Mercury is the firstplanet from theSun and the smallest in theSolar System. In English, it is named after theancient Roman godMercurius (Mercury), god of commerce and communication, and the messenger of the gods. Mercury is classified as aterrestrial planet, with roughly the same surfacegravity asMars. The surface of Mercury is heavilycratered, as a result of countlessimpact events that have accumulated over billions of years. Its largest crater,Caloris Planitia, has a diameter of 1,550 km (960 mi), which is about one-third the diameter of the planet (4,880 km or 3,030 mi). Similarly to theEarth'sMoon, Mercury's surface displays an expansiverupes system generated fromthrust faults and brightray systems formed byimpact event remnants.
Mercury'ssidereal year (88.0 Earth days) andsidereal day (58.65 Earth days) are in a 3:2 ratio. This relationship is calledspin–orbit resonance, andsidereal here means "relative to the stars". Consequently, one solar day (sunrise to sunrise) on Mercury lasts for around 176 Earth days: twice the planet's sidereal year. This means that one side of Mercury will remain in sunlight for one Mercurian year of 88 Earth days; while during the next orbit, that side will be in darkness all the time until the next sunrise after another 88 Earth days.
As of the early 2020s, many broad details of Mercury's geological history are still under investigation or pending data from space probes. Like other planets in the Solar System, Mercury was formed approximately 4.5 billion years ago. Itsmantle is highly homogeneous, which suggests that Mercury had amagma ocean early in its history, like the Moon. According to currentmodels, Mercury may have a solidsilicate crust and mantle overlying a solid outer core, a deeper liquid core layer, and a solid inner core. There are many competing hypotheses about Mercury's origins and development, some of which incorporate collision withplanetesimals and rock vaporization.
Nomenclature
Historically, humans knew Mercury by different names depending on whether it was an evening star or a morning star. By about 350 BC, theancient Greeks had realized the two stars were one.[20] They knew the planet asΣτίλβωνStilbōn, meaning "twinkling", andἙρμήςHermēs, for its fleeting motion,[21] a name that is retained in modernGreek (ΕρμήςErmis).[22] The Romans named the planet after the swift-footed Roman messenger god,Mercury (LatinMercurius), whom they equated with the Greek Hermes, because it moves across the sky faster than any other planet,[20][23] though some associated the planet withApollo instead, as detailed byPliny the Elder.[24] Theastronomical symbol for Mercury is a stylized version of Hermes'caduceus; aChristian cross was added in the 16th century:.[25][26]
Mercury is one of fourterrestrial planets in theSolar System, which means it is a rocky body like Earth. It is the smallest planet in the Solar System, with anequatorialradius of 2,439.7 kilometres (1,516.0 mi).[4] Mercury is alsosmaller—albeit more massive—than the largestnatural satellites in the Solar System,Ganymede andTitan. Mercury consists of approximately 70% metallic and 30%silicate material.[27]
Internal structure
Mercury's internal structure and magnetic field
Mercury appears to have a solid silicatecrust and mantle overlying a solid, metallic outer core layer, a deeper liquid core layer, and a solid inner core.[28][29] The composition of the iron-rich core remains uncertain, but it likely contains nickel, silicon and perhaps sulfur and carbon, plus trace amounts of other elements.[30] 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.[4] If the effect ofgravitational 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.[31] 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 thecore, 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.[32]
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 amoment of inertia factor of0.346±0.014.[9][33] 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.[34][35] The mantle-crust layer is in total 420 km (260 mi) thick.[36] Projections differ as to the size of the crust specifically; data from theMariner 10 andMESSENGER probes suggests a thickness of 35 km (22 mi), whereas anAiry isostacy model suggests a thickness of 26 ± 11 km (16.2 ± 6.8 mi).[37][38][39] 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.[40][41][42]
Mercury's core has a higher iron content than that of any other 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 commonchondrite meteorites, thought to be typical of the Solar System's rocky matter, and a mass approximately 2.25 times its current mass.[43] Early in the Solar System's history, Mercury may have been struck by aplanetesimal of approximately1⁄6 Mercury's mass and several thousand kilometers across.[43] The impact would have stripped away much of the original crust and mantle, leaving the core behind as a relatively major component.[43] A similar process, known as thegiant impact hypothesis, has been proposed to explain the formation of Earth's Moon.[43]
Alternatively, Mercury may have formed from thesolar nebula before the Sun's energy output had stabilized. It would initially have had twice its present mass, but as theprotosun contracted, temperatures near Mercury could have been between 2,500 and 3,500 K and possibly even as high as 10,000 K.[44] 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 thesolar wind.[44] A third hypothesis proposes that the solar nebula causeddrag on the particles from which Mercury wasaccreting, which meant that lighter particles were lost from the accreting material and not gathered by Mercury.[45]
Each hypothesis predicts a different surface composition, and two space missions have been tasked with making observations of this composition. The firstMESSENGER, 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.[46]BepiColombo, which will arrive at Mercury in 2025, will make observations to test these hypotheses.[47] The findings so far would seem to favor the third hypothesis; however, further analysis of the data is needed.[48]
