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 twilight. Comparatively little is known about the planet: the only spacecraft to approach Mercury was Mariner 10 from 1974 to 1975, which mapped only 40%–45% of the planet's surface.

Physically, Mercury is similar in appearance to the Moon as it is heavily cratered. It has no natural satellites and no substantial atmosphere. The planet has a large iron core which generates a magnetic field about 1% as strong as that of the Earth. Surface temperatures on Mercury range from about 90 to 700 K (-180 to 430°C), with the subsolar point being the hottest and the bottoms of craters near the poles being the coldest.

The Romans named the planet after the fleet-footed messenger god Mercury, probably for its fast apparent motion in the twilight sky. The astronomical symbol for Mercury, displayed at the top of the infobox, is a stylized version of the god's head and winged hat atop his caduceus, an ancient astrological symbol. Before the 5th century BC, Greek astronomers believed the planet to be two separate objects: one visible only at sunrise, the other only at sunset. In India, the planet was named Budha (बुध), after the son of Chandra (the Moon). The Chinese, Korean, Japanese, and Vietnamese cultures refer to the planet as the water star (水星), based on the Five Elements. The Hebrews named it Kokhav Hamah (כוכב חמה), "the star of the hot one" ("the hot one" being the Sun).

Structure

Mercury is one of the four terrestrial planets, meaning that like the Earth it is a rocky body. 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. When corrected for gravitational compression, Mercury is in fact denser than Earth, with an uncompressed density of 5.3 g/cm³ versus Earth's 4.4 g/cm³.^[1]

Internal structure: core, mantle and crust

Mercury's high density can be used to infer details of its inner structure. While the Earth's high density results partly from compression at the core, Mercury is much smaller and its inner regions are not nearly so compressed. Therefore, for it to have such a high density, its core must be large and rich in iron.^[2] Geologists estimate that Mercury's core occupies about 42% of its volume. (Earth's core occupies about 17% of its volume.)

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 ^[3] (alternative theories are discussed below).

Mercury's crust is thought to be 100–200 km thick. One very distinctive feature of Mercury's surface is numerous ridges, some extending over several hundred kilometers. It is believed that these were formed as Mercury's core and mantle cooled and contracted after the crust had solidified.^[4]

Mercury has a higher iron content than any other major planet in our solar system. Several theories have been proposed to explain Mercury's high metallicity. The most widely accepted theory is that Mercury originally had a metal-silicate ratio similar to common chondrite meteors and a mass approximately 2.25 times its current mass; but that 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.^[3] A similar theory 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 vaporized at such temperatures, forming an atmosphere of "rock vapor" which could have been carried away by the solar wind.^[5]

A third theory suggests 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.^[6] 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

Mercury's surface is 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. The small number of unmanned missions to Mercury means that its geology 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 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 crater is the enormous Caloris Basin, with a diameter of 1300 km. 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 geomorphologic 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.^[7] Alternatively, it has been suggested that this terrain formed as a result of the convergence of ejecta at this basin's antipode.

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.^[8] 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.^[9]

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 (353.9°F, 178.9°C), but it ranges from 90 K (-297.7°F, -183.2°C) to 700 K (800.3°F, 426.9°C), due to the absence of an atmosphere; by comparison, the temperature on Earth varies by only about 150 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.^[10] 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 weighs 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.^[11]

Atmosphere

Mercury is too small for its gravity to retain any significant atmosphere over long periods of time; it has a tenuous atmosphere containing hydrogen, helium, oxygen, sodium, calcium and potassium. The 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.^[12]

Magnetic field

Despite its slow rotation, Mercury has a relatively strong magnetic field, with a magnetic field strength 1% as strong as the Earth's. It is possible that this magnetic field is generated in a manner similar to Earth's, by a dynamo of circulating liquid core material. However, scientists are unsure whether Mercury's core could still be liquid,^[13] although it could perhaps be kept liquid by tidal effects during periods of high orbital eccentricity. It is also possible that Mercury's magnetic field is a remnant of an earlier dynamo effect that has now ceased, with the magnetic field becoming "frozen" in solidified magnetic materials.

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 the orbit. The diagram on the left illustrates the effects of the eccentricity, showing Mercury’s orbit with a circular orbit with 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, illustrates the varying heliocentric distance. This varying distance to the Sun, combined with a unique 3:2 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.

