The US space agency's Mars rover Opportunity has discovered evidence that its landing site may once have been a shoreline at the edge of a sea or large body of water.
BBC News Online looks at the significance of the find.
Surely we know there was once water on Mars now, what's new about this announcement?
When Nasa scientists made their announcement in early March, they knew that Opportunity's landing site at Meridiani Planum had been soaked in water at some time during the past.
Their evidence came from mineral and chemical analysis of the outcrop of light coloured rock in Eagle Crater, where Opportunity came to a stop.
But they needed to do more work to find out if there had once been a standing body of water, such as a sea, in the area and whether rocks in the outcrop were sediments laid down in this water.
The latest findings appear to confirm that there was indeed a salty sea at Meridiani Planum and that the layered rocks in the outcrop are sediments.
How do they know this?
Examination of the rocks with a microscopic camera on the rover shows that some layers in the rocks lie at angles to the main layers. These patterns are called cross-beds and are made by rippling of sediment grains by moving water or wind.
Geologists sometimes have a hard time telling the difference between cross-beds formed in water and those formed in wind. But Nasa scientists say that if the sediments form in a moving fluid, such as water, the cross-bedding does have distinctive features.
LAID DOWN IN WATER
There is persuasive evidence for the rocks' watery past
Some rock layers have cross-beds that could have been formed by the action of wind. But others show festoons, smile-shaped curves which are considered by many geologists to be reliable indicators of the shifting of rippled sediments under a current of water.
The types of ripples in the rock are consistent with sediments shaped by water flowing at a speed of 10 to 50cm/s.
So why do scientists think there was once a beach there?
Mission scientist John Grotzinger believes the types of ripples seen in the Mars rocks are like those that form at salt flats or playas on Earth, which occur at the edge of oceans or in desert basins. These environments are sometimes covered by shallow water but are sometimes also dry.
Further evidence for this comes from the outcrop's chemical composition. When the researchers looked at the rocks with a scientific instrument on the rover called an Alpha Particle X-ray Spectrometer (APXS), they found the element bromine in the rock.
The proportion of bromine compared with other similar elements such as chlorine went up or down by as much as a factor of 10 in some places. On Earth this is characteristic of rocks that are formed by the evaporation of salty water.
Principal scientific investigator Steve Squyres believes this is indicative of a body of water that came and went over time.
Do they know how big this body of water was?
No, the scientists have no way of telling how deep the water at Meridiani Planum was or how far it extended across the region.
But to produce the ripples in the rock, scientists think the water must have been at least 5cm (two inches) deep and possibly much deeper.
What are the chances that there was once life there?
Scientists can only speculate about this point. On Earth, environments like the one that once existed at Meridiani Planum almost always harbour life. But scientists do not know enough about how common life is in the Universe to say whether it is likely it evolved on our neighbour Mars.
If minerals precipitate out of liquid water, as happened at Meridiani Planum, this process can trap a record of whatever was in that water in rock. That makes Meridiani Planum an excellent place to look for fossilised evidence of life.
But if life did evolve in environments like this on Mars, it is likely to have been microbial. The rovers do not have the right scientific instruments to look for evidence of life on this tiny scale.
Only future missions will be able to resolve the question of whether life really did evolve on Mars. Specifically, scientists want to return rocks to Earth for study. Meridiani Planum is an excellent target for a future sample return mission.