Mercury's surface is similar in appearance to that of the Moon, showing extensivemare-like plains and heavy cratering, indicating that it has been geologically inactive for billions of years. It is moreheterogeneous than the surface ofMars or the Moon, both of which contain significant stretches of similar geology, such asmaria and plateaus.[49]Albedo features are areas of markedly different reflectivity, which include impact craters, the resulting ejecta, andray systems. Larger albedo features correspond to higher reflectivity plains.[50] Mercury has "wrinkle-ridges" (dorsa), Moon-likehighlands, mountains (montes), plains (planitiae), escarpments (rupes), and valleys (valles).[51][52]
MASCS spectrum scan of Mercury's surface byMESSENGER
The planet's mantle is chemically heterogeneous, suggesting the planet went through amagma ocean phase early in its history. Crystallization of minerals and convective overturn resulted in a 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 earlychemically reducing conditions than is found on other terrestrial planets. The surface is dominated by iron-poorpyroxene andolivine, as represented byenstatite andforsterite, respectively, along with sodium-richplagioclase 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.[53][54]
Names for features on Mercury come from a variety of sources and are set according to theIAUplanetary nomenclature system. 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 orfossae are named for works of architecture. Montes are named for the word "hot" in a variety of languages.Plains or planitiae are named forMercury in various languages.Escarpments orrupēs are named for ships of scientific expeditions. Valleys or valles are named for abandoned cities, towns, or settlements of antiquity.[55]
Mercury was heavily bombarded by comets andasteroids during and shortly following its formation 4.6 billion years ago, as well as during a possibly separate subsequent episode called theLate Heavy Bombardment that ended 3.8 billion years ago.[56] Mercury received impacts over its entire surface during this period of intense crater formation,[52] facilitated by the lack of anyatmosphere to slow impactors down.[57] During this time Mercury wasvolcanically active; basins were filled bymagma, producing smooth plains similar to the maria found on the Moon.[58][59] One of the most unusual craters isApollodorus, or "the Spider", which hosts a series of radiating troughs extending outwards from its impact site.[60]
Craters on Mercury range in diameter from small bowl-shaped cavities tomulti-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.[61] According toInternational 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.[62]
Overhead view of Caloris Basin
Perspective view of Caloris Basin – high (red); low (blue)
The largest known crater isCaloris Planitia, or Caloris Basin, with a diameter of 1,550 km (960 mi).[63] The impact that created the Caloris Basin was so powerful that it causedlava eruptions and left a concentric mountainous ring ~2 km (1.2 mi) tall surrounding theimpact 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 were volcanic lava flows induced by the impact or a large sheet of impact melt.[61]
At theantipode 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.[64] Alternatively, it has been suggested that this terrain formed as a result of the convergence of ejecta at this basin's antipode.[65]
Tolstoj basin is along the bottom of this image of Mercury's limb
Overall, 46 impact basins have been identified.[66] A notable basin is the 400 km (250 mi)-wide, multi-ringTolstoj Basin that has an ejecta blanket extending up to 500 km (310 mi) 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 (388 mi)-diameter rim.[61] Like the Moon, the surface of Mercury has likely incurred the effects ofspace weathering processes, including solar wind andmicrometeorite impacts.[67]
Plains
There are two geologically distinct plains regions on Mercury.[61][68] Gently rolling, hillyplains in the regions between craters are Mercury's oldest visible surfaces,[61] 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 (19 mi) in diameter.[68]
Smooth plains are widespread flat areas that fill depressions of various sizes and bear a strong resemblance to 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 localization and rounded, lobate shape of these plains strongly support volcanic origins.[61] 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.[61]
Compressional features
An unusual feature of Mercury's surface is the numerous compression folds, orrupes, that crisscross the plains. These exist on the Moon, but are much more prominent on Mercury.[69] As Mercury's interior cooled, it contracted and its surface began to deform, creatingwrinkle ridges andlobate scarps associated withthrust faults. The scarps can reach lengths of 1,000 km (620 mi) and heights of 3 km (1.9 mi).[70] These compressional features can be seen on top of other features, such as craters and smooth plains, indicating they are more recent.[71] Mapping of the features has suggested a total shrinkage of Mercury's radius in the range of ~1–7 km (0.62–4.35 mi).[72] Most activity along the major thrust systems probably ended about 3.6–3.7 billion years ago.[73] Small-scale thrust fault scarps have been found, tens of meters in height and with lengths in the range of a few kilometers, 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.[70][72]
Volcanism
Picasso crater—the large arc-shaped pit located on the eastern side of its floor is postulated to have formed when subsurface magma subsided or drained, causing the surface to collapse into the resulting void.
