Mars is a cold, dry place. Yet there is abundant evidence that fluvial (river) processes have carved the planet's surface; witness deep canyons, streamlined islands, and drainage networks. Most of these features formed more than 3 billion years ago, and a long line of research has led to the "warm and wet early Mars" hypothesis. The idea is that early Mars had a thicker atmosphere with an enhanced greenhouse effect that allowed stable liquid water and a hydrologic cycle to exist (1). The search for life on Mars, or at least conditions suitable for life, is predicated on this idea. Until now, no observations have unambiguously identified and characterized river-lain sediments, although the Mars Exploration Rovers turned up some evidence of a watery past (2). As the first major finding from the Mars Science Laboratory mission and its car-sized rover, Curiosity, Williams et al. (3) report on page 1068 of this issue the discovery of conglomerates on Mars—pebbles mixed with sand and turned to rock—resulting from ancient river deposits. The finding provides the clearest view yet on the nature of early martian rivers and should provide momentum for Curiosity's mission moving forward.
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