It's one thing to spot stuff from orbit above an alien world and quite another to get in close.
Earlier Mars Reconnaissance Orbiter imagery of Gale Crater, now home to NASA's Curiosity rover, had shown signs of what appeared to be something akin to an “alluvial fan”—a sign that at some previous time there had been a flow of liquid water washing into and across a section of the crater floor.
In September, Curiosity came across the ground truth, including a rocky outcrop that is made of gravelly pebbles (clasts) cemented together into a crusty conglomerate. This is a chunk of uptilted, uplifted ancient stream bed. The pebbles probably originated from the crater rim a few hundred meters higher up, and their range of sizes, somewhat rounded shapes and placement all point toward their having been washed and rolled in water that was somewhere between ankle and hip deep.
It is a remarkable discovery. Water has always been a prime contender for carving and depositing these structures, and now it really does seem that it once flowed, albeit perhaps temporarily, on the planetary surface to leave this formation of gravel fixed into a mudlike cement. Today Gale Crater may be drier than the driest desert on Earth, but a long time ago there was, at least, a brief respite as water gurgled and sparkled in the sunlight on Mars.
Adapted from Scharf's Life Unbounded blog at blogs.ScientificAmerican.com/life-unbounded