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 be retrograde. Four days after perihelion, the Sun's normal apparent motion resumes.

Advance of perihelion

When it was discovered, the slow precession of Mercury's orbit around the Sun could not be completely explained by Newtonian mechanics, and for many years 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 Uranus' orbit 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 a full explanation for the observed precession. Mercury's precession showed the effects of mass dilation, providing a crucial observational confirmation of one of Einstein's theories—Mercury is slightly heavier at perihelion than it is at aphelion because it is moving faster, and so it slightly "overshoots" the perihelion position predicted by Newtonian gravity. The effect is very small: the Mercurian relativistic perihelion advance excess is just 43 arcseconds per century. The effect is even smaller for other planets, being 8.6 arcseconds per century for Venus, 3.8 for Earth, and 1.3 for Mars.

Research indicates 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.^[14]

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.

Observation

Mercury's apparent magnitude varies between about -2.0 - brighter than Sirius - and 5.5.^[15] Observation of Mercury is complicated by its proximity to the Sun, as it is lost in the Sun's glare for much of the time. Mercury can be observed for only a brief period during either morning or evening twilight. The Hubble Space Telescope cannot observe Mercury at all.

Mercury exhibits moonlike 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 half-moon phase occurs at greatest elongation, when Mercury rises earliest before the Sun when at greatest elongation west, and setting latest after the Sun when at greatest elongation east (its separation from the Sun ranging from 18.5° if it is at perihelion at the time of the greatest elongation to 28.3° if it is at aphelion).

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 degree of 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 South 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 half full 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.

Studies of Mercury

Early astronomers

Mercury has been known since at least the 3rd millennium BC, when it was known to the Sumerians of Mesopotamia as Ubu-idim-gud-ud, among other names. The Babylonians (2000–1000 BC) succeeded the Sumerians, and early Babylonians may have recorded observations of the planet: although no records have survived, late Babylonian records from the 7th century BC refer to much earlier records. The Babylonians called the planet Nabu or Nebu after the messenger to the Gods in their mythology.^[16]

The ancient Greeks gave the planet two names: Apollo when it was visible in the morning sky and Hermes when visible in the evening. However, Greek astronomers came to understand that the two names referred to the same body, with Pythagoras being the first to propose the idea.^[17]

Ground-based telescopic research

The first telescopic observations of Mercury were made by Galileo in the early 17th century. Although he observed phases when he looked at Venus, his telescope was not powerful enough to see the phases of Mercury. In 1631 Pierre Gassendi made the first observations of the transit of a planet across the Sun when he saw a transit of Mercury predicted by Johannes Kepler. In 1639 Giovanni Zupi used a telescope to discover that the planet had orbital phases similar to Venus and the Moon. The observation demonstrated conclusively that Mercury orbited around the Sun.

A very rare event in astronomy is the passage of one planet in front of another (occultation), as seen from Earth. Mercury and Venus occult each other every few centuries, and the event of May 28, 1737 is the only one historically observed, having been seen by John Bevis at the Royal Greenwich Observatory.^[18] The next occultation of Mercury by Venus will be in 2133.

The difficulties inherent in observing Mercury mean that it has been far less studied than the other planets. In 1800 Johann Schröter made observations of surface features, but erroneously estimated the planet's rotational period at about 24 hours. 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.^[19] This phenomenon is known as synchronous rotation and is also shown by Earth's Moon.

The theory that Mercury's rotation was synchronous became widely held, and it was a significant shock to astronomers when radio observations made in the 1960s questioned this. 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, but in 1965 radar observations showed conclusively that the planet's rotational period was about 59 days. Italian astronomer Giuseppe Colombo noted that this value was about two-thirds of Mercury's orbital period, and proposed that a different form of tidal locking had occurred in which the planet's orbital and rotational periods were locked into a 3:2 rather than a 1:1 resonance.^[20] Data from Mariner 10 subsequently confirmed this view.[5]

Ground-based observations did not shed much further light on the innermost planet, and it was not until space probes visited Mercury that many of its most fundamental properties became known. However, recent technological advances have led to improved ground-based observations: in 2000, high-resolution lucky imaging from the Mount Wilson Observatory 60-inch telescope provided the first detailed views of the parts of Mercury which were not imaged in the Mariner missions.^[21]

Research with space probes

Reaching Mercury from Earth poses significant technical challenges, since the planet orbits so much closer to the Sun than does the Earth. A Mercury-bound spacecraft launched from Earth must travel over 91 million kilometers into the Sun's gravitational potential well. Starting from the Earth's orbital speed of 30 km/s, the change in velocity (delta-v) the spacecraft must make to enter into a Hohmann transfer orbit that passes near Mercury is large compared to other planetary missions.

