WHY DID MARS DRY OUT?

Billions of years ago, Mars had rivers and lakes; today it lacks both. Possible causes are loss of atmosphere (mostly CO2), loss of H2O (to deep burial or to space), or loss of the capacity for non-CO2 warming (e.g. [1,2]). We hypothesize that clues to the cause of Mars’ final drying out are encoded in shifts in the distribution of water-involved features on Mars’ surface as surface liquid water waned (e.g. [3,4]). For example, the lower elevation (after correcting for resurfacing) of Mars’ late-stage alluvial-fan sourcing catchments relative to the elevation of the catchments that fed Mars’ earlier valley networks might be explained as the result of (i) a reduction in pCO2; (ii) a reduction in the strength of Mars’ greenhouse effect; or (iii) a shift from surface runoff to groundwater control, marking an intermediate stage in loss of H2O from Mars (Fig. 1) (e.g. [5]). Preliminary results are used to illustrate this approach.