This Mariner 10 image shows Discovery Rupes, the sinuous dark feature running through the craters at the center of the slide. Many such features were discovered in the Mariner images of Mercury and are interpreted to be enormous thrust faults where part of the mercurian crust was pushed over an adjacent part by compressional forces. The abundance and length of the thrust faults indicate that the radius of Mercury decreased by 1–2 kilometers (0.6–1.2 miles) after the solidification and impact cratering of the surface. This volume change probably was due to the cooling of the planet, following the formation of a metallic core three-fourths the size of the planet.
(Digital mosaic of Mariner 10 images by P. Schenk, Lunar and Planetary Institute.)