Mercury (planet)

Mercury is the innermost and smallest planet in the Solar System,[a] orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits. The perihelion of Mercury's orbit precesses around the Sun at an excess of 43 arcseconds per century; a phenomenon that was explained in the 20th century by Albert Einstein's General Theory of Relativity.[11] Mercury is bright when viewed from Earth, ranging from −2.3 to 5.7 in apparent magnitude, but is not easily seen as its greatest angular separation from the Sun is only 28.3°. Since Mercury is normally lost in the glare of the Sun, unless there is a solar eclipse it can be viewed from Earth's Northern Hemisphere only in morning or evening twilight, while its extreme elongations occur in declinations south of the celestial equator, such that it can be seen at favorable apparitions from moderate latitudes in the Southern Hemisphere in a fully dark sky.

Comparatively little is known about Mercury; ground-based telescopes reveal only an illuminated crescent with limited detail. The first of two spacecraft to visit the planet was Mariner 10, which mapped about 45% of the planet’s surface from 1974 to 1975. The second is the MESSENGER spacecraft, which attained orbit around Mercury on March 17, 2011,[12] where it will begin mapping the rest of the planet.

Mercury is similar in appearance to the Moon: it is heavily cratered with regions of smooth plains, has no natural satellites and no substantial atmosphere. However, unlike the Moon, it has a large iron core, which generates a magnetic field about 1% as strong as that of the Earth.[13] It is an exceptionally dense planet due to the large relative size of its core. Surface temperatures range from about 90 to 700 K (−183 °C to 427 °C),[14] with the subsolar point being the hottest and the bottoms of craters near the poles being the coldest.

Recorded observations of Mercury date back to at least the first millennium BC. Before the 4th century BC, Greek astronomers believed the planet to be two separate objects: one visible only at sunrise, which they called Apollo; the other visible only at sunset, which they called Hermes.[15] The English name for the planet comes from the Romans, who named it after the Roman god Mercury, which they equated with the Greek Hermes (Ἑρμῆς). The astronomical symbol for Mercury is a stylized version of Hermes' caduceus

Internal structure

Mercury is one of four terrestrial planets in the Solar System, and is a rocky body like the Earth. It is the smallest planet in the Solar System, with an equatorial radius of 2,439.7 km.[3] Mercury is even 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.[17] Mercury'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, the materials of which Mercury is made would be denser, with an uncompressed density of 5.3 g/cm³ versus Earth’s 4.4 g/cm³.[18]

Mercury’s density can be used to infer details of its inner structure. While the Earth’s high density results appreciably from gravitational compression, particularly at the core, Mercury is much smaller and its inner regions are not nearly as strongly compressed. Therefore, for it to have such a high density, its core must be large and rich in iron.[19]
1. Crust: 100-300 km thick
2. Mantle: 600 km thick
3. Core: 1,800 km radius

Geologists estimate that Mercury’s core occupies about 42% of its volume; for Earth this proportion is 17%. Recent research strongly suggests Mercury has a molten core.[20][21] Surrounding the core is a 500–700 km mantle consisting of silicates.[22][23] Based on data from the Mariner 10 mission and Earth-based observation, Mercury’s crust is believed to be 100–300 km thick.[24] One distinctive feature of Mercury’s surface is the presence of numerous narrow ridges, extending up to several hundred kilometers in length. It is believed that these were formed as Mercury’s core and mantle cooled and contracted at a time when the crust had already solidified.[25]

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.[26] However, early in the Solar System’s history, Mercury may have been struck by a planetesimal of approximately 1/6 that mass and several hundred kilometers across.[26] The impact would have stripped away much of the original crust and mantle, leaving the core behind as a relatively major component.[26] A similar process, known as the giant impact hypothesis, has been proposed to explain the formation of Earth’s Moon.[26]

Alternatively, Mercury may have formed from the solar nebula before the Sun's energy output had stabilized. The planet 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 (Celsius equivalents about 273 degrees less), and possibly even as high as 10,000 K.[27] Much of Mercury’s surface rock could have been vaporized at such temperatures, forming an atmosphere of "rock vapor" which could have been carried away by the solar wind.[27]

