6. Mercury: a battered world
A modified Moon-like surface
Craters like the Moon
At first glance, Mercury closely resembles the Moon, for both worlds are small, heavily cratered, and without a significant atmosphere to cause erosion. Like the Moon, the planet Mercury has highlands that are pockmarked with impact craters, ranging in diameter from impact basins a million meters across to craters only 100 meters in diameter, the limit of Mariner 10's photographic resolution. The ubiquitous craters on Mercury strongly resemble their lunar counterparts, indicating that they were formed by meteoritic impact. As on the Moon, there are small bowl-shaped craters, larger craters with terraces and central peaks, relatively young craters with bright rays, and huge impact basins on Mercury.
Intercrater highland plains
In many important respects, Mercury's resemblance to the Moon is superficial. The planet's most densely cratered surfaces are not as heavily cratered as the lunar highlands, and Mercury does not contain regions of overlapping large craters and basins. Also unlike the Moon, the heavily cratered terrain on Mercury is interspersed with large regions of gently rolling, intercrater plains.
Smooth lowland plains
Widespread areas of Mercury are covered by relatively flat, sparsely cratered terrain called the smooth lowland plains. They are younger than the intercrater highland plains, and they are about 2 thousand meters lower. Unlike the dark maria on the Moon, the smooth lowland plains on Mercury are about the same brightness or color as the heavily cratered terrain and intercrater plains in the highlands of Mercury.
The smooth plains occur within and around the Caloris basin and on the floors of other basins, but a careful study of crater densities suggests that the smooth plains are younger than the Caloris impact. This age difference suggests that the lowland plains on Mercury are volcanic eruptions from the interior, rather than surface material melted and thrown out during basin-forming impacts. An investigation of the colors of sunlight reflected from the surface, and by implication the minerals it contains, support the view that some of the smooth plains originated by volcanic outflow.
Rupes - cliffs or scarps
The most remarkable geological features on Mercury are its winding cliffs or scarps, that are widely distributed over the planet. These unique features have been named rupes, Latin for "rock or cliff", each preceded by the name of a ship of discovery or a scientific expedition. An example is Dicovery Rupes, named after the Captain James Cook's (1728-1779) ship on his third and last voyage to the Pacific from 1776-80. The long cliff snakes its way across pre-existing craters and plains, attaining a length of 350 thousand meters and rising to 4 thousand meters, as high as the Pyrenees.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University