There is fresh evidence pointing to life on Mars in the distant past, US scientists claim.
In two new studies, the scientists report that Mars once hosted vast lakes, flowing rivers and a variety of other wet environments that had the "potential to support life".
They reached this conclusion on the basis of data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) and two other instruments on board NASA's Mars Reconnaissance Orbiter (MRO).
One study, published in Thursday's issue of the international science journal Nature, shows that vast regions of the ancient highlands of Mars, which cover about half the planet, contain clay minerals that can form only in the presence of water.
Volcanic lavas buried the clay-rich regions during subsequent drier periods of the planet's history, but impact craters later exposed them at thousands of locations across the planet.
The clay-like minerals, called phyllosilicates, preserve a record of the interaction of water with rocks dating back to the earliest years of the solar system (about 4.6 billion to 3.8 billion years ago) when Earth, the moon and Mars sustained a cosmic bombardment by comets and asteroids.
Rocks of this age have largely been destroyed on Earth but "the phyllosilicate-containing rocks on Mars preserve a unique record of liquid water environments - possibly suitable for life - in the early solar system", a statement by the Johns Hopkins University said, quoting its scientists who built the instrument CRISM.
In most locations the rocks are lightly altered by liquid water, but in a few places they have been so altered that a great deal of water must have flushed though the rocks and soil, the statement said.
A companion study, published in Nature Geosciences, finds that the wet conditions persisted for a long time. Thousands to millions of years after the clays were formed, a system of river channels eroded them out of the highlands and concentrated them in a delta where the river emptied into a crater lake about 40 km in diameter.
According to authors of this study, the distribution of clays inside the ancient lake bed showed that water must have been standing there for thousands of years. "Clays are wonderful at trapping and preserving organic matter. So if life ever existed in this region there's a chance of its chemistry being preserved in the delta," the statement said.
By combining data from CRISM and the two other instruments on board the orbiter, the team has identified three principal classes of water-related minerals suggesting that different types of watery environments created them.
"The big surprise from these new results is how pervasive and long-lasting Mars' water was, and how diverse the wet environments were," said the statement.
Scott Murchie, CRISM's principal investigator, was quoted as saying: "Our whole team is turning our findings into a list of sites where future missions could land to look for organic chemistry and perhaps determine whether life ever existed on Mars."