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.[28] Each hypothesis predicts a different surface composition, and two upcoming space missions, MESSENGER and BepiColombo, both aim to make observations to test them

Surface geology

Mercury’s surface is overall very 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. Since our knowledge of Mercury's geology has been based on the 1975 Mariner flyby and terrestrial observations, it is the least understood of the terrestrial planets.[21] As data from the recent MESSENGER flyby is processed this knowledge will increase. For example, an unusual crater with radiating troughs has been discovered which scientists are calling "the spider."[31]

Albedo features refer to areas of markedly different reflectivity, as seen by telescopic observation. Mercury possesses Dorsa (also called "wrinkle-ridges"), Moon-like highlands, Montes (mountains), Planitiae, or plains, Rupes (escarpments), and Valles (valleys).[32][33]

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 came to an end 3.8 billion years ago.[34] During this period of intense crater formation, the planet received impacts over its entire surface,[33] facilitated by the lack of any atmosphere to slow impactors down.[35] During this time the planet was volcanically active; basins such as the Caloris Basin were filled by magma from within the planet, which produced smooth plains similar to the maria found on the Moon.[36][37]

Data from the October 2008 flyby of MESSENGER gave researchers a greater appreciation for the jumbled nature of Mercury's surface. Mercury's surface is more heterogeneous than either Mars' or the Moon's, both of which contain significant stretches of similar geology, such as maria and plateaus.[38]
Impact basins and craters
Mercury’s Caloris Basin is one of the largest impact features in the Solar System.

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.[39]

The largest known crater is Caloris Basin, with a diameter of 1,550 km,.[40] The impact that created the Caloris Basin was so powerful that it caused lava eruptions and left a concentric ring over 2 km tall surrounding the impact crater. 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 the planet, converging at the basin’s antipode (180 degrees away). The resulting high stresses fractured the surface.[41] Alternatively, it has been suggested that this terrain formed as a result of the convergence of ejecta at this basin’s antipode.[42]

Overall, about 15 impact basins have been identified on the imaged part of Mercury. A notable basin is the 400 km wide, multi-ring Tolstoj Basin which 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.[39] Like the Moon, the surface of Mercury has likely incurred the effects of space weathering processes, including Solar wind and micrometeorite impacts.[43]
Plains
The so-called “Weird Terrain” was formed by the Caloris Basin impact at its antipodal point.

There are two geologically distinct plains regions on Mercury.[39][44] Gently rolling, hilly plains in the regions between craters are Mercury's oldest visible surfaces,[39] 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.[44] It is not clear whether they are of volcanic or impact origin.[44] The inter-crater plains are distributed roughly uniformly over the entire surface of the planet.

Smooth plains are widespread flat areas which fill depressions of various sizes and bear a strong resemblance to the lunar maria. Notably, they fill a wide ring surrounding the Caloris Basin. 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.[39] All the Mercurian smooth plains formed significantly later than the Caloris basin, as evidenced by appreciably smaller crater densities than on the Caloris ejecta blanket.[39] 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 lavas induced by the impact, or a large sheet of impact melt.[39]

One unusual feature of the planet’s surface is the numerous compression folds, or rupes, which crisscross the plains. As the planet’s interior cooled, it may have contracted and its surface began to deform, creating these features. The folds can be seen on top of other features, such as craters and smoother plains, indicating that the folds are more recent.[45] Mercury’s surface is flexed by significant tidal bulges raised by the Sun—the Sun’s tides on Mercury are about 17 times stronger than the Moon’s on Earth.

