Grid-mapping Hellas Planitia, Mars - Insights into distribution, evolution and geomorphology of (Peri)-glacial, fluvial and lacustrine landforms in Mars' deepest basin

摘要

Traditional maps of Hellas Planitia, the most prominent impact basin on Mars, have focused on the delineation of continuous surface units. We applied the newly developed grid-mapping method in order to quantitatively analyze the distribution and geostatistics of selected (peri)-glacial, fluvial, and lacustrine landforms. The study area was subdivided in grid cells with a mesh size of 20 x 20 km, and more than 10,000 grids have been inspected manually in a GIS environment at a mapping scale of 1:30,000. Each grid has been checked for the presence or absence of a landform. Thus, we were able to statistically evaluate the geographical behavior of landforms with respect to elevation, slope inclination, aspect, and other parameters. We searched for 24 pre-selected landforms. However, only 15 of them had a sufficient abundance for scientific research. Whereas the latitude-dependent mantle is widespread in most of Hellas, it was found to be mostly missing in the northeastern part, likely a result of desiccating winds circulating clockwise within the basin. The location and morphologic expression of scalloped terrain also seems to be influenced by winds, as the local orientation of scalloped depressions appears to be aligned along the dominant wind direction, indicating that insolation is not the only factor controlling their formation. Hellas Planitia has been suggested as the site of a former sea. We also searched each grid for the presence of possible shorelines. Despite the small scale of our mapping, no clear evidence for coastal landforms has been detected. Our results reveal a distinctive asymmetry with respect to fluvial channels and Noachian light toned sediments along the rim of the impact basin. While the northern rim shows a high density of both channels and sediments, the southern counterpart basically lacks channels and light-toned deposits. We suggest different climatic conditions for this imbalance, as the northern part of Hellas likely experienced higher temperatures throughout most of Mars' evolution, while the colder conditions at the southern rim may have prohibited aqueous processes, preventing the development of channels and related sediments. As Hellas contains the deepest areas of the planet's surface, and thus the highest air pressure, its climatic environment can exceed the triple point of water until today, making it a potential habitat. However, our results have shown that the basin floor displays only a very low density of landforms that may indicate liquid water and ice, and especially gullies and viscous-flow features are scarce. The high air pressure and relatively mild temperatures in Hellas decrease the relative atmospheric water content, resulting in a desiccated air and soil, and hence, may explain the lack of viscous-flow features and gullies. All these findings extended our knowledge not only of Hellas Planitia, but of the screened landforms themselves too, In conclusion, small-scale grid-mapping made it possible to recognize large-scale patterns and distributions in Hellas Planitia.