner 10 ran out of fuel Because its orbit could no longer be accurately controlled mission controllers instructed the probe to shut down 199 Mariner 10 is thought to be still orbiting the Sun passing close to Mercury every few months 200 MESSENGER Main article MESSENGER Estimated details of the impact of MESSENGER on April 30 2015 A second NASA mission to Mercury named MESSENGER MErcury Surface Space ENvironment GEochemistry and Ranging was launched on August 3 2004 It made a fly by of Earth in August 2005 and of Venus in October 2006 and June 2007 to place it onto the correct trajectory to reach an orbit around Mercury 201 A first fly by of Mercury occurred on January 14 2008 a second on October 6 2008 202 and a third on September 29 2009 203 Most of the hemisphere not imaged by Mariner 10 was mapped during these fly bys The probe successfully entered an elliptical orbit around the planet on March 18 2011 The first orbital image of Mercury was obtained on March 29 2011 The probe finished a one year mapping mission 202 and then entered a one year extended mission into 2013 In addition to continued observations and mapping of Mercury MESSENGER observed the 2012 solar maximum 204 The mission was designed to clear up six key issues Mercury s high density its geological history the nature of its magnetic field the structure of its core whether it has ice at its poles and where its tenuous atmosphere comes from To this end the probe carried imaging devices that gathered much higher resolution images of much more of Mercury than Mariner 10 assorted spectrometers to determine abundances of elements in the crust and magnetometers and devices to measure velocities of charged particles Measurements of changes in the probe s orbital velocity were expected to be used to infer details of the planet s interior structure 205 MESSENGER s final maneuver was on April 24 2015 and it crashed into Mercury s surface on April 30 2015 206 207 208 The spacecraft s impact with Mercury occurred near 3 26 pm EDT on April 30 2015 leaving a crater estimated to be 16 m 52 ft in diameter 209 BepiColombo Main article BepiColombo The European Space Agency and the Japanese Space Agency developed and launched a joint mission called BepiColombo which will orbit Mercury with two probes one to map the planet and the other to study its magnetosphere 210 Launched on October 20 2018 BepiColombo is expected to reach Mercury in 2025 211 It will release a magnetometer probe into an elliptical orbit then chemical rockets will fire to deposit the mapper probe into a circular orbit Both probes will operate for one terrestrial year 210 The mapper probe carries an array of spectrometers similar to those on MESSENGER and will study the planet at many different wavelengths including infrared ultraviolet X ray and gamma ray 212 BepiColombo conducted the first of its six planned Mercury flybys on October 1 2021 213 See also Solar System portal Outer space portal Astronomy portalOutline of Mercury planet Budha a deity identified with the planet in Hindu astrology Colonization of Mercury Mercury in astrology Mercury in fictionNotes In astronomy the words rotation and revolution have different meanings Rotation is the turning of a body about an axis that passes through the body as in Earth rotates once a day Revolution is motion around a centre that is external to the body usually in orbit as in Earth takes a year for each revolution around the Sun The verbs rotate and revolve mean doing rotation and revolution respectively Pluto was considered a planet from its discovery in 1930 to 2006 but after that it has been reclassified as a dwarf planet Pluto s orbital eccentricity is greater than Mercury s Pluto is also smaller than Mercury but was thought to be larger until 1976 The Sun s total angular displacement during its apparent retrograde motion as seen from the surface of Mercury is 1 23 while the Sun s angular diameter when the apparent retrograde motion begins and ends is 1 71 increasing to 1 73 at perihelion midway through the retrograde motion It is important to be clear about the meaning of closeness In the astronomical literature the term closest planets often means the two planets that approach each other most closely In other words the orbits of the two planets approach each other most closely However this does not mean that the two planets are closest over time For example essentially because Mercury is closer to the Sun than Venus Mercury spends more time in proximity to Earth it could therefore be said that Mercury is the planet that is closest to Earth when averaged over time However using this time average definition of closeness as noted above it turns out that Mercury is the closest planet to all other planets in the solar system For that reason arguably the proximity definition is not particularly helpful An episode of the BBC Radio 4 programme More or Less explains the different notions of proximity well 19 Some sources precede the cuneiform transcription with MUL MUL is a cuneiform sign that was used in the Sumerian language to designate a