Megaripples, mid-scale bedforms caused by wind action, have been widely studied and considered largely inactive relics of past climates, with few exceptions. A new paper by Planetary Science Institute researcher Matthew Chojnacki shows that abundant populations of megaripples have been identified in the north polar region of Mars and have been shown to migrate with dunes and ripples.
Mega-ripples on Mars are about 1-2 meters high and 5-40 meters apart, where their size ranges from ripples about 40 centimeters high with 1-5 meter spacing and dunes that can reach hundreds of meters in height with a spacing of 100 to 300 meters. While megaripple migration rates are slow by comparison (average of 0.13 meters per Earth year), some of the nearby ripples have been found to migrate an average equivalent of 9.6 meters per year in just 22 days in summer du North, unprecedented rates for March. These high rates of sand movement help explain megaripple activity.
“Using repeated HiRISE images acquired over long durations – six Martian years or 13 Earth years – we examined the dynamic activity of polar bedforms. We found that the thin Martian atmosphere can mobilize mega-jaws to large grains, overturning earlier notions that they were static relic landforms of a past climate.We mapped megaripples and adjacent bedforms across the North Pole sand seas, the most extensive collection vast array of dune fields on Mars,” said Chojnacki, lead author of “Widespread Megaripple Activity Across the North Polar Ergs of Mars” which appears in Journal of Geophysical Research: Planets.
Part of the uncertainty when studying planetary polar landforms is the long, cold polar winter that eventually blankets the region in carbon dioxide and water ice. For wind-driven bedforms, such as Megariples, this means that they are unable to migrate for nearly half the year. “However, it appears that the late spring and summer winds coming down from the polar cap more than compensate for these other periods of inactivity,” Chojnacki said.
Megariples were found to be widespread throughout the region and migrating at relatively high rates compared to other sites on Mars that are at lower latitudes. This increased activity is likely related to the larger sand fluxes found for nearby dunes that are driven by DST seasonal winds when polar ice sublimates. This supports the idea that much of the Martian surface is actively modified and not just ancient or static.” said Chojnacki. “In contrast, other megariples appear to be stabilized, a likely result of intergranular ice in low wind areas.”
Order hidden in the windswept sand
Matthew Chojnacki et al, Widespread megaripple activity across Mars’ north polar ergs, Journal of Geophysical Research: Planets (2021). DOI: 10.1029/2021JE006970
Provided by the Institute of Planetary Sciences
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