Nilosyrtis Mensae is an ancient terrain with a wonderful variety of landforms and rock types.
What is this circular landform? It probably got its shape from an impact crater long ago, but was subsequently eroded and filled in and eroded again, so that now it is a low mesa surrounded by a boulder-rich geologic unit.
Written by: Alfred McEwen (audio by Tre Gibbs) (24 October 2012)
More info and image formats at http://hirise.lpl.arizona.edu/ESP_028957_2085
Image: NASA/JPL/University of Arizona