The atmosphere on ancient Mars was too thin to keep its surface consistently warm, which could be the reason why it had no water constantly in liquid form, suggests a new study published in Nature Geoscience on Monday.

Previous studies concluded that the planet's topography indicates liquid water has flooded Mars in the distant past. However, evidence increasingly suggests that those episodes reflect occasional warm spells, not a consistently hospitable phase in the planet's history.

Researchers from Princeton University said the size of the planet's craters showed ancient Mars did not have a thick atmosphere. If Mars had once possessed a denser atmosphere, they contend, small objects would have broken up as they passed through it, rather than surviving largely intact to blast craters.

Using images from the Mars Reconnaissance Orbiter, the researchers catalogued more than 300 craters pockmarking an 84,000-square-kilometre area near the planet's equator.

A total of 10 percent of the definite craters in that terrain, which has not changed much geologically for about 3.6 billion years, had diameters of 50 metres or less, and roughly 10 percent of features presumed to be the remnants of ancient craters were 21 metres across or smaller.

Then, the team used computer simulations of incoming objects pummelling Mars, trying the scenario with a range of atmospheric densities. According to the team's analysis, the thickness of the atmosphere was less than one-third of what some teams say would be needed to consistently keep Mars' surface above freezing.

According to researchers, this new study suggested that Mars was only intermittently warm, which bolstered previous studies that suggest early Mars was icy.