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A paper published on Saturday in the Journal of Geophysical Research – Planets of the American Geophysical Union, scientists suggest that the polygonal shaped cracks seen in the crater floors on many Martian craters could be evidence of ancient lakes. The evidence also suggests that the lakes were formed by the impact that created the crater. The energy of the impact melted underground ice to form a temporary lake inside the crater, which eventually dries out, leaving behind the polygons. From the abstract:
We propose desiccation to be a dominant mechanism for the formation of Crater Floor Polygons without ruling out thermal contraction as a possible contributor in some cases. This implies that lakes or water-rich sediments occupied the craters in the past. Many such aqueous environments have no apparent external source of water, and thus, hydrothermal processes occurring shortly after the impact event may be viable explanations for the observed evidence.