star or planet but it is not considered part of the actual name The 4 is a reference number in the Sumero Akkadian transliteration system to designate which of several syllables a certain cuneiform sign is most likely designating References Mercurian Lexico UK English Dictionary Oxford University Press Archived from the original on March 27 2020 Mercurial Lexico UK English Dictionary UK English Dictionary Oxford University Press Archived from the original on December 22 2019 a b c d e f g h i j k l m Williams David R November 25 2020 Mercury Fact Sheet NASA Retrieved April 19 2021 Souami D Souchay J July 2012 The solar system s invariable plane Astronomy amp Astrophysics 543 11 Bibcode 2012A amp A 543A 133S doi 10 1051 0004 6361 201219011 A133 Yeomans Donald K April 7 2008 HORIZONS Web Interface for Mercury Major Body JPL Horizons On Line Ephemeris System Retrieved April 7 2008 Select Ephemeris Type Orbital Elements Time Span 2000 01 01 12 00 to 2000 01 02 Target Body Mercury and 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2455 2457 Bibcode 2000AJ 119 2455D doi 10 1086 301328 S2CID 121483006 Harmon John K Slade Martin A Butler Bryan J Head III James W Rice Melissa S Campbell Donald B 2007 Mercury Radar images of the equatorial and midlatitude zones Icarus 187 2 374 405 Bibcode 2007Icar 187 374H doi 10 1016 j icarus 2006 09 026 Webster Guy June 10 2014 Mercury Passes in Front of the Sun as Seen From Mars NASA Retrieved June 10 2014 Zacny Kris July 2 2015 Inner Solar System Prospective Energy and Material Resources Springer International Publishing p 154 ISBN 9783319195698 Wagner Sam Wie Bong November 2015 Hybrid Algorithm for Multiple Gravity Assist and Impulsive Delta V Maneuvers Journal of Guidance Control and Dynamics 38 11 2096 2107 Bibcode 2015JGCD 38 2096W doi 10 2514 1 G000874 Mercury PDF NASA Jet Propulsion Laboratory May 5 2008 Retrieved April 26 2021 Leipold Manfred E Seboldt W Lingner Stephan Borg Erik Herrmann Axel Siegfried Pabsch Arno Wagner O Bruckner Johannes 1996 Mercury sun synchronous 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and Planetary Science Letters 218 3 4 261 268 Bibcode 2004E amp PSL 218 261A doi 10 1016 S0012 821X 03 00682 4 Dunne James A amp Burgess Eric 1978 Chapter Eight The Voyage of Mariner 10 Mission to Venus and Mercury NASA History Office Grayzeck Ed April 2 2008 Mariner 10 NSSDC Master Catalog NASA Retrieved April 7 2008 MESSENGER Engine Burn Puts Spacecraft on Track for Venus SpaceRef com 2005 Retrieved March 2 2006 a b Countdown to MESSENGER s Closest Approach with Mercury Johns Hopkins University Applied Physics Laboratory January 14 2008 Archived from the original on May 13 2013 Retrieved May 30 2008 MESSENGER Gains Critical Gravity Assist for Mercury Orbital Observations MESSENGER Mission News September 30 2009 Archived from the original on May 10 2013 Retrieved September 30 2009 NASA extends spacecraft s Mercury mission United Press International November 15 2011 Retrieved November 16 2011 MESSENGER Fact Sheet PDF Applied Physics Laboratory February 2011 Retrieved August 21 2017 Wall Mike March 29 2015 NASA Mercury Probe Trying to Survive for Another Month Space com Retrieved April 4 2015 Chang Kenneth April 27 2015 NASA s Messenger Mission Is Set to Crash Into Mercury The New York Times Archived from the original on April 29 2015 Retrieved April 27 2015 Corum Jonathan April 30 2015 Messenger s Collision Course With Mercury The New York Times Retrieved April 30 2015 Best Determination of MESSENGER s Impact Location MESSENGER Featured Images Johns Hopkins Applied Physics Lab June 3 2015 Retrieved April 29 2015 a b ESA gives go ahead to build BepiColombo European Space Agency February 26 2007 Retrieved May 29 2008 BepiColombo Fact Sheet European Space Agency December 1 2016 Retrieved December 19 2016 Objectives European Space Agency February 21 2006 Retrieved May 29 2008 Warren Haygen October 24 2021 BepiColombo completes first Mercury flyby science provides insight into planet s unique environment NASA Spaceflight Retrieved October 8 2022 External linksListen to this article 41 minutes source source track This audio file was created from a revision of this article dated 16 January 2008 2008 01 16 and does not reflect subsequent edits Audio help More spoken articles Mercury at Wikipedia s sister projects Definitions from Wiktionary Media from Commons Quotations from Wikiquote Texts from Wikisource Textbooks from Wikibooks Resources from Wikiversity Data from Wikidata Atlas of Mercury NASA 1978 SP 423 Mercury nomenclature and map with feature names from the USGS IAU Gazetteer of Planetary Nomenclature Equirectangular map of Mercury by Applied Coherent Technology Corp 3D globe of Mercury by Google Mercury at Solarviews com Mercury by Astronomy Cast MESSENGER mission web site BepiColombo mission web site Portals Astronomy Stars Spaceflight Outer space Solar system Retrieved from https en wikipedia org w index php title Mercury planet amp oldid 1135550469, wikipedia, wiki, book, books, library,

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