The potential energy liberated by moving down the Sun's potential well becomes kinetic energy; requiring another large delta-v to do anything other than rapidly pass by Mercury. In order to land safely or enter a stable orbit since the planet has very little atmosphere, the approaching spacecraft cannot use aerobraking and must rely on rocket motors. A trip to Mercury actually requires more rocket fuel than that required to escape the solar system completely. As a result, only one space probe has visited the planet so far.

Mariner 10

The only spacecraft to approach Mercury so far has been NASA's Mariner 10 (1974–75).^[17] The spacecraft used the gravity of Venus to adjust its orbital velocity so that it could approach Mercury—the first spacecraft to use this gravitational "slingshot" effect. Mariner 10 provided the first close-up images of Mercury's surface, which immediately showed its heavily cratered nature, and also revealed many other types of geological features, such as the giant scarps which were later ascribed to the effect of the planet shrinking slightly early in its geological history. Unfortunately, the same face of the planet was lit at each of Mariner 10's close approaches, resulting in less than 45% of the planet's surface being mapped.

The spacecraft made three close approaches to Mercury, the closest of which took it to within 327 km of the surface. 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. The Moon's magnetic field, on the other hand, is so weak that the solar wind reaches the surface. However, the origin of Mercury's magnetic field is still the subject of several competing theories.

Just a few days after its final close approach, Mariner 10 ran out of fuel, its orbit could no longer be accurately controlled and mission controllers instructed the probe to shut itself down. Mariner 10 is thought to be still orbiting the Sun, still passing close to Mercury every few months.^[22]

MESSENGER

A second NASA mission to Mercury, named MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging), was launched on August 3, 2004, from the Cape Canaveral Air Force Station aboard a Boeing Delta 2 rocket. The MESSENGER spacecraft will make several close approaches to planets to place it onto the correct trajectory to reach an orbit around Mercury. It made a close approach to the Earth in February 2005, and to Venus in October 2006. Another Venusian encounter will follow in 2007, followed by three close approaches to Mercury in 2008 and 2009, after which it will enter orbit around the planet in March 2011.

The mission is designed to shed light on six key issues: Mercury's high density, its geological history, the nature of its magnetic field, the structure of its core, whether it really has ice at its poles, and where its tenuous atmosphere comes from. To this end, the probe is carrying imaging devices which will gather much higher resolution images of much more of the planet than Mariner 10, assorted spectrometers to determine abundances of elements in the crust, and magnetometers and devices to measure velocities of charged particles. Detailed measurements of tiny changes in the probe's velocity as it orbits will be used to infer details of the planet's interior structure.^[23]

BepiColombo

Japan is planning a joint mission with the European Space Agency called BepiColombo, which will orbit Mercury with two probes: one to map the planet and the other to study its magnetosphere. An original plan to include a lander has been shelved. Russian Soyuz rockets will launch the probes in 2013. As with MESSENGER, the BepiColombo probes will make close approaches to other planets en route to Mercury, passing the Moon and Venus and making several approaches to Mercury before entering orbit. The probes will reach Mercury in about 2019, orbiting and charting its surface and magnetosphere for a year.

The probes will carry a similar array of spectrometers to those on MESSENGER, and will study the planet at many different wavelengths including infrared, ultraviolet, X-ray and gamma ray. Apart from intensively studying the planet itself, mission planners also hope to use the probe's proximity to the Sun to test the predictions of General Relativity theory with improved accuracy.

The mission is named after Giuseppe (Bepi) Colombo, the scientist who first determined the nature of Mercury's orbital resonance with the Sun and who was also involved in the planning of Mariner 10's gravity-assisted trajectory to the planet in 1974.^[24]