Early Mars valley networks require decoupled surface water and groundwater

<title>Abstract</title> <p>The ancient surface of Mars records both runoff carved valley networks and evidence for groundwater activity, yet how much surface water reached regional aquifers remains uncertain. We use valley network elevations to constrain groundwater table height, comparing them with transient simulations that alternate between recharge and quiescence. Across 365 simulations spanning recharge intensity, duration, and spacing, the groundwater response is controlled by a single parameter: time averaged recharge over mean hydraulic conductivity. Compatibility with valley network incision, which requires groundwater to remain below valley floors, is met only when regional recharge is a small fraction of precipitation estimates. Within a 100 Myr valley network forming interval, only low recharge fractions permit many runoff episodes, whereas the terrestrial median recharge fraction of 22% would limit cumulative wet time to only a few of the longest episodes. A single ≥100 yr warm event, the minimum duration in coupled climate photochemical models, would exceed this entire budget. We infer that valley network forming runoff was at most weakly coupled to regional groundwater recharge. A weakly coupled surface and subsurface water cycle suggests caution is merited in inter-comparisons of the hydrology implied by surface and subsurface geologic and geomorphic observations on Mars.</p>