Shale: This photograph was taken by NASA's Mars Rover Curiosity in 2012 using its mast camera. It shows a portion of an outcrop inside the Gale Crater. This view shows an area about one meter wide. The color has been balanced to make the scene look as if it were on Earth.
Visible in this image are rocks that are very similar to the shales found on Earth. They are fine-grained, thinly layered and fissile (meaning they easily break into thin sheets). Rocks on Earth that break this way are usually made up of clay minerals or mica grains that settled out of an aqueous suspension. Their plate-shaped grains deposited on the bottom in a parallel orientation. This gives the rock the ability to be split into thin layers. Clay minerals are known to be abundant on Mars, so it is likely that these rocks are composed of clay minerals.
Martian impact craters are a great place to observe rocks because the impact blasted a hole in the planet's surface with outcrops exposed in the crater walls. In this scene, large amounts of fine-grained sediments can be seen covering the ground. Sediments on the surface of Mars are a product of millions of years of asteroid impacts and mechanical weathering. They are reworked by the wind today, and in the past, they were moved, deposited, and reworked by flowing water. Image by NASA. Enlarge image.