The nearest major planet to the Sun and the smallest of the terrestrial planets.
Telescopic observation of Mercury from the Earth is very difficult, partly because of its small size and partly because it can never be more than 28í from the Sun on the celestial sphere since its orbit lies well inside the Earth's. For the same reason, Mercury (like Venus, the other inferior planet) exhibits a cycle of phases, similar to those of the Moon. Hardly any surface detail can be discerned and very little was known about the planet until the flybys of Mariner 10 in 1974 and 1975. The space probe was put in an orbit around the Sun such that it encountered Mercury three times before it ran out of attitude-control gas. The images returned have allowed about 35 per cent of the surface of Mercury to be mapped.
Ancient, heavily cratered terrain accounts for 70 per cent of the area surveyed. The most significant single feature is the Caloris Basin, a huge impact crater with a diameter of 1,300 kilometres - a quarter the diameter of the planet. The basin has been filled by a relatively smooth plain, and terrain of the same type covers parts of the ejecta blanket. The impact took place 3,800 million years ago and produced a temporary revival of the volcanic activity that had mostly ceased 100 million years earlier, creating the smoother areas inside and around the basin. At the point on Mercury diametrically opposite the impact site, there is curious chaotic terrain that must have been created by the shock wave.
Characteristic features found on Mercury are lobate scarps (rupes), which take the form of cliffs between a few hundred and 3,000 metres high, believed to have formed when the planetary crust shrank as it cooled. In places they cut across craters.
The planet's rotation period is such that a ╘day' on Mercury lasts two ╘years'. This leads to immense temperature contrasts: at perihelion, the subsolar point reaches 430íC; the nighttime temperature plunges to -170íC.
The high daytime temperatures and the small mass of the planet make it impossible for an atmosphere to be retained. The small amounts of sodium and helium detected may be the product of micrometeorite impacts and radioactive decay of surface rocks or may be captured from the solar wind.
The average density of Mercury is only slightly less than that of the Earth. Taking account of its smaller size and lower interior pressure leads to the conclusion that Mercury has a substantial iron core accounting for 70 per cent of its mass and 75 per cent of its total diameter. There is also a magnetic field of about 1 per cent the strength of the Earth's field, providing further evidence for the metallic core.