There is evidence forpyroclastic flows on Mercury from low-profileshield volcanoes.[74][75][76] Fifty-one pyroclastic deposits have been identified,[77] where 90% of them are found within impact craters.[77] 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.[77]
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 (5.0 mi) in diameter. It is thus a "compound volcano".[78] The vent floors are at least 1 km (0.62 mi) 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.[78] Scientists could not quantify the age of the volcanic complex system but reported that it could be on the order of a billion years.[78]
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.[79]
The surface temperature of Mercury ranges from 100 to 700 K (−173 to 427 °C; −280 to 800 °F).[80] It never rises above 180 K at the poles,[15] due to the absence of an atmosphere and a steep temperature gradient between the equator and the poles. Atperihelion, the equatorialsubsolar point is located at latitude 0°W or 180°W, and it climbs to a temperature of about700 K. Duringaphelion, this occurs at 90° or 270°W and reaches only550 K.[81] On the dark side of the planet, temperatures average110 K.[15][82] The intensity ofsunlight on Mercury's surface ranges between 4.59 and 10.61 times thesolar constant (1,370 W·m−2).[83]
Although daylight temperatures at the surface of Mercury are 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.[84] This creates acold trap where ice can accumulate. Water ice strongly reflectsradar, and observations by the 70-meterGoldstone Solar System Radar and theVLA in the early 1990s revealed that there are patches of high radarreflection near the poles.[85] Although ice was not the only possible cause of these reflective regions, astronomers thought it to be the most likely explanation.[86] The presence ofwater ice was confirmed usingMESSENGER images of craters at the north pole.[79]
The icy crater regions are estimated to contain about 1014–1015 kg of ice,[87] and may be covered by a layer ofregolith that inhibitssublimation.[88] By comparison, theAntarctic ice sheet on Earth has a mass of about 4×1018 kg, and Mars's south polar cap contains about 1016 kg of water.[87] The origin of the ice on Mercury is not yet known, but the two most likely sources are fromoutgassing of water from the planet's interior and deposition by impacts of comets.[87]
Mercury is too small and hot for itsgravity to retain any significantatmosphere over long periods of time; it does have a tenuous surface-boundedexosphere[89] at a surface pressure of less than approximately 0.5 nPa (0.005 picobars).[4] It includeshydrogen,helium,oxygen,sodium,calcium,potassium,magnesium,silicon, andhydroxide, among others.[18][19] This exosphere is not stable—atoms are continuously lost and replenished from a variety of sources.Hydrogen atoms andhelium atoms probably come from the solar wind,diffusing into Mercury'smagnetosphere before later escaping back into space. Theradioactive decay of elements within Mercury's crust is another source of helium, as well as sodium and potassium. 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−, andH3O+ was a surprise.[90][91] 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.[92][93]
Sodium, potassium, and calcium were discovered in the atmosphere during the 1980s–1990s, and are thought to result primarily from the vaporization of surface rock struck by micrometeorite impacts[94] including presently fromComet Encke.[95] In 2008, magnesium was discovered byMESSENGER.[96] 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.[97]
According to NASA, Mercury is not a suitable planet for Earth-like life. It has asurface boundary exosphere instead of a layered atmosphere, extreme temperatures, and high solar radiation. It is unlikely that any living beings can withstand those conditions.[98] Some parts of the subsurface of Mercury may have beenhabitable, and perhapslife forms, albeit likely primitivemicroorganisms, may have existed on the planet.[99][100][101]
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 byMariner 10, it is about 1.1% the strength ofEarth's. The magnetic-field strength at Mercury's equator is about300nT.[102][103] Like that of Earth, Mercury's magnetic field isdipolar[97] and nearly aligned with the planet's spin axis (10° dipolar tilt, compared to 11° for Earth).[104] Measurements from both theMariner 10 andMESSENGER space probes have indicated that the strength and shape of the magnetic field are stable.[104]
It is likely that this magnetic field is generated by adynamo effect, in a manner similar to the magnetic field of Earth.[105][106] This dynamo effect would result from the circulation of the planet's iron-rich liquid core. Particularly strongtidal 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.[107][108]
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,[97] is strong enough to trap solar windplasma. This contributes to the space weathering of the planet's surface.[104] Observations taken by theMariner 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.[97]
