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Mercury is the closest planet to the Sun. With a diameter of 4880 km, it is the second smallest. It orbits the Sun in a markedly elliptical orbit (with an eccentricity of 0.21) in 88 days. Its distance from the Sun varies between 0.31 and 0.47 Astronomical Units (1 au is the mean distance of the Earth from the Sun). Mercury has a rotation period of 58.6 days, which is two thirds of its orbital period.
Most of our knowledge of Mercury's surface was determined during the close approaches of Mariner 10 (so far the only space probe to visit the planet) in 1974/5. The surface looks very like that of the Moon, with many craters of very different sizes and lava plains called maria. There are also shallow cliff-like structures, which are not seen on the Moon, believed to result from wrinkling of the surface as the planet cooled and shrank. The largest craters on Mercury are well preserved and, as they are probably about 3 or 4 billion years old, indicate that there has been no migration of plates (like we see on the Earth) since then.
As Mercury is so close to the Sun it has very high mid-day temperatures, approaching 450°C. However it has almost no atmosphere, so consequently has night-time temperatures as low as –180°C.
Mercury has a small magnetic field which is evidence that it probably has a large nickel-iron core.
Mercury has no seasons, as the Earth and Mars have; instead it has a seasonal variation with longitude on the planets surface. This is due to the coupling between the rotational and orbital periods. The longitudes near 0° and 180° receive two and a half times as much radiation overall as do those near 90° and 270°.
Mercury’s axis is almost perpendicular to the plane of it’s orbit around the Sun. This means that the Sun’s rays always strike at a very shallow angle to the surface at the poles and the floors of the deepest craters are never exposed to sunlight. Earth-based radar has detected an extremely reflective material in these regions which may be sublimed sulphur or even water ice.
In a small telescope Mercury can be seen (even in the day-time) to have a small disc between 5 and 15 arcseconds across. The disc shows phases like those of the Moon. It appears full at Superior Conjunction (when Mercury is at its furthest from the Earth, behind the Sun) and new at Inferior Conjunction (when it is between the Earth and the Sun).
Mercury has no moon.
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