Exploring The Planets - Mercury - The Surface of Mercury

A Terrestrial Planet Like Earth's Moon?

The surface of Mercury, at first glance, looks very much like that of the Moon. There are, however, some important differences. On Mercury, the most heavily cratered regions, called cratered terrain, have fewer craters than the highlands of the Moon. This is because Mercury has more extensive intercrater plains than bury some of the cratered terrain. These plains may be made of ejecta material from large impact basins or they may be volcanic in origin.
Image courtesy of M.S. Robinson

Caloris Basin -- Impact Site

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This is one of the largest impact basins in the solar system and the largest feature yet observed on Mercury. The Caloris Basin is 1300 kilometers (810 miles) in diameter. Only half of the basin was imaged by Mariner 10, the other half was hidden by darkness. After the impact, the basin was flooded by lava. Ridges and fractures formed when the volcanic rock contracted and stretched as it settled under its own weight.
Image courtesy of M.S. Robinson

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A Shrinking Planet?

The surface of Mercury has landforms that indicate its crust may have contracted. They are long, sinuous scarps or cliffs called lobate scarps. The white arrows indicate their locations. These scarps appear to be the surface expression of thrust faults, where the crust is broken along an inclined plane and pushed upward. What caused Mercury's crust to shrink? As the interior of the planet cooled it contracted. Gravity then forced the crust to adjust to a smaller interior.
Image courtesy of M.S. Robinson

Discovery Rupes	73k JPEG
The largest known lobate scarp on Mercury is Discovery Rupes ("rupes" is Latin for cliff). Discovery is about 550 kilometers (350 miles) long and as high as 1.5 kilometers (1 mile). Notice that the walls and floors of two impact craters have been deformed by the thrust fault that formed the scarp. Image courtesy of T.R. Watters, M.S. Robinson, and A.C. Cook

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Digital Elevation Model

How do we know how high Discovery Rupes is? One way is to use two images that provide a stereo (3-D) view of the feature. Using a computer, the heights (elevations) of all the points covered by a stereo pair can be calculated and a digital elevation model made. This Mariner 10 image shows the area covered by the stereo pair and the digital elevation model is color-coded to show the highest elevations in white and lowest elevations in black. The white arrows in the image indicate the location of part of Discovery Rupes.
Image courtesy of T.R. Watters, M.S. Robinson, and AC Cook