The unusual features of the southern edge of Acidalia Planitia have HiRISE mission scientists scratching their heads.
The irregular depressions with unique rims can’t have been caused by volcanic activity and can’t be impact craters. Nor can they be windblown formations because the pits hold large boulders that even strong Martian winds couldn’t move.
What the scientists do believe is that the large basin was once a huge ocean, which may have caused depositions of sediment in outflow channels. It’s also possible that the unusual topography was caused by shallow lenses of water ice that left behind small basins.
But neither conjecture explains why the depressions sport such peculiar raised rims.
HiRISE principal investigator, Alfred McEwen, planetary geologist at the University of Arizona, says, “Ancient glaciation is another possibility, perhaps depositing ice-rich debris next to topographic obstacles.
“Future images of this region may provide clues, but for now this is a mystery.”
Source: Discovery News, May 7, 2013 The Planetary Society, August 4, 2010 Photo credit NASA/JPL/MSSS