Deltas Along the Crustal Dichotomy of Mars Likely Record Large Lakes, Not a Global Ocean

Deltaic deposits are important for constraining the hydrologic history of Mars as they occur globally and require sustained liquid surface water to form. Of particular interest are deltas along the martian crustal dichotomy as these were previously interpreted to have formed in a global ocean. A high concentration of these previously mapped deltas occurs in and near Gale Crater, where recent orbital and rover data has shown that multiple generations of lakes likely existed. Here we use updated visual and topographic data to further probe whether deltas in the Gale crater region ( 1000 km east and west of Gale) formed contemporaneously to deltas within Gale and to what extent they record past water bodies. Our geomorphic analysis was performed using digital elevation models (DEMs) derived from Context Camera ( 6 m/pixel) stereopairs using the Ames Stereo Pipeline. These were supplemented with a blended Mars Orbiter Laser Altimeter and High-Resolution Stereo Camera DEM ( 200 m/pixel) for regional topographic context.

We investigated 24 candidate deltaic landforms, twelve of which have been characterized in previous studies. Deltas were differentiated from other depositional features, such as alluvial fans and landslide deposits, based on their arcuate form, low-gradient tops, and steeply-dipping fronts. They also tended to originate from short, stubby, v-shaped valleys with amphitheater-shaped terminations. Delta front elevations were found to vary by 2620 m, ranging from -3980 m to -1360 m. The deltaic deposits were rarely isolated, rather they typically were clustered in groups of 2 to 5 deltas. In each of these groups the deltas had similar front elevations to one another; within 30 to 200 m range. Using delta front elevations as water level proxies, we found evidence for at least eight distinct enclosed basins into which these features formed. Based on modern day topography, the spatial extent and depth of hypothesized paleo-lakes in these enclosed basins was reconstructed. While several of these deltas did not correspond to an enclosed basin, these data suggest that most dichotomy deltas in the Gale region record large lakes, not a global ocean, which has significant implications for the size of Mars' past global hydrosphere.