To determine plausible groundwater recharge rates on early Mars, we develop analytic and numerical solutions for an unconfined steady-state aquifer beneath the southern highlands. We show that the aquifer’s mean hydraulic conductivity, $K$, is the primary constraint on the plausible magnitude of mean steady recharge, $r$. By restricting groundwater upwelling to Arabia Terra, using a mean hydraulic conductivity of, $K$ ${\sim}10^{-7}$ m/s, and varying shoreline elevations and recharge distributions, the mean recharge must be order of $10^{-2}$ mm/yr. Recharge for other values of $K$ can be estimated as $r$ ${\sim}10^{-5}\,K$. Our value is near the low end of previous recharge estimates and two orders-of-magnitude below the smallest precipitation estimates. This suggests that, for a steady hydrologic cycle, most precipitation forms runoff, not groundwater recharge. It is also plausible the transient aquifer response to recharge is sufficiently slow that no upwelling occurs prior to cessation of climatic excursions causing precipitation.

Distinguishing single-thread and braided channel patterns and reconstructing paleo-water discharge from the sedimentary rock record has proven to be difficult. This is because only remnants of the river channels are preserved, often accounting for a small fraction of the overall stratigraphy. Instead, channel belt deposits—the amalgamation of deposits from many individual channels—are observed more often. Identifying channel patterns from geologic records is important for testing a range of hypotheses, including whether single-thread rivers were rare prior to the arrival of land plants in the Silurian. Here, we develop new quantitative metrics using distributions of channel geometry and channel belt properties with the ultimate goal of distinguishing channel types in the rock record. Metrics measured include width, sinuosity, radius of curvature, wavelength, and amplitude, and are measured from modern fluvial systems in the US via remote sensing techniques. An entropy-based braided index is used to quantify river branch number, which is directly related to a conventional braided index. Preliminary finding suggest that channel belt width and wavelength are well correlated with river branch number and water discharge via power-law relations, respectively. Specifically, mean river branch number is inversely related to channel belt wavelength, normalized by mean total channel width. In addition, mean channel belt width is directly related to water discharge at 2% exceedance flow event. This could imply that extreme flow events may contribute significantly to channel belt formation, more so than typical bankfull flow events (5–25% exceedance flows) based on threshold channel theory. Analyses presented here will aid improved interpretation of sedimentary deposits on Earth and other planets to infer past surface conditions.

Fluvio-lacustrine features on the martian surface attest to a climate that was radically different in the past. Since climate models have difficulty sustaining a liquid hydrosphere at the surface, multiple cycles of runoff episodes may have characterized the ancient Mars climate. A fundamental question thus remains: what was the duration of these runoff-producing episodes? Here we use morphometric measurements from newly identified coupled lake systems (containing both an open- and a closed-basin lake). We combined hydrological balances with precipitation outputs from climate models, and found that breaching runoff episodes likely lasted 10^2–10^5 yr; other episodes may have been shorter but could not be longer. Runoff episode durations are model-dependent and spatially variable, and no climate model scenario can satisfy a unique duration for all coupled systems. In the near future, these quantitative constraints on early Mars lake persistence may be tested through in situ observations from Perseverance rover.