This is a false color image of a mosaic of Mercury.
Click on image for full size
Courtesy of NASA.

Evolution of Mercury

Mercury, like the other planets, is believed to have formed in the earliest stage of the evolution of the solar system as dust came together to form even larger clumps and eventually small planets or "planetesimals". Eventually, the largest clump's pull of gravity "swept up" the remaining smaller clumps, producing craters on the young planet's surface. Because Mercury is so close to the Sun, many of the lightest elements in the original solar dust cloud were probably not plentiful in the innermost solar system. As a result, the material left to form the planet was rich in metals and other elements and minerals that are solids at high temperature. Under the influence of gravity, the planet continued to shrink, eventually melting much of the planet and allowing separation of materials by their density. Sinking of the heaviest elements to the center of the planet led to the formation of an iron-rich molten core. Movement of this molten metal in Mercury's interior is probably responsible for the magentic field Mariner 10 measured at Mercury. The planet cooled quickly, because of its small size, and it probably has a thick rocky layer or "lithosphere" extending hundreds of km into the planet from the surface - possibly even to the core. As the planet continued to cool, it shrunk slightly, causing compression of the surface and eventually thrust faults, where one portion of rock slips up to partially cover another. These thrust faults are probably responsible for the formation of wrinkled ridges which were observed by Mariner 10.

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