The giant planets, their satellites, the asteroids and the comets formed very early. In the inner portions of the nebula, within a couple of AU of the Sun, a collection of dry rocky planetesimals that had survived the early solar winds, began slowly to assemble into larger bodies. Moonsized objects appeared. Finally several the size of Mercury and Mars dominated the scene but most were swept up by the two bodies that became Earthand Venus. A couple escaped into stable orbits and survive as Mercury and Mars.

Mercury

Although there is a common perception that Mercury is so close to the Sun that it is difficult to observe, it is often clearly visible away from city lights as a morning or evening star. Mercury is unique on account of its high density that tells us that it has a high content of metallic iron relative to rock. In contrast, the Moon has the reverse; a low content of metal relative to rock. Explanations for the peculiar nature of both bodies have a long history and much effort has been expended in attempts to fit one or both into overall schemes of planetary formation.

Like the Moon, the real significance of the anomalous nature of Mercury has only recently been appreciated; both these objects have played the role of red herrings in our attempts to understand the solar system [1].