MERCURY: LIKE A DRIED AND CRATERED APPLE
Observations from the earth had hinted that Mercury might look like our moon. But it was the Mariner 10 flyby on March 29, 1974, and the two subsequeht flybys on September 21,1974, and March 16, 1975, that showed that the planet is indeed heavily crate red like the moon. Although from photographs the overall surface of Mercury is remarkably similar to that of the moon, there are significant differences - differences that suggest a somewhat different surface evolution from that of the other terrestrial planets.
The surface of Mercury is pockmarked with craters ranging from hundreds of meters up to hundreds of kilometers. Some of the bright craters have extensive ray systems like those on the moon. Compared to the moon, Mercury and Mars are deficient in craters in the range of a few tens of kilometers, and in their place we see the extensive intercrater plains.
Like the moon, both Mercury and Mars have dark maria; while those for Mars and the moon are almost indistinguishable, the maria on Mercury exhibit small but significant differences. Twenty or so of Mercury's maria are several hundred kilometers across, while Caloris, the largest basin is more than 1000 kilometers wide. Its interior surface resembles the famous Orientale Basin on the moon. The basins on Mercury were created during the heavy bombardment period by a period of volcanism that filled them with lava. Mercury and the moon have remained virtually unaltered since this period of lava flooding 3 to 4 billion years ago. Mercury is also like the moon in a way not yet fully explained for either body: The craters cluster in one region, and the maria in another.
But there are some conspicuous differences between Mercury and the moon. Craters on the lunar highlands are densely packed, with rims of young craters overlying old craters, and the mare regions are sharply bounded. On Mercury, by contrast, craters are often interspersed with relatively smooth plains, giving the terrain a speckled appearance. Mercury has relatively few craters in the range of a few tens of kilometers, as mentioned above. Scarps or cliffs a few kilometers high and often hundreds of kilometers long are distributed widely over the old and heavily crate red regions. They cut across plains and craters alike. Unlike the scarps on the moon, they may have been formed when the crust of Mercury cooled and shrank, like wrinkles on the skin of an old apple.
