EARTH'S SURFACE
The evidence from the heavy-bombardment period in the inner solar system has been erased from the earth's surface. Even the oldest rocks, which are about 3.8 billion years old, are of no help in identifying this early impact cratering since they are mostly isolated outcroppings, many covered with ice. The earth also has
basaltic plains, like the maria on the moon, Mercury, and Mars- not old ones, as on the other terrestrial planets, but very young. The major ones on earth are formed by the addition of new material to the lithospheric plates at the midoceanic ridges.
It is unlikely that the earth ever wel1t through an appearance I.ike that of the moon, Mercury, and Mars, in which maria were formed from lava's flooding a huge impact basin. In all probability the thermaltectonic activity of the earth's surface has always been too great to have allowed maria to form. Thus if we could watch a time-lapse movie of the evolution of the surfaces of all the terrestrial planets, it is unlikely that they would all start out the same and begin to depart from each other later. Rather, the surfaces of the moon and Mercury, which have thick lithospheresthicker than those of Mars, Venus, and the earthhave not been fractured and then deformed by convection. This is because the moon and Mercury should have cooled very quickly, extinguishing any tectonic activity if it ever existed. And the earth has probably always shown vigorous tectonic activity, with Venus's less than that of the earth and Mars even less. Thus the moon and Mercury have the oldest surfaces; Mars's surface is old but with some youngish features; Venus one guesses to have a mixture of old and young; and the earth's surface is comparatively very young.
