Mercury never strays far from the Sun in the sky. Like Venus it appears as an evening or a morning object. To see it you have to look low down on the horizon and know where to look. Little detail can be seen on it even through the world's largest telescopes.

Only one spacecraft has visited Mercury. Mariner 10 first flew to Venus and used its gravity to reach tiny Mercury which is only slightly larger than our moon.

After its encounter with Venus Mariner 10 entered solar orbit and encountered Mercury on three occasions making passes of the planet in March and September 1974 and March 1975 observing the same area of the planet each time. In all, 75% of the planet was mapped.

Mariner 10

Mariner 10 could see detail about a kilometre across but in small selected areas details about 100-500 metres across. This means that our photographs of Mercury are about the same that we had of the Moon before the advent of the space age.

Most astronomers expected Mercury to look very much like the Moon and they weren't disappointed. They saw mountains and craters everywhere with flat less cratered terrain where geologists believe volcanic lavas once flowed.

A closer look however reveals some distinctly Mercurian landforms. There are hundreds of long cliffs stretching across the surface which geologists call lobate scarps.

These cliffs, some more than a mile high are believed they formed when the crust of Mercury shrank and wrinkled as its interior heat source waned soon after its formation.

Using computers scientists have reprocessed Mariner's images and produced a new mosaic of the planet.