Paleolake Inlet Valley Formation: Factors Controlling Which Craters Breached on Early Mars

The ancient surface of Mars is dominated by degraded impact craters with reduced or eliminated rim relief. Some degraded craters have an inlet valley, while many remain fluvially isolated. Despite controlling Martian fluvial connectivity, few constraints exist on why some—but not all—degraded craters possess inlets. We compared a suite of properties around degraded Martian craters with and without inlets to ascertain what topographic and hydrologic factors influenced inlet formation. Slope and surface roughness are similar, but topographic inset within the catchment, drainage density, and potential contributing areas diverge for breached and non‐breached craters. We suggest that the importance of basin hydrology‐related factors over topographic factors is the result of the former less frequently surpassing inlet incision thresholds than the latter. We conclude that greater topographic inset (i.e., craters deeper within regional depressions) promoted higher discharge, and that inlet valley formation was ultimately controlled by Mars' crater‐dominated topography. Plain Language Summary The ancient surface of Mars is dominated by impact craters that have been modified by mass wasting, aeolian infilling, water activity, and subsequent impact cratering. Erosion has lowered or eliminated the relief of many craters' rims, resulting in “degraded” craters. On early Mars, water activity also led to the formation of branching valley networks, which flow into some Martian impact craters. However, many degraded craters remain without an inlet valley. Inlet formation reveals that water activity could overcome the disruption of topography caused by impact crater formation. Yet, little is known about factors that would promote or limit inlet valley formation and it is unclear why some—but not all—degraded craters were breached by inlet valleys on Mars. We compared a suite of properties around degraded Martian craters with and without inlets to find out what topographic (e.g., slope) and hydrologic factors (e.g., the number of valleys per area) affected inlet formation. We find that hydrology‐related factors are very different between craters with and without inlets, while topographic factors are similar. Our results suggest that inlet valley formation was ultimately controlled by hydrologic routing on Mars' crater‐dominated surface. Key Points We compared slope, roughness, drainage density, and catchment geometry around degraded Martian craters with and without an inlet v