Mercury is the innermost and smallest planet in the solar system, orbiting the Sun once every 88 days. It ranges in brightness from about −2.0 to 5.5 in apparent magnitude, but is not easily seen as its greatest angular separation from the Sun (greatest elongation) is only 28.3°.

It can only be seen in morning or evening twilight.

Mercury is one of the four terrestrial planets, being a rocky body like the Earth. It is the smallest of the four, with a diameter of 4879 km at its equator. Mercury consists of approximately 70% metallic and 30% silicate material. The density of the planet is the second highest in the solar system at 5.43 g/cm³, only slightly less than Earth’s density. 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.[5]
1. Crust - 100–200 km thick
1. Crust - 100–200 km thick
2. Mantle - 600 km thick
3. Core - 1,800 km radius

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.[6] 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.[7]

Surrounding the core is a 600 km mantle. It is generally thought that early in Mercury’s history, a giant impact with a body several hundred kilometers across stripped the planet of much of its original mantle material, resulting in the relatively thin mantle compared to the sizable core.[8] (alternative theories are discussed below)

Mercury’s crust is thought to be 100–200 km thick. One distinctive feature of Mercury’s surface are numerous narrow ridges, some extending over several hundred kilometers. 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.[9]

Mercury has a higher iron content than any other major planet in our 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 meteors (thought to be typical of average solar system rocky matter) and a mass approximately 2.25 times its current mass. However, early in the solar system’s history, Mercury was struck by a planetesimal of approximately 1/6 that mass. The impact would have stripped away much of the original crust and mantle, leaving the core behind as a relatively major component.[8] A similar process has been proposed to explain the formation of Earth’s Moon (see giant impact theory).

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 2500 and 3500 K, and possibly even as high as 10000 K. 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.[10]

A third theory 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.[11] Each of these theories predicts a different surface composition, and two upcoming space missions, MESSENGER and BepiColombo, both aim to take observations that will allow the theories to be tested.

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 is based on only a single spacecraft flyby, it is the least well understood of the terrestrial planets. Surface features are given the following names:

* Albedo features — areas of markedly different reflectivity
* Dorsa — ridges (see List of ridges on Mercury)
* Montes — mountains (see List of mountains on Mercury)
* Planitiae — plains (see List of plains on Mercury)
* Rupes — scarps (see List of scarps on Mercury)
* Valles — valleys (see List of valleys on Mercury)

During and shortly following the formation of Mercury, it was heavily bombarded by comets and asteroids for a period called the late heavy bombardment, that came to an end 3.8 billion years ago. During this period of intense crater formation, the planet received impacts over its entire surface, facilitated by the lack of any atmosphere to slow impactors down. 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.

Craters on Mercury range in diameter from a few meters to hundreds of kilometers across. The largest known craters are the enormous Caloris Basin, with a diameter of 1300 km, and the Skinakas Basin with a diameter of 1600 km, but known only from low resolution Earth-based imaging of the non-Mariner-imaged hemisphere. The impact which 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 the origin of this geomorphological unit is that shock waves generated during the impact traveled around the planet, and when they converged at the basin’s antipode (180 degrees away) the high stresses were capable of fracturing the surface.[12] Alternatively, it has been suggested that this terrain formed as a result of the convergence of ejecta at this basin’s antipode.
The so-called “Weird Terrain” was formed by the Caloris Basin impact at its antipodal point.

The plains of Mercury have two distinct ages: the younger plains are less heavily cratered and probably formed when lava flows buried earlier terrain. One unusual feature of the planet’s surface is the numerous compression folds which crisscross the plains. It is thought that as the planet’s interior cooled, it contracted and its surface began to deform. The folds can be seen on top of other features, such as craters and smoother plains, indicating that they are more recent.[13] Mercury’s surface is also flexed by significant tidal bulges raised by the Sun—the Sun’s tides on Mercury are about 17% stronger than the Moon’s on Earth.[14]

Like the Moon, the surface of Mercury has likely incurred the effects of space weathering processes. Solar wind and micrometeorite impacts can darken the albedo and alter the reflectance properties of the surface.

The mean surface temperature of Mercury is 452 K (179 °C), but it ranges from 90 K (−183 °C) to 700 K (427 °C), due to the absence of an atmosphere; by comparison, the temperature on Earth varies by only about 80 K. The sunlight on Mercury’s surface is 6.5 times as intense as it is on Earth, with a solar constant value of 9.13 kW/m².

