Four billion years ago, the Martian surface may have been cold and dry — not warm, watery and more Earthlike than it is today, as many scientists have suggested.
Instead of saturating the dusty surface, fluids appeared only occasionally, quickly shaping channels and other landforms that bear watery footprints. And tucked beneath the planet’s reddish, rocky sands lurked a warm and wet subterranean environment, a potential incubator powered by hydrothermal activity and revealed when meteorite impacts blasted telltale minerals from the planet’s crust.
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