Surface conditions and "atmosphere" (exosphere)

The mean surface temperature of Mercury is 442.5 K,[3] but it ranges from 100 K to 700 K[47] 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 then drops to 550 K at aphelion.[48] On the dark side of the planet, temperatures average 110 K.[49] The intensity of sunlight on Mercury’s surface ranges between 4.59 and 10.61 times the solar constant (1,370 W·m−2).[50]

Although the temperature at the surface of Mercury is generally extremely high, observations strongly suggest that ice 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.[51] Water ice strongly reflects radar, and observations by the 70 m Goldstone telescope and the VLA in the early 1990s revealed that there are patches of very high radar reflection near the poles.[52] While ice is not the only possible cause of these reflective regions, astronomers believe it is the most likely.[53]

The icy regions are believed to contain about 1014–1015 kg of ice,[54] and may be covered by a layer of regolith that inhibits sublimation.[55] By comparison, the Antarctic ice sheet on Earth has a mass of about 4×1018 kg, and Mars' south polar cap contains about 1016 kg of water.[54] 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.[54]

Mercury is too small for its gravity to retain any significant atmosphere over long periods of time; however, it does have a "tenuous surface-bounded exosphere"[56] containing hydrogen, helium, oxygen, sodium, calcium, potassium and others. This exosphere is not stable—atoms are continuously lost and replenished from a variety of sources. Hydrogen 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 H2O+ was a surprise.[57][58] 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.[59][60]

Sodium, potassium and calcium were discovered in the atmosphere during the 1980–1990s, and are believed to result primarily from the vaporization of surface rock struck by micrometeorite impacts.[61] In 2008 magnesium was discovered by MESSENGER probe.[62] 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

Magnetic field and magnetosphere

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% as strong as the Earth’s. The magnetic field strength at the Mercurian equator is about 300 nT.[64][65] Like that of Earth, Mercury's magnetic field is dipolar.[63] Unlike Earth, however, Mercury's poles are nearly aligned with the planet's spin axis.[66] Measurements from both the Mariner 10 and MESSENGER space probes have indicated that the strength and shape of the magnetic field are stable.[66]

It is likely that this magnetic field is generated by way of a dynamo effect, in a manner similar to the magnetic field of Earth.[67][68] This dynamo effect would result from the circulation of the planet's iron-rich liquid core. Particularly strong tidal effects caused by the planet's high orbital eccentricity would serve to keep the core in the liquid state necessary for this dynamo effect.[69]

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 the Earth,[63] is strong enough to trap solar wind plasma. This contributes to the space weathering of the planet's surface.[66] Observations taken by the Mariner 10 spacecraft detected this low energy plasma in the magnetosphere of the planet's nightside. Bursts of energetic particles were detected in the planet's magnetotail, which indicates a dynamic quality to the planet's magnetosphere.[63]

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 'tornadoes' form when magnetic fields carried by the solar wind connect to Mercury's magnetic field. As the solar wind blows past Mercury's field, these joined magnetic fields are carried with it and twist up into vortex-like structures. 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.[70]

The process of linking interplanetary and planetary magnetic fields, called magnetic reconnection, is common throughout the cosmos. It occurs in Earth's magnetic field, where it generates magnetic tornadoes as well. However, the MESSENGER observations show the reconnection rate is ten times higher at Mercury. Mercury's proximity to the Sun only accounts for about a third of the reconnection rate observed by MESSENGER.

Orbit and rotation

Mercury has the most eccentric orbit of all the planets; its eccentricity is 0.21 with its distance from the Sun ranging from 46 to 70 million kilometers. It takes 88 days to complete an orbit. The diagram on the right illustrates the effects of the eccentricity, showing Mercury’s orbit overlaid with a circular orbit having the same semi-major axis. The higher velocity of the planet when it is near perihelion is clear from the greater distance it covers in each 5-day interval. The size of the spheres, inversely proportional to their distance from the Sun, is used to illustrate the varying heliocentric distance. This varying distance to the Sun, combined with a 3:2 spin-orbit resonance of the planet’s rotation around its axis, result in complex variations of the surface temperature.[17] This resonance makes a single day on Mercury last exactly two Mercury years, or about 176 Earth days.[71]

Mercury’s orbit is inclined by 7 degrees to the plane of Earth’s orbit (the ecliptic), as shown in the diagram on the right. 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 the Earth and the Sun. This occurs about every seven years on average.[72]
Orbit of Mercury as seen from the ascending node (bottom) and from 10° above (top)

