Seldom, outside the treasure hunts of childhood, has the discovery of a pebble caused such a stir. But the first evidence of gravel beds on Mars is reason enough to get very excited indeed.

Most obviously, the smoothly eroded rocks discovered by Nasa's rover Curiosity indicate the presence of water at some point in the past. Although scientists have been confident for some time that water did once exist on Mars, only now, thanks to the findings in the Gale Crater, do we have the incontrovertible proof.

Nor is that the full extent of what we have learned. Analysis of the size and shape of the rocks suggests that they were formed by a fast‑flowing stream anywhere between a few inches and several feet deep. Very much like, in fact, the streams and rivers on Earth.

The implications could hardly be more profound: the Red Planet's pebbles conjure images of a watery world where life could easily have thrived, some three-and-a-half or maybe four billion years ago. More portentous still, if life developed on Mars as well as Earth, the entire universe might well be teeming with it. All this, yet the two-year mission has barely begun. Just wait until Curiosity really gets going.