Data acquired on Mars from the Mars Exploration Rover Spirit’s mini-thermal emission spectrometer shows the light, or spectral, signature of an as-of-yet unidentified mineral that contains bound water in its crystal structure. Minerals such as gypsum and zeolites are possible candidates. If water is now bound inside crystals underground, it once flowed freely on the surface, meaning there’s a large chance that there was once?and maybe still is?life on Mars.

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