Evidence for fluvial and glacial activities within impact craters that excavated into a Noachian volcanic dome on Mars

Abstract

Impact craters on Mars preserve diverse records of volcanic, fluvial, and glacial activities. Enigmatically, the preservation of these major activities or records altogether within impact craters is rare. We report one such new observation of impact craters that formed on a volcanic dome studied using data from the Mars Reconnaissance Orbiter's (MRO) Context Camera (CTX), High-Resolution Imaging Science Experiment (HiRISE), and Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), Mars Global Surveyor's Mars Orbiter Laser Altimeter (MOLA), and Mars Express' High-Resolution Stereo Camera (HRSC). A ~ 20 km diameter impact crater, informally named as Degana-A, is formed within the ~50 km diameter impact crater Degana. Mineralogical analysis reveals exposures of low-calcium pyroxene and olivine deposits, which occupy Degana-A's eastern walls, leading to the idea that pristine Noachian bedrocks might be exposed from beneath the volcanic dome. Degana-A floor is completely covered by alluvial fans from all sides with distributaries. Within Degana crater, multiple fans are observed along its eastern to southern side only. The most likely source of water was the accumulation of snow on Degana crater walls, which possibly melted as a result of the impact of Degana-A. We observed a ~ 1 km wide breach on the eastern wall of Degana-A and the estimated maximum flow velocity is ~2 m/s and a run-off ~2.25 mm/h. Over the south-facing walls, multiple moraine-like ridges superposed the fans, which suggests overprinting by glacial activities. The presence of fans and superposed moraine-like ridges located at the mid-latitudes (~23�S) implies atmosphere-derived snow/ice precipitation was possible. Chronologically, the dome is of Noachian age, whereas Degana crater formed in the Hesperian period and crater retention on the fans indicates late Hesperian to Amazonian ages. Overall, the preserved Noachian crustal material underneath a volcanic dome is rarely exposed in its pristine context, which offers a rare window into early igneous processes. This intriguing location also witnessed a climatic transition as implied by water/ice derived landforms formed by non-coeval events.