Mercury is the closest planet to the Sun and the smallest of the Inner Planets, probably because the heat of the nearby Sun as Mercury formed, about 4.6 billion years ago, prevented most of the gases present in the vicinity from becoming part of the protoplanet. When the planet is visible on Earth's horizon just after sunset or before dawn, it is obscured by the haze and dust in our atmosphere. Only radar telescopes gave any hint of Mercury's surface conditions prior to the voyage of Mariner 10. The photographs Mariner 10 radioed back to Earth revealed an ancient, heavily cratered surface, closely resembling our own Moon. The pictures also showed huge cliffs criss-crossing the planet. These apparently were created when Mercury's interior cooled and shrank, buckling the planet's crust. The cliffs are as high as 3 kilometres (2 miles) and as long as 500 kilometres (310 miles). Some cut through the rings of craters and other features in such a way as to indicate that they were formed by compression. It is estimated that the surface area of Mercury shrank by about 0.1% (or a decrease of about 1 km in the planet's radius).
Mercury, being scarcely more than moon-size itself, has no natural satellites.
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