A rock outcrop dubbed "Missoula," near Marias Pass on Mars, is seen in this image mosaic taken by Curiosity's MAHLI camera on sol 1031 (July 1, 2015). The area pictured is about 40 centimeters across. Pale mudstone (bottom of outcrop) meets coarser sandstone (top) in this geological contact zone, which has piqued the interest of Mars scientists. White mineral veins that fill fractures in the lower rock unit abruptly end when they meet the upper rock unit. Such clues 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.