High and Dry: Billion?Year Trends in the Aridity of River?Forming Climates on Mars

Mars' wet-to-dry transition is a major environmental catastrophe, yet the spatial pattern, tempo, and cause of drying are poorly constrained. We built globally-distributed database constraints on Mars late-stage paleolake size relative to catchment area (aridity index), found evidence for climate zonation as was out. Aridity increased over time in southern midlatitude highlands, where lakes became proportionally small modern Nevada. Meanwhile, intermittently wetter climates persisted equatorial northern-midlatitude lowlands. This consistent with change greenhouse effect that left highlands too cold liquid water except during brief melt season, or alternatively fall groundwater table. The data switch unknown dependence aridity index elevation, from high-and-wet early on, high-and-dry later. These results sharpen our view surface conditions increasingly stressing life.