NASA’s Mars rover Opportunity discovered a mineral vein telling of a warmer, wetter and potentially more conducive to life past for the seemingly cold, dry and desolate Red Planet. The mineral vein, found in the crater Endeavor, is composed of a calcium sulfate called gypsum. Unlike some other forms of calcium sulfates, gypsum retains water in high temperatures after precipitating, in other words its presence indicates the existence of water. 
via Mars Daily

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