Spirit acquired this mosaic while investigating the area east of the elevated plateau known as "Home Plate" in the "Columbia Hills." The trench exposed a patch of nearly pure silica, with the composition of opal. It could have come from either a hot-spring environment or an environment called a fumarole, in which acidic, volcanic steam rises through cracks. Either way, its formation involved water, and on Earth, both of these types of settings teem with microbial life.
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Credit: NASA/JPL-Caltech