Despite the generally extremely high temperature of its surface, observations strongly suggest that ice exists on Mercury. The floors of some deep craters near the poles are never exposed to direct sunlight, and temperatures there remain far lower than the global average. Water ice strongly reflects radar, and observations reveal that there are patches of very high radar reflection near the poles.[15] While ice is not the only possible cause of these reflective regions, astronomers believe it is the most likely.

The icy regions are believed to be covered to a depth of only a few meters, and contain about 1014–1015 kg of ice. 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. 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.

Atmosphere

Mercury is too small for its gravity to retain any significant atmosphere over long periods of time; however, it does have a tenuous atmosphere containing hydrogen, helium, oxygen, sodium, calcium and potassium. This atmosphere 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. Water vapor is probably present, being brought to Mercury by comets impacting on its surface.

Magnetosphere

Despite its slow 59-day-long rotation, Mercury has a relatively strong, and apparently global, magnetic field. It is about 1.1% as strong as the Earth’s.[18][19] It is likely that this magnetic field is generated in a manner similar to Earth’s, by a dynamo of circulating liquid core material.[20] A mechanism that has been suggested for keeping it liquid are particularly strong tidal effects during periods of high orbital eccentricity.

Mercury’s magnetic field is strong enough to deflect the solar wind around the planet, creating a magnetosphere inside which the solar wind does not penetrate. This is in contrast to the situation on the Moon, which has a magnetic field too weak to stop the solar wind impacting on its surface and so lacks a magnetosphere.

Orbit and rotation

The orbit of Mercury is the most eccentric of the major planets, with the planet’s distance from the Sun ranging from 46,000,000 to 70,000,000 kilometers. It takes 88 days to complete an orbit. The diagram on the left illustrates the effects of the eccentricity, showing Mercury’s orbit overlain 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.

Mercury’s orbit is inclined by 7° to the plane of Earth’s orbit (the ecliptic), as shown in the diagram on the left. 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.

Mercury’s axial tilt is only 0.01 degrees. This is over 300 times smaller than that of Jupiter, which is the second smallest axial tilt of all planets at 3.1 degrees. This means an observer at Mercury’s equator during local noon would never see the sun more than 1/100 of one degree north or south of the zenith. Conversely, at the poles the Sun never rises more than 0.01° above the horizon.

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 prior to 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.

Advance of perihelion

It was noticed in the 19th century 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 (notably by the French mathematician Le Verrier). It was hypothesized that another planet might exist in an orbit even closer to the Sun to account for this perturbation (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 great faith in this explanation, and the hypothetical planet was even named Vulcan. However, 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, being 8.62 arcseconds per century for Venus, 3.84 for Earth, 1.35 for Mars, and 10.05 for 1566 Icarus.

Spin–orbit resonance

For many years it was thought that Mercury was synchronously tidally locked with the Sun, rotating once for each orbit and keeping the same face directed towards the Sun at all times, 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. The original reason astronomers thought it was synchronously locked was because whenever Mercury was best placed for observation, it was always at the same point in its 3:2 resonance, hence showing the same face. 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. A sidereal day (the period of rotation) lasts about 58.7 Earth days.

Orbital simulations indicate that the eccentricity of Mercury’s orbit varies chaotically from 0 (circular) to a very high 0.47 over millions of years. 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.

Observation

Mercury’s apparent magnitude varies between about −2.0—brighter than Sirius—and 5.5.[24] 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.

Like the Moon, Mercury exhibits phases as seen from Earth, being “new” at inferior conjunction and “full” at superior conjunction. The planet is rendered invisible on both of these occasions by virtue of its rising and setting in concert with the Sun in each case. The first and last quarter phases occur at greatest elongation east and west, respectively, when Mercury's separation from the Sun ranges anywhere from 18.5° at perihelion to 28.3° at aphelion. At greatest elongation west, Mercury rises earliest before the Sun, and at greatest elongation east, it sets latest after the Sun.

Mercury attains inferior conjunction every 116 days on average, but this interval can range from 111 days to 121 days due to the planet’s eccentric orbit. 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 also arises from the planet’s high orbital eccentricity.

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 when the Southern Hemisphere is having its late winter season. 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. 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.

Mercury is brightest as seen from Earth when it is at a gibbous phase, between either quarter phase and full. Although the planet is further away from Earth when it is gibbous than when it is a crescent, the greater illuminated area visible more than compensates for the greater distance. The opposite is true for Venus, which appears brightest when it is a thin crescent, because it is much closer to Earth than when gibbous.