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