Solar System/Mercury

Mercury is the smallest planet and the closest planet to the Sun. With a diameter of just 4879.4 kilometers (38% of that of Earth), it is smaller than the two largest planetary moons in the solar system — Ganymede of Jupiter and Titan of Saturn. Mercury has no moons. Mercury and Venus are the only two planets with zero oblateness: its polar diameter is the same as its equatorial diameter. Mercury is one of the four terrestrial planets (planets with a solid surface). If you were standing on Mercury, you would weigh just 38% of what you weigh on Earth.

True color photograph of Mercury.

Orbit

Orbit of Mercury.

Mercury orbits the Sun once every 87.97 Earth-days, with an orbital eccentricity of 0.21 — far greater than that of any other planet. As a result of this eccentricity, the planet moves notably faster near perihelion (closest point to the Sun) than at aphelion (farthest point from the Sun). Mercury's distance from the Sun varies during its elliptical orbit from 0.31 AU (Earth-Sun distances) at perihelion to 0.47 AU at aphelion, with an average over the entire orbit of 0.39 AU.

Rotation

Mercury is unique in having essentially no axial tilt whatsoever — its tilt is 0.00°. It rotates prograde (in the direction of its orbit around the Sun) about its axis relative to the distant stars once every 58.65 Earth-days. Because its orbital period is not substantially greater than its rotational period (almost exactly 1.5 times as great), Mercury is the only planet on which a solar day (noon to noon) lasts for more than one local year — specifically, a solar day lasts for almost precisely two solar orbits. Furthermore, the fact that the highly elliptical orbit causes the planet to speed up and slow down over the course of the orbit, combined with the similarity in the lengths of the rotational period and orbital period, results in the Sun temporarily reversing direction across the sky when the planet is near perihelion. This occurs because near perihelion the planet is actually revolving around the Sun faster than it is rotating. Mercury is the only planet where this happens, and there are certain places on the planet (where perihelion occurs around sunrise) where the Sun rises temporarily (in the east), reverses direction and sets temporarily in the east, and then rises for good, while there are other places (where perihelion occurs around sunset) where the Sun sets temporarily (in the west), reverses direction and temporarily rises in the west, and then sets for good.

Atmosphere

Emissions from Mercury.

The mean surface temperature of Mercury is 442.5 K,[3] but it ranges from 100 K to 700 K[44] 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.[45] On the dark side of the planet, temperatures average 110 K.[46] The intensity of sunlight on Mercury’s surface ranges between 4.59 and 10.61 times the solar constant (1,370 W·m−2).[47]

Despite the generally extremely high temperature of its surface, 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.[48] 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.[49] While ice is not the only possible cause of these reflective regions, astronomers believe it is the most likely.[50]

The icy regions are believed to contain about 1014–1015 kg of ice,[51] and may be covered by a layer of regolith that inhibits sublimation.[52] 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.[51] 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.[51]

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"[53] 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.[54][55] 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.[56][57]

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.[58] In 2008 magnesium was discovered by MESSENGER probe.[59] 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.[60]

Internal Structure

Internal diagram of Mercury with magnetic field.

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.[14] 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.[15]

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.[16]

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.[17][18] Surrounding the core is a 500–700 km mantle consisting of silicates.[19][20] Based on data from the Mariner 10 mission and Earth-based observation, Mercury’s crust is believed to be 100–300 km thick.[21] One distinctive feature of Mercury’s surface is the presence of numerous narrow ridges, and these can extend up to 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.[22]

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 meteors, thought to be typical of the Solar System's rocky matter, and a mass approximately 2.25 times its current mass.[23] 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.[23] The impact would have stripped away much of the original crust and mantle, leaving the core behind as a relatively major component.[23] A similar process has been proposed to explain the formation of Earth’s Moon (see giant impact theory).[23]

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.[24] 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.[24]

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.[25] Each hypothesis predicts a different surface composition, and two upcoming space missions, MESSENGER and BepiColombo, both aim to make observations to test them.[26][27]

Surface

Mariner 10's "Sail Power"

Mariner diagram

The first spacecraft to visit Mercury was NASA’s Mariner 10 (1974–75).[12] 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.[120] 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 which were later ascribed to the effect of the planet shrinking slightly as its iron core cools.[123] Unfortunately, due to the length of Mariner 10's orbital period, the same face of the planet was lit at each of Mariner 10’s close approaches. This made observation of both sides of the planet impossible,[124] and resulted in the mapping of less than 45% of the planet’s surface.[125]

On March 27, 1974, two days before its first flyby of Mercury, Mariner 10's instruments began registering large amounts of unexpected ultraviolet radiation near Mercury. This led to the tentative identification of Mercury's moon. Shortly afterward, the source of the excess UV was identified as the star 31 Crateris, and Mercury's moon passed into astronomy's history books as a footnote.

The spacecraft made three close approaches to Mercury, the closest of which took it to within 327 km of the surface.[126] At the first close approach, instruments detected a magnetic field, to the great surprise of planetary geologists—Mercury’s rotation was expected to be much too slow to generate a significant dynamo effect. The second close approach was primarily used for imaging, but at the third approach, extensive magnetic data were obtained. The data revealed that the planet’s magnetic field is much like the Earth’s, which deflects the solar wind around the planet. However, the origin of Mercury’s magnetic field is still the subject of several competing theories.[127]

On March 24, 1975, just eight days after its final close approach, Mariner 10 ran out of fuel. Since its orbit could no longer be accurately controlled, mission controllers instructed the probe to shut down.[128] Mariner 10 is thought to be still orbiting the Sun, passing close to Mercury every few months.[129]