What is that “purplish” colored stuff (in HiRISE IRB color) with the polygonal fractures in this observation? Scientists think it contains chlorides, like sodium chloride or table salt, or maybe chloride of calcium or magnesium.
These salts can be deposited during the evaporation or water in lakes or playas. These deposits are billions of years old, so it is truly old salt, not a fish tale. [More at link]