The "Hottah" rock outcrop, an ancient stream or river bed (NASA/JPL-Caltech/MSSS)

It's one thing to spot stuff from orbit above an alien world, 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' spreading downwards from the crater rim. It was extremely tempting to conclude that at some earlier time there had been a flow or flood of liquid water washing into and across a section of the crater floor.

Close up of the crusty stream bed, a larger 'clast' is circled (NASA/JPL-Caltech/MSSS)

Now Curiosity has come 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 up-tilted, 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 towards their having been washed and rolled in water that was somewhere between ankle and hip deep.

The view from orbit, color coded to show relative elevation. The alluvial fan from the Gale Crater rim is highlighted (NASA/JPL-Caltech/UofA)

It is a remarkable discovery. A great deal of speculation has been made about the nature of channel-like features across the surface of Mars. 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 mud-like 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.