Surface Features

The adjacent figure is a mosaic of images from Mariner 10 of Mercury's surface. There are three major types of surface features on Mercury:

  • Smooth plains that resemble somewhat lunar maria, though there is little evidence for the lava flows evident in the maria.
  • Intercrater plains, which are pocked with small craters and occupy about 70 percent of the surface that has been examined.
  • Rugged highlands that bear some resemblance to the corresponding regions on the Moon.

    • Craters and Volcanoes
    Generally, Mercury is heavily cratered but not as much as the Moon. In addition, although we expect that Mercury had a volcanic past, we find little of the evidence for past volcanism common on the Moon. (Though some newer analyses of the old Mariner 10 data suggest that some plains could be volcanic in origin.) These facts are not well understood, but suggest that the history of Mercury was not exactly the same as the history of the Moon, though they share many similarities.

    Examples of Surface Features
    The following images illustrate three features from the surface of Mercury: a large impact basin called Caloris that is similar to large maria on the Moon (the dark feature centered on the left edge of the left image and outlined with dashed lines), jumbled surface features called "weird terrain," and a long cliff-like feature called a scarp (the almost vertical feature near the center of the right image and indicated by the arrow).

    Origin of these Surface Features
    The Caloris Basin is the only large impact basin found on Mercury. Only part of it can be seen in the image because it lies on the terminator (line dividing lighted and dark hemispheres of the planet). The "weird terrain" lies almost exactly on the opposite side of Mercury from the Caloris Basin. It is speculated that seismic waves from the impact creating Caloris travelled through the planet and were focused on the opposite side, creating the jumbling and twisting of the surface seen in the area of "weird terrain." Scarps are common features on Mercury. They can be hundreds of kilometers long and often lie on top of craters, indicating that they were formed after the meteor impacts that formed the craters. The scarps are thought to have resulted from "wrinkling" of the surface as it cooled and perhaps from tidal stresses induced by the Sun.