White Rock Fingers on Mars . ... Credit Copyright: THEMIS , Mars Odyssey Team, ASU , JPL , NASA Explanation: What caused this unusual white rock formation on Mars? Intrigued by the possibility that they could be salt deposits left over as an ancient lakebed dried-up , detailed studies of these fingers now indicate a more mundane origin: volcanic ash . ... The above picture was taken with the Thermal Emission Imaging System on the Mars Odyssey spacecraft currently orbiting Mars . ...