Hematite is a mineralogical marker of ancient climate change on Mars

The ancient climate of Mars changed from warm to cold surface conditions. This climate transition is demonstrated by geomorphological evidence but lacks suitable mineralogical indicators. We investigated the crystallographic properties of hematite (iron oxide) in Gale crater measured by the Curiosity rover and compared them with laboratory experiments. Hematite crystallite sizes are about 5 to 65 nm in the oldest sedimentary rocks investigated by the rover (the Murray formation) and less than 10 nm in the younger overlying strata (the Mirador and Carolyn Shoemaker formations). We attribute the larger crystallites in the Murray formation to postdepositional coarsening by groundwater in warm and wet conditions that persisted for several million years. Hematite with small crystallites co-occurs with goethite (iron oxyhydroxide) in the overlying layers, consistent with colder and water-limited conditions.