Scientists have developed a rough guide for determining the age of planetary surfaces. They count the craters that have pocked a planet and compare the crater density to a similar area of the moon.

Short of chemically analyzing actual surface samples in the laboratory (a feat so far accomplished only with samples from the moon), this is the only reliable method scientists have to go by. The moon is the Rosetta Stone -- the secret decoder ring of surface ages.

From chemical analysis of lunar samples returned during the Apollo era, it is known that the gray lava fields on the moon formed some 3.6 billion years ago. Since then, the surfaces have been exposed continuously to bombardment from the space debris.