For decades Mars has teased scientists with whispers of water's presence. Valleys and basins and rivers long dry point to the planet's hydrous past. The accumulation of condensation on surface landers and the detection of vast subterranean ice deposits suggest the stuff still lingers in gaseous and solid states. But liquid water has proved more elusive. Evidence to date suggests it flows seasonally, descending steep slopes in transient trickles every Martian summer.

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