INCISION OF PALEOLAKE OUTLET CANYONS ON MARS FROM OVERFLOW FLOODING (Invited Presentation)

The ancient surface of Mars preserves a record of >200 paleolake basins that were drained by incised outlet canyons. For each of these paleolakes, transition from an originally closed basin to a hydrologically open lake required that ponded water within the basin was able to overflow the confining topography (typically a crater rim) and in the process cut a hydraulic connection between the basin interior and the exterior terrain. Some of the largest martian valley systems have main stems that are paleolake outlet canyons, and studies of individual breached basins have suggested these canyons were incised rapidly during overflow flooding. However, it has also been hypothesized that incision of paleolake outlet canyons was the result of multiple overflow floods, or longer-term outflow that balanced inflow to the basins.

Here we present a study of outlet geometry for 24 open paleolake basins on Mars to test these competing hypotheses. Using orbital topography data we measured outlet geometry (breach depth and cross-sectional area, and volume of material excavated from the outlet canyon) and estimated the volume of water drained during progressive breach incision. Our results show that drained volume is a strong predictor of all the measured outlet geometric properties. Based on our results, we conclude that the studied outlet canyons formed during highly erosive, single episodes of lake overflow flooding. The studied basins have spatial and size distributions that are representative of the full catalog of open martian paleolakes, and we hypothesize that rapid outlet canyon incision from overflow flooding was important for most breached paleolakes on Mars. Finally, we compare our results to observations of breached lake basins on Earth, and show that outlet canyons have consistent geometric scaling relationships for the two planets. This points to the first order control of basin size and the lake overflow process on the resultant outlet erosion, regardless of variable boundary conditions (e.g., lithology).