California's Mono Lake is an earthly stand-in for the Mars of four billion years

California's Mono Lake is an earthly stand-in for the Mars of four billion years ago, when the planet was starting to lose its surface water. As the lake's waters slowly evaporate and recede, they reveal tufas, strangely shaped towers of calcium carbonate. Scientists use the lake to test hardware bound for Mars, and they study microbes that live in its salty, alkaline waters.

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