Water ice and water vapor have already been shown to exist on Mars' surface, and there has long been evidence of liquid water having flowed across Mars' surface billions of years ago. Scientists believe that the images sent by MGS show layered terrains that they suspect were left by water deposits below an ice layer or ancient shallow seas. However, it was assumed that the current atmosphere of Mars was so thin that any water on the surface would quickly freeze or evaporate. Indeed, in the first pictures of Mars taken in the 1960s and those sent from the MGS in August of 1999, the canyon showed no signs to the contrary

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