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According to a new paper, scientists now think that the rough and broken cap layer of the Greenheugh pediment that Curiosity is presently traveling across was originally a dune field periodically washed by water runoff, which with time eventually hardened into sandstone.
That broken terrain, dubbed “gator-back terrain” by the Curiosity science team, is shown clearly in the image to the right, taken on March 20, 2022. From the paper’s abstract:
The Greenheugh pediment is capped by a unit of broadly uniform thickness which represents the remains of the Stimson dune field that existed <2.5 Ga (mid- to late-Hesperian). ChemCam geochemical data shows that the sands deposited at the Greenheugh capping unit were sourced from a nearby olivine-rich unit. Surface waters then cemented the windblown sand deposits, ponding at the unconformity with the underlying mudstone unit, creating concretions towards the base. Episodes of groundwater circulation did not affect the rocks at Greenheugh as much as they did at other Stimson localities with the exception of acid-sulfate alteration that occurred along the unconformity. These results suggest that the ancient Stimson dune field was a dynamic environment, incorporating grains from the surrounding geological units on Mt Sharp. Furthermore, liquid water was stable at the surface in the Hesperian and was available for multiple diagenetic events along bedrock weaknesses.
In other words, material from Mount Sharp formed the dune fields, all of which were reshaped by groundwater circulation, with the dunes higher on the mountain seeing less groundwater.
The biggest uncertainty of these findings is explaining how surface liquid water could exist on Mars. Scientists have yet to develop an accepted model that would allow it. Another possibility would be the recent data that suggests Gale Crater was filled with glaciers. If so, scientists would need to figure out how the interaction of a Martian glacier might have geologically changed those dunes in a manner similar to groundwater.
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