Geologic Evolution of Dao Vallis, Mars

Three major outflow channel systems (Dao, Harmakhis, and Reull Vallis) extend through the cratered highlands and sedimentary plains of the eastern Hellas region of Mars. These valles represent a stage in regional geologic history intermediate to formation of channels and valleys within highland terrains in the Late Noachian and Early Hesperian Epochs and debris aprons and gullies in the Amazonian Period. Dao Vallis, along with its tributary Niger Vallis, extends for ~1200 km from the eastern margin of Hadriaca Patera into Hellas Planitia, where a depositional lobe is observed on the basin floor. The Dao Vallis system (~6-50 km wide) is characterized by two steep-walled source depressions, regions of subsided plains, and prominent central canyons whose walls display gullies with associated depositional aprons covering parts of canyon floors. The present study is designed to utilize MOC images, MOLA topographic data, and THEMIS daytime and nighttime images to evaluate and refine the geologic evolution of Dao Vallis as defined in earlier Viking-based studies. Dao Vallis has previously been interpreted to have formed by collapse of volcanic and sedimentary plains, potentially triggered by volcano-ice interactions in the Martian subsurface. Small channels and lineations parallel to canyon walls provide evidence for surface flow of fluids. Evidence for subsurface flow is present in zones of subsided plains that separate the central canyons of Dao and Niger Valles from their source depressions. Later stages of the geologic history were dominated by mass-wasting from canyon walls, which may have significantly increased canyon width in places. Comparison of MOC, THEMIS, and Viking images suggests a consistency of geologic processes across a range of spatial scales, as evidence for collapse, mass-wasting, and fluvial erosion and deposition is observed in all datasets. The higher spatial resolution of MOC and THEMIS permits a more detailed understanding of Dao Vallis' evolution, in particular its later stages which are dominated by small-scale activity along canyon walls. Canyon walls are diverse morphologically and include ice-rich mantling deposits, viscous debris flows, slump blocks and associated erosional grooves, and alcove-gully-apron systems that dissect and redistribute mantling and wall materials. Canyon floors include coherent blocks and rounded knobs emplaced via wall failure, surrounded by unconsolidated materials displaying both smooth and lineated surfaces. The unconsolidated floor materials may have been derived from canyon walls and/or the blocks and knobs. Lineation patterns extend away from source slopes and show deflection around topographic obstacles on the canyon floor. Dune fields are also observed but many areas of the canyon floor are dune free. Comparison of daytime and nighttime THEMIS data allow variations in the thermophysical properties of deposits to be delineated and dusty and rocky regions to be identified. Dao Vallis' floor deposits shows a high degree of variability and an overall increase in rockiness relative to the surrounding geologic units. This suggests that canyon floor deposits are not significantly mantled and is consistent with recent and possibly current transport of wall material into the canyon. Preliminary analyses of THEMIS data and the combination of Viking, MGS, and Odyssey data demonstrate the potential for robust analyses of Dao Vallis and other canyon systems on Mars. Future work will include systematic correlation of geomorphic signatures with MOLA topographic characteristics to further assess the formation and evolution of Dao Vallis.