 Evidence for ancient rivers, a lake, and dune fields have all been observed in the preserved strata by rovers at Meridiani Planum and Gale crater.One group of researchers proposed that some of the layers on Mars were caused by groundwater rising to the surface in many places, especially inside of craters.According to the theory, groundwater with dissolved minerals came to the surface, in and later around craters, and helped to form layers by adding minerals (especially sulfate) and cementing sediments.