Rocky outcrop on Mars. Curiosity rover image of a rock outcrop called 'Missoula', on the surface of Mars, showing where paler mudstone (bottom) meets coarser sandstone (top). This geological contact zone is of interest to Mars scientists due to the white mineral veins that fill fractures in the lower rock unit and abruptly end where they meet the upper rock unit. Clues like this help scientists understand the possible timing of geological events. First, the fine sediment that now forms the lower unit would have hardened into rock. It then would have fractured, and groundwater would have deposited calcium sulfate minerals into the fractures. Next, the coarser sediment that forms the upper unit would have been deposited. Imaged by Curiosity's Mars Hand Lens Imager (MAHLI), in the 'Marias Pass' area of Mount Sharp, Mars, on 11th July 2015.