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 to800 km wide or a third of the radius of the planet. These twisted magnetic flux tubes, technically known asflux transfer events, form open windows in the planet's magnetic shield through which the solar wind may enter and directly impact Mercury's surface viamagnetic reconnection.[109] This also occurs in Earth's magnetic field. TheMESSENGER observations showed the reconnection rate was ten times higher at Mercury, but its proximity to the Sun only accounts for about a third of the reconnection rate observed byMESSENGER.[109]
Orbit, rotation, and longitude
Orbit of Mercury (2006)
Animation of Mercury's and Earth's revolution around the Sun
Mercury has the mosteccentric 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 samesemi-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 bytidal bulges raised by theSun that are about 17 times stronger than the Moon's on Earth.[110] Combined with a 3:2spin–orbit resonance of the planet's rotation around its axis, it also results in complex variations of the surface temperature.[27] The resonance makes a singlesolar day (the length between twomeridian transits of the Sun) on Mercury last exactly two Mercury years, or about 176 Earth days.[111]
Mercury's orbit is inclined by 7 degrees to the plane of Earth's orbit (theecliptic), the largest of all eight known solar planets.[112] 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.[113]
Mercury'saxial tilt is almost zero,[114] with the best measured value as low as 0.027 degrees.[115] This is significantly smaller than that ofJupiter, 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 2.1arcminutes above the horizon.[115] By comparison, theangular size of the Sun as seen from Mercury ranges from1+1⁄4 to 2 degrees across.[116]
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 sameMercurian day.[a] This is because approximately four Earth days before perihelion, Mercury's angularorbital velocity equals its angularrotational velocity so that the Sun'sapparent 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 aretrograde direction. Four Earth days after perihelion, the Sun's normal apparent motion resumes.[27] A similar effect would have occurred if Mercury had been in synchronous rotation: the alternating gain and loss of rotation over a revolution would have caused a libration of 23.65° in longitude.[117]
For the same reason, there are two points on Mercury's equator, 180 degrees apart inlongitude, 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 todiurnal temperature lag, at 0.4 Mercury days and 0.8 Mercury years past sunrise.[118] 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.[119]
Mercury attains an inferior conjunction (nearest approach to Earth) every 116 Earth days on average,[4] 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.[120] Its period of retrograde motion as seen from Earth can vary from 8 to 15 days on either side of an inferior conjunction. This large range arises from the planet's high orbital eccentricity.[27] Essentially, because Mercury is closest to the Sun, when taking an average over time, Mercury is most often the closest planet to the Earth,[121][122] and—in that measure—it is the closest planet to each of the other planets in the Solar System.[123][124][125][b]
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, byMariner 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, calledHun Kal, which provides the exact reference point for measuring longitude.[126][127] 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.[128] 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, theMESSENGER project uses an east-positive convention.[129]
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 synchronouslytidally 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 stationary in Mercury's sky.[130]
The 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.[131] 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 always point 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.[131]
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.[27] Asidereal day (the period of rotation) lasts about 58.7 Earth days.[27]
Simulations indicate that the orbital eccentricity of Mercury varieschaotically from nearly zero (circular) to more than 0.45 over millions of years due toperturbations from the other planets.[27][132] 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.[133] 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.[134]
Numerical simulations show that a futuresecularorbital resonant interaction with the perihelion of 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.[135][136]
