Evidence suggest that Mars was wet, but it was probably not consistently warm enough for making the water flow, a thrilling study reveals.
Signs of flowing water on Mars include layered sediments presumed to have been laid down in ancient lakes, as well as rugged canyons and lowlands apparently sculpted by massive floods.
These prompted researchers to suggest that the red planet, now frigid and dry, was warm and wet throughout its early history.
But that would have required an atmosphere much thicker than today's, a prospect that now seems unlikely, said Edwin Kite, a planetary scientist at Princeton University in the US.
The evidence against the idea that ancient Mars held a thick atmosphere for more than a few millennia at a time lies in the sizes of the planet's craters, said the researchers.
If Mars had once possessed a denser atmosphere, they said, small objects would have broken up as they passed through it, as they do in Earth's atmosphere, rather than surviving largely intact to blast craters.
The researchers used images from the Mars Reconnaissance Orbiter, to catalogue more than 300 craters pockmarking an 84,000-square-kilometre area near the planet's equator.
"It's not the size of the smallest craters, but the size distribution of the entire population that's important," Kite said.
"The thickness of the atmosphere was less than one-third what some teams say would be needed to consistently keep Mars' surface above freezing," said Sanjoy Som, an astrobiologist with the Blue Marble Space Institute of Science in the US.
The study appeared in the journal Nature Geoscience.
