Clay minerals often form at or near rocky planetary surfaces under low temperature hydrous conditions. Careful identification of the types of clay minerals and consideration of their geological context can yield information on ancient environmental conditions, including inferences of atmospheric composition. However, this task is complicated by multiple potential sources of clays and overprinting by subsequent diagenetic or hydrothermal processes. Dr Bristow will discuss approaches to paleoenvironmental interpretation of clay mineral-bearing sedimentary rocks with a focus on recent findings from the Mars Science Laboratory (MSL) rover Curiosity. MSL’s discovery of clay minerals in ancient lake sediments at Yellowknife Bay is critical evidence of habitable conditions in the past. Determination of the origins of the clay minerals at Yellowknife Bay provides constraints on the longevity of aqueous conditions, redox state within sediments, and potential chemical energy sources for organisms.