Mercury’s axial tilt is almost zero,[73] with the best measured value as low as 0.027 degrees.[7] 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.[7]

At certain points on Mercury’s surface, an observer would be able to see the Sun rise about halfway, then reverse and set before rising again, all within the same Mercurian day. This is because approximately four days before perihelion, Mercury’s angular orbital velocity exactly equals its angular rotational velocity so that the Sun’s apparent motion ceases; at perihelion, Mercury’s angular orbital velocity then exceeds the angular rotational velocity. Thus, the Sun appears to move in a retrograde direction. Four days after perihelion, the Sun’s normal apparent motion resumes at these points.[17]
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 the Earth. However, 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.[74]

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 (the length between two meridian transits of the Sun) lasts about 176 Earth days.[17] A sidereal day (the period of rotation) lasts about 58.7 Earth days.[17]

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.[17][75] This is thought to explain Mercury’s 3:2 spin-orbit resonance (rather than the more usual 1:1), since this state is more likely to arise during a period of high eccentricity.[76] Numerical simulations show that a resonant orbital interaction with Jupiter may cause the eccentricity of Mercury's orbit to increase to the point where the planet may collide with Venus within the next five billion years.[77]
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.[78] (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 even named Vulcan. However, no such planet was ever found.[79]

The perihelion precession of Mercury is 5600 arc seconds per century. Newtonian mechanics, taking into account all the effects from the other planets, predicts a precession of 5557 seconds of arc per century.[80] In the early 20th century, Albert Einstein’s General Theory of Relativity provided the explanation for the observed precession. The effect is very small: the Mercurian relativistic perihelion advance excess is just 42.98 arcseconds per century, therefore it requires a little over twelve million orbits for a full excess turn. Similar, but much smaller, effects operate for other planets: 8.62 arcseconds per century for Venus, 3.84 for Earth, 1.35 for Mars, and 10.05 for 1566 Icarus.[

Observation

Mercury’s apparent magnitude varies between −2.6[9] (brighter than the brightest star Sirius) and about +5.7. The extremes occur when Mercury is close to the Sun in the sky.[9] [10] 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. The Hubble Space Telescope cannot observe Mercury at all, due to safety procedures which prevent its pointing too close to the Sun.[84]

Like the Moon, 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 relative nearness to the Sun. The first and last quarter phases occur at greatest elongation east and west, respectively, when Mercury's separation from the Sun ranges anywhere from 17.9° at perihelion to 27.8° at aphelion.[85][86] At greatest elongation west, Mercury rises at its earliest before the Sun, and at greatest elongation east, it sets at its latest after the Sun.[87]

Mercury attains inferior conjunction every 116 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 close as 77.3 million km to the Earth.[3] In 871 AD, the nearest approach was the first in about 41,000 years to be closer than 82.2 Gm, something that has happened 68 times since then. After much longer gaps, the next approach to within 82.1 Gm is in 2679, and to 82 Gm in 4487. But it will not be closer to Earth than 80 Gm until 28,622.[clarification needed][88] 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.[17]

Mercury is more often easily visible from Earth’s Southern Hemisphere than from its Northern Hemisphere; this is because its maximum possible elongations west of the Sun always occur when it is early autumn in the Southern Hemisphere, while its maximum possible eastern elongations happen during late winter in the Southern Hemisphere.[87] In both of these cases, the angle Mercury strikes with the ecliptic is maximized, allowing it to rise several hours before the Sun in the former instance and not set until several hours after sundown in the latter in countries located at southern temperate zone latitudes, such as Argentina and New Zealand.[87] By contrast, at northern temperate latitudes, Mercury is never above the horizon of a more-or-less fully dark night sky. Mercury can, like several other planets and the brightest stars, be seen during a total solar eclipse.[89]

Mercury is brightest as seen from Earth when it is at a full phase. Although the planet is farthest away from Earth when it is full the greater illuminated area that is visible and the opposition brightness surge more than compensate for the distance.[9] The opposite is true for Venus, which appears brightest when it is a crescent, because it is much closer to Earth than when gibbous.