The US space agency's Mars rover Opportunity has discovered evidence that its landing site near the planet's equator was once awash with water.
BBC News Online looks at the significance of the find.
What precisely has the rover uncovered?
Opportunity has been studying soil and layered rocks in a shallow crater at a place known as Meridiani Planum. The rover has detected "unequivocal" evidence that the area was once "drenched in liquid water".
It can tell this from the chemistry and physical appearance of the rocks. For example, the rover found very high concentrations of sulphur salts. On Earth, rocks with as much salt as these Mars rocks either have formed in water or, after formation, have been highly altered by long exposure to water.
But we already knew Mars was once a wet world?
Yes. Orbital evidence over more than 30 years has pointed to a warmer wetter Mars in ancient times. Most scientists now agree that running water cut the huge channels and gullies that mark the planet's surface.
The US Mars Odyssey probe and, more recently, Europe's Mars Express orbiter have also returned data to show substantial quantities of water-ice are still retained on the planet in the polar regions. But if the orbiters have shown us what water can do on the large scale, the rover Opportunity has now provided direct evidence of what water can do on Mars on the fine scale.
What does the water evidence mean for life?
Nasa's rovers are not equipped to look for life on the planet today. They carry no instruments to do biology experiments. They were sent to Mars to see if conditions once existed that could have supported life.
Opportunity has now demonstrated this was the case at Meridiani Planum. But scientists still do not know how much water was in the area. Were these rocks merely altered by water passing through them, or did they actually form in water? Answering these questions will tell us if Meridiani Planum once contained something perhaps as big as a sea. Opportunity will keep working at these issues.
What does this mean for future Mars exploration?
One point is very clear: rocks need to be returned from Mars for thorough analysis in Earth laboratories. Nasa says the rover discovery has made this a priority and Meridiani Planum could now be the target of a future sample-return mission. If life did exist there once then its signature may still be retained in the rocks. But there is only so much scientists can glean from remote instruments on a rover operated at a distance of 250 million km.
To get definitive answers on the life question, researchers would need to run sophisticated tests on untainted samples, and that can only be done on Earth. The age of the rocks needs to be tested as well to tell us when the water acted on the Martian surface. Spacecraft will be despatched in the coming years to haul Martian rocks back to our planet.