In 1859, the French mathematician and astronomerUrbain Le Verrier reported that the slowprecession of Mercury's orbit around the Sun could not be completely explained byNewtonian 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.[137] Other explanations considered included a slight oblateness of the Sun. The success of the search forNeptune based on its perturbations of the orbit ofUranus led astronomers to place faith in this possible explanation, and the hypothetical planet was namedVulcan, but no such planet was ever found.[138]
The observedperihelion precession of Mercury is 5,600arcseconds (1.5556°) per century relative to Earth, or574.10±0.65 arcseconds per century[139] relative to the inertialICRF. Newtonian mechanics, taking into account all the effects from the other planets and including 0.0254 arcseconds per century due to the oblateness of the Sun, predicts a precession of 5,557 arcseconds (1.5436°) per century relative to Earth, or531.63±0.69 arcseconds per century relative to ICRF.[139] In the early 20th century,Albert Einstein'sgeneral 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: just42.980±0.001 arcseconds per century (or 0.43 arcsecond per year, or 0.1035 arcsecond per orbital period) for Mercury; it therefore requires a little over 12.5 million orbits, or 3 million years, for a full excess turn. Similar, but much smaller, effects exist for other Solar System bodies: 8.6247 arcseconds per century for Venus, 3.8387 for Earth, 1.351 for Mars, and 10.05 for1566 Icarus.[140][141]
Observation
Image mosaic byMariner 10, 1974
Mercury'sapparent magnitude is calculated to vary between −2.48 (brighter thanSirius) aroundsuperior conjunction and +7.25 (below the limit of naked-eye visibility) aroundinferior conjunction.[16] 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.[16] 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.[142]
Ground-based telescope observations of Mercury reveal only an illuminated partial disk with limited detail. TheHubble Space Telescope cannot observe Mercury at all, due to safety procedures that prevent its pointing too close to the Sun.[143] Because the shift of 0.15 revolutions of Earth in a Mercurian year makes up a seven-Mercurian-year cycle (0.15 × 7 ≈ 1.0), in the seventh Mercurian year, Mercury follows almost exactly (earlier by 7 days) the sequence of phenomena it showed seven Mercurian years before.[144]
Like the Moon and Venus, Mercury exhibitsphases as seen from Earth. It is "new" atinferior 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,[142] except at 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 theopposition brightness surge more than compensates for the distance.[145] The opposite is true for Venus, which appears brightest when it is acrescent, because it is much closer to Earth than whengibbous.[145][146]
False-color map showing the maximum temperatures of the north polar regionMercury (lower left) as seen fromSan Jose, California with Venus and the Moon.
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 greatestelongation 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.[144][147] At greatestwestern elongation, Mercury rises at its earliest before sunrise, and at greatesteastern elongation, it sets at its latest after sunset.[148]
Mercury is more often and easily visible from theSouthern Hemisphere than from theNorthern. 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.[148] 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.[148]
An alternate method for viewing Mercury involves observing the planet with atelescope 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.[149] This method bypasses the limitation of twilight observing when the ecliptic is located at a low elevation (e.g. on autumn evenings). The planet is higher in the sky and less atmospheric effects affect the view of the planet. Mercury can be viewed as close as 4° to the Sun near superior conjunction when it is almost at its brightest.
Mercury can, like several other planets and the brightest stars, be seen during a totalsolar eclipse.[150]
Observation history
Ancient astronomers
Mercury, fromLiber astronomiae, 1550
The earliest known recorded observations of Mercury are from theMUL.APIN tablets. These observations were most likely made by anAssyrian astronomer around the 14th century BC.[151] Thecuneiform name used to designate Mercury on the MUL.APIN tablets is transcribed as UDU.IDIM.GU\U4.UD ("the jumping planet").[c][152]Babylonian records of Mercury date back to the 1st millennium BC. TheBabylonians called the planetNabu after the messenger to the gods intheir mythology.[153]
TheGreco-Egyptian[154] astronomerPtolemy wrote about the possibility of planetary transits across the face of the Sun in his workPlanetary Hypotheses. He suggested that no transits had been observed either because planets such as Mercury were too small to see, or because transits were too infrequent.[155]
Ibn al-Shatir's model for the appearances of Mercury, showing the multiplication ofepicycles using theTusi couple, thus eliminating the Ptolemaic eccentrics andequant.
