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Today’s cool image, to the right, takes us to the equatorial regions of Mars, a region that today appears quite arid and dry based on all the orbital and rover/lander data so far gathered. The photo and its complex geology however provides us a hint that once liquid water did exist here. At least, that is the hypothesis that scientists presently favor, though making it fit this complex geology is not simple or straightforward.
The mosaic to the right is made from four context camera images taken by Mars Reconnaissance Orbiter (MRO). It shows a very complicated series of depressions — one of which vaguely resembles a crater — that appear to have been washed out by some past erosion process, though that process could not have been that simple because of the fissures and cracks that dominate the floor of the circular feature.
I contacted Chris Okubo of the U.S. Geological Survey, who had requested a high resolution image from MRO of a small part of this mosaic, as indicated by the white box, to ask him what we are looking at. His answer was appropriately noncommittal:
There hasn’t been a lot of work done on these, but similar landforms have been previously interpreted as spring mounds that developed within the crater filling sediments.
He then directed me to this 2014 paper [pdf], which analyzed similar features in the same general region of Mars, and put forth some preliminary theories as to the processes that created this geology.
The map to the right gives the geographic context. The white cross marks the location of the mosaic above. The second image, rotated, cropped, and reduced to post here, was taken on April 30, 2020 by MRO’s high resolution camera and is the photo requested by Okubo.
The 2016 paper put forth the following:
We propose that these features are genetically related to groundwater upwelling that ascended from deep fractures and faults.
… Where the groundwater reached the surface, arguably in the southern sectors of craters, the sediments package is thicker and the [layers] are characterized by abundant prominent morphologies such as mounds, furrows ridge-and-trough structures. Furrows and ridge-and-trough structures developed on flat topography are apparently devoid of gravity flow structures. … The groundwater, pumped by the regional hydraulic head, flowing through the fractures induced by the impact, reached the surface and gave rise to the mound clusters. Then the water probably filled the flat bottom of the crater and generated furrows and ridge-and-trough structures during the water table standstill.
The location of this image, as indicated by the map, is at the eastern end of Valles Marineris, in the region where theorized catastrophic floods resurged out into the northern lowland plain of Chryse Planitia. When those floods took place, this location likely had an ample liquid water table, with that water quite energized from the floods. This, according to the above hypothesis, would have provided the force for the upwelling from below that produced the small mounds seen in the close-up, from which water and sediments were pushed up. When that flood energy dissipated and these depressions were then filled with the slowly disappearing standing water, it acted to widen the cracks and fissures.
This is only a theory, with many assumptions and gaps in knowledge, the biggest assumption of all being the belief that Mars’ atmosphere was once warm and thick enough to have allowed liquid water on its surface. Right now no model convincingly makes that possible, even though the geology here and in many other places on Mars strongly suggests it must have been the case.
Above all, the geology in these photos argues that Mars once had both subsurface and surface liquid water at its equator. Whether that water still exists as deep underground ice, remains unknown.