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Mercury.
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| MERCURY - THE WINGED MESSENGER | |
| Mercury, the innermost planet of the Solar System, was regarded by the Romans as the god of travel and by the Greeks as the "winged messenger" of the gods. | |
| Mercury is a difficult object to see in the night sky, as it is always located near the Sun and can only be seen close to the horizon at sunrise and sunset. Nevertheless, Mercury has been known to ancient peoples since at least 3000BC. | |
| Very little is known about Mercury - less than half of its surface has been mapped by spacecraft. No spacecraft has visited Mercury recently, and the planet is too close to the Sun to observe safely with the Hubble Space Telescope. Nevertheless the study of Mercury's surface continues today. | |
 The orbit of Mercury. |
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| An eccentric orbit | |
| The orbit of Mercury is highly eccentric, and consequently Mercury's distance from the Sun varies between 46 and 70 million kilometres. The point of closest approach to the Sun, perihelion, itself moves (precesses) slowly around the Sun. However, the predicted rate of precession never matched observations, and this lead to a fruitless search for the planet Vulcan - it was believed that Vulcan's gravity was responsible for affecting Mercury's orbit. Eventually, Einstein's Theory of Relativity correctly predicted Mercury's orbital precession - a successful prediction that helped establish Relativity. | |
| Mercury's rotation rate (length of day) has been established with radar observations. Mercury rotates three times on its own axis during every two Mercurian years. Usually, when an object has a rotation period synchronised with its orbital period, it is on a 1:1 basis - Mercury is the only known Solar System object with a 3:2 synchronisation. | |
| Does Mercury have metal at its core? | |
| Although Mercury is a small planet, it has a high density, similar to the density of the Earth, and greater than the Moon's. The high density, and the existence of a magnetic field, implies that Mercury is metal-rich. In fact, Mercury may have a metal core making up most of the planet's volume. The question is, did this structure arise out of the solar nebula when Mercury was formed, or did Mercury lose most of its crust during an impact catastrophe? | |
 The interior structure of Mercury. |
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| Weak magnetic field | |
| Mercury has a weak magnetic field, only about 1% the strength of Earth's. The cause of the field is currently unknown, though it may possibly indicate that Mercury has a molten or semi-molten core. | |
 Mercury compared to Earth. |
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| Mercury is one of the smallest planets in the Solar System - only Pluto is smaller. Saturn's moon Titan, and Jupiter's moon Ganymede, are both larger than Mercury. | |
| Escaping atmosphere | |
| Mercury's small mass and low gravity gives the planet a very low escape velocity, making it easier for an atmosphere to escape. When coupled with Mercury's close proximity to the Sun and the high temperatures, it becomes impossible for Mercury to retain an atmosphere. | |
 The atmosphere of Mercury. |
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| A trace atmosphere has been detected, consisting mainly of helium and sodium, and with a pressure less than a trillionth the pressure of Earth's atmosphere. The source of the gases is either the solar wind pouring out from the Sun and flowing around Mercury, or possibly a slow release of gases from the Mercurian surface. It is likely that Mercury loses the gases as fast as it releases them from the surface or captures them from the Sun. | |
| The cratered surface | |
| Mercury has a cratered surface, like our Moon, which was shaped by intense meteorite bombardment during the early history of the solar system. The formation of craters was accompanied by the eruption of lavas to create volcanic plains. Although it seems that nothing very interesting has happened since, the geological story of Mercury is not as simple as it might at first appear. There remain many questions which even today cannot be answered satisfactorily. | |
 The surface of Mercury. |
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| More about Mercury's surface. | |
| Exploring Mercury | |
| There has been very little exploration of Mercury. The only spacecraft to visit, Mariner 10, captured imagery covering less than half of the planet's surface. | |
| More about the exploration of Mercury | |
| Heat from the Sun | |
| Earth's atmosphere acts as a thermal blanket, preventing large temperature variations between day and night. Mercury has no such protection from the Sun, and the temperature variations experienced are the most extreme in the Solar System. Mercury's close proximity to the Sun causes daytime temperatures to rise to 420oC, hot enough to melt zinc. At night, the lack of a substantial atmosphere allows the heat to radiate away into space, and temperatures drop to below -170oC, cold enough to freeze carbon dioxide. | |
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