Inancient 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 theFive Phases system of metaphysics.[156] ModernChinese,Korean,Japanese andVietnamese cultures refer to the planet literally as the "water star" (水星), based on theFive elements.[157][158][159]Hindu mythology used the nameBudha for Mercury, and this god was thought to preside over Wednesday.[160] The godOdin (or Woden) ofGermanic paganism was associated with the planet Mercury and Wednesday.[161] TheMaya 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 theunderworld.[162] Mercury was sometimes known asStilbon (Greek: Στίλβων) meaning 'the shining, glittering'.[163]
In India, theKerala school astronomerNilakantha Somayaji in the 15th century developed a partially heliocentric planetary model in which Mercury orbits the Sun, which in turn orbits Earth, similar to theTychonic system later proposed byTycho Brahe in the late 16th century.[168]
Ground-based telescopic research
The first telescopic observations of Mercury were made byThomas Harriot andGalileo 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 byJohannes 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 the Sun.[27]
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 byJohn Bevis at theRoyal Greenwich Observatory.[170] The next occultation of Mercury by Venus will be on December 3, 2133.[171]
The difficulties inherent in observing Mercury meant 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 assynchronous rotation. The effort to map the surface of Mercury was continued byEugenios Antoniadi, who published a book in 1934 that included both maps and his own observations.[97] Many of the planet's surface features, particularly thealbedo features, take their names from Antoniadi's map.[174]
In June 1962, Soviet scientists at theInstitute of Radio-engineering and Electronics of theUSSR Academy of Sciences, led byVladimir 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 AmericansGordon H. Pettengill and Rolf B. Dyce, using the 300-metre-wide (330 yd)Arecibo radio telescope inPuerto 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]
In 1965, Italian astronomerGiuseppe 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 fromMariner 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 everysecond 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, technological advances have led to improved ground-based observations. In 2000, high-resolutionlucky imaging observations were conducted by theMount Wilson Observatory 1.5-metre (4.9 ft) Hale telescope. They provided the first views that resolved surface features on the parts of Mercury that were not imaged in theMariner 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]
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 asunspot.
Elongation is the angle between the Sun and the planet, with Earth as the reference point. Mercury appears close to the Sun.
Water ice (yellow) at Mercury's north polar region
MESSENGER being prepared for launchMercury transiting theSun as viewed by the Mars roverCuriosity (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 gravitationalpotential well. Mercury has anorbital speed of 47.4 km/s (29.5 mi/s), whereas Earth's orbital speed is 29.8 km/s (18.5 mi/s).[112] Therefore, the spacecraft must make a larger change invelocity (delta-v) to get to Mercury and then enter orbit,[188] as compared to the delta-v required for, say,Mars planetary missions.
Thepotential energy liberated by moving down the Sun's potential well becomeskinetic energy, requiring a delta-v change to do anything other than pass by Mercury. Some portion of thisdelta-v budget can be provided from agravity 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 toescape the Solar System completely. As a result, only three space probes have visited it so far.[190] A proposed alternative approach would use asolar sail to attain a Mercury-synchronous orbit around the Sun.[191]
The first spacecraft to visit Mercury was NASA'sMariner 10 (1974–1975).[20] 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 ofMariner 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 significantdynamo 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 theMariner 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]
Estimated details of the impact ofMESSENGER on April 30, 2015
A second NASA mission to Mercury, namedMESSENGER (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 byMariner 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 2012solar maximum.[204]
Topography of Mercury based on MDIS (Mercury Dual Imaging System) data
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 thanMariner 10, assortedspectrometers to determine the abundances of elements in the crust, andmagnetometers 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 at 3:26:01 p.m.EDT on April 30, 2015, leaving a crater estimated to be 16 m (52 ft) in diameter.[209]
TheEuropean Space Agency and theJapanese Space Agency developed and launched a joint mission calledBepiColombo, 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 onMESSENGER, and will study the planet at many different wavelengths includinginfrared,ultraviolet,X-ray andgamma ray.[212]BepiColombo conducted the first of its six planned Mercury flybys on October 1, 2021,[213] and the sixth was completed on January 9, 2025. The spacecraft will enter the planet's orbit in 2026.[214]
On March 5, 2024, NASA released images of transits of the moonDeimos, the moonPhobos and the planet Mercury as viewed by thePerseverance rover on the planet Mars.
Transits viewed from Mars by thePerseverance rover
^ 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).
^In 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 doesnot mean that the two planets are closest over a long period of 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, it turns out that using this time-average definition of 'closeness', Mercury can be the "closest planet" toall other planets in the solar system.
^ 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.
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