|Larger image with caption|
|Mean radius||57,910,000 km|
|Orbital period||87d 23.3h|
|Synodic period||115.88 days|
|Avg. Orbital Speed||47.9 km/s|
|Number of satellites||0|
|Equatorial diameter||4879.4 km|
|Surface area||7.5 × 107 km2|
|Mean density||5.43 g/cm3|
|Surface gravity||2.78 m/s2|
|Rotation period||58d 15.5088h|
|Escape Speed||4.3 km/s|
|Avg. Surface temp.: Day||623 K|
|Avg. Surface temp.: Night||103 K|
Mercury is the closest planet to the Sun. This is the second-smallest planet within the Terran system. Upon the celestial sphere, Mercury ranges from -0.4 to 5.5, in apparent magnitude. The only spacecraft to approach Mercury was Mariner 10 (1974).
Mercury has no natural satellites and only trace amounts of atmosphere. Its surface appears very similar to Earth's Moon, but the planet has a much larger iron core and is therefore much denser; Mercury's composition is approximately 70% metallic and 30% silicate.
Several theories have been proposed to explain this high metallicity. One theory suggests 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 planetoid of approximately 1/6 that mass. The impact would have been sufficient to strip away much of the original crust and mantle of Mercury, leaving only its metal-rich core behind. A similar mechanism has been proposed to explain the formation of Earth's Moon, see giant impact theory. Alternately, Mercury may have formed very early in the history of the solar system, before the Sun's energy output had stabilized. Mercury starts out with approximately twice its current mass in this theory as well, but as the proto-Sun contracted temperatures in the vicinity of Mercury could have been between 2500K and 3500K, and possibly even as high as 10000K for a brief period. Much of Mercury's surface rock would have vaporized at these temperatures, forming an atmosphere of rock vapor which would have been carried away by the nebular wind[?].
Mercury has a relatively strong magnetosphere, with 1% of the magnetic field strength of Earth. It is possible that this magnetic field is generated in a manner similar to Earth's, by a dynamo of circulating liquid core material; current estimates suggest that Mercury's core is not hot enough to liquefy nickel-iron, but it is possible that materials with a lower melting point such as sulfur may be responsible. Mercury's slow rotation also presents problems with this theory. It is also possible that Mercury's magnetic field is a remnant of an eariler dynamo effect that has now ceased, the magnetic field becoming "frozen" in solidified magnetic materials.
The atmosphere of Mercury is extremely thin. Indeed, gas molecules in Mercury's atmosphere collide with the surface of the planet more frequently than they collide with each other; for most practical purposes Mercury can be said to be airless. The atoms that compose Mercury's atmosphere are continually being lost to space, with the average "lifespan" of a potassium or sodium atom being approximately 3 hours during the Mercurian day (and only half that at perihelion). The lost atmosphere is continually being replenished by several mechanisms; solar wind capture by Mercury's magnetic field, vapor produced by micrometer impacts on Mercury's surface, and direct thermal evaporation or sputtering.
Mercury's most distinctive known surface feature is Caloris Basin, a impact crater ~1350km in diameter. The planet is marked with huge curved kilometer-tall cliffs and lobate scarps hundreds of kilometers long, which apparently formed billions of years ago as Mercury's core cooled and shrank causing the crust to wrinkle. The majority of Mercury's surface is covered with plains of two distinct ages; the younger plains are less heavily cratered and probably formed when lava flows buried earlier terrain.
At certain points on Mercury's surface, an observer would be able to see the Sun rise twice in the same Mercurian day. This is because approximately four days prior to perihelion Mercury's orbital velocity exactly equals its rotational velocity, so that the Sun's motion through the sky ceases, and at perihelion Mercury's orbital velocity exceeds its rotational velocity, causing the Sun's motion in the sky to reverse. Four days after perihelion the Sun's normal east-to-west motion resumes. An observer in the right place could as a result see the Sun rise about halfway, then reverse and set on the same horizon, and then rise again to complete its normal circuit across the sky.
The mean surface temperature of Mercury is 452 K, but it ranges between 90 and 700 K. The sunlight on Mercury's surface is 6.3 times more intense than that on Earth, a total irradiance of 3566 W/m2.
Mercury has been known of since at least the time of the Sumerians (3rd millennium BC), who called it Ubu-idim-gud-ud. The earliest recorded detailed observations were made by the Babylonians, who called it gu-ad or gu-utu. It was given two names by the ancient Greeks, Apollo when visible in the morning sky and Hermes when visible in the evening, but Greek astronomers knew that the two names referred to the same body. Heraclitus even believed that Mercury and Venus orbited the Sun, not the Earth. Observation of Mercury is severely complicated by its proximity to the Sun; it is only visible from Earth at sunrise or sunset.
The orbit of Mercury is highly eccentric, ranging from 46 million kilometers to 70 million kilometers in radius. The slow precession of this orbit around the sun could not be completely explained by Newtonian Classical Mechanics, and for some time it was thought that another planet might be present in an orbit even closer to the sun (sometimes referred to as Vulcan) to account for this perturbation. Einstein's General Theory of Relativity provided the explanation for this small discrepancy instead, however.
Mercury is one of the least-studied of the solar system's planets. It was visited by only one spacecraft, Mariner 10, which flew past it on three occasions in 1974 and 1975. Only 45% of the planet's surface was mapped, and it is located too near the Sun for existing telescopes to conduct further mapping from Earth. Until radar observations in 1965 proved otherwise it was thought that Mercury was tidally locked with the Sun, rotating once for each orbit and keeping the same face directed towards the sun at all times. Instead, Mercury is in a 3:2 resonance, rotating three times for every two revolutions around the Sun; the eccentrity of Mercury's orbit makes this resonance stable. The original reason astronomers thought it was tidally locked was because whenever Mercury was best placed for observation, it was always at the same point in its 3:2 resonance, so showing the same face, which would be also the case if it was totally locked.
Recent radar observations have suggested that water ice may be present in small quantities at Mercury's North Pole. Such water, if it exists, is likely located at the permanently shaded bottoms of craters where it is deposited by comet impacts. A new mission to Mercury has been approved by NASA, named Messenger, which will launch in 2004 to orbit Mercury starting in 2009.
Japan is planning a joint mission with the European Space Agency that would be the first to land a probe on Mercury. The mission entails three probes, two that would orbit and one that would land, to map the topography and study the origins of the planet. Russian Soyuz rockets would launch the probes starting in 2010. The probes would reach Mercury about four years later, with one of them landing on the planet, and the other two orbiting and charting its surface for a year.