Abstract:
Long-term monitoring of dune/non-dune gullies has substantiated extensive and diversified morphological changes whose timing correlates with the time of removal of CO2 frost, thereby implying seasonal control on the active processes. We present evidence for present-day changes in sand-filled gully channels consistent with ripples. These channels are ubiquitous in the mid-latitudes and found in gullies formed over a variety of substrates on crater walls. We investigated 1483 gullied craters having coverage of at least two High Resolution Imaging Science Experiment (HiRISE) images between 30°S and 75°S and identified 98 craters (6.6% of the total) with sand-filled gully channels. Among these 98 craters, sand-filled gully channels in 25 craters (25.5% of the total) exhibit morphological changes indicative of avulsion, narrow channel incision, formation of channels with potential terminal pits, channel widening and extension, downslope flow of mobilized sand inside channels, dark flows and rapid boulder movement inside channels, and local sand rearranging and erasing of channels. Investigation of the sequence of events revealed CO2 frost inside the sand-filled gully channels in the image acquired during winter and prior to the image in which recent changes have been observed. This suggests a connection with the sublimating frost inside the channels and a seasonal control that subsequently produces extensive changes in the sand-filled gully channels during defrosting seasons in approximately a Mars year. Comparison of changes on both sand-filled gully channels and dune gully channels substantiates that the currently active processes have similar effects on the loose, unconsolidated substrate in both the gully types. For example, CO2 frost processes are able to erode both sand-filled gully channels and dune gully channels to form linear channels with terminal pits. In contrast, we have not found similar evidence of linear channels with terminal pits on bedrock and/or LDM substrate on the crater wall suggesting that the bedrock and LDM substrate may not be able to be eroded by CO2 frost processes in a manner similar to the loose, unconsolidated substrate in sand-filled gully channels investigated in this study. Together, the results signify that the sand-filled gully channels represent a significant component of present-day modification of gully channels on Mars.