Two scientists went looking for water on Mars. After closely studying the Martian terrain, they think they might have found it–covering about a third of the planet, 3.5 billion years ago.

In a study published yesterday in Nature Geoscience, Gaetano Di Achille and Brian M. Hynek detail their hunt, which included looking at data from NASA’s Mars Orbiter Laser Altimeter (MOLA), a probe that studied the topography of the planet’s surface for four and a half years, starting in the late 1990s.

Scientists have debated whether Mars once supported oceans for over two decades, and, as the authors claim in their study’s abstract, these oceans remain one of the “largest uncertainties in Mars research.”

The authors of this study, who started out speculating on how water might have formed the apparent deltas and valleys on the planet, eventually looked at the altitudes of these features to determine if they could have been linked to a large body of water.

Gaetano Di Achille and Brian Hynek … had been building a database of Martian river deltas and valleys to examine how they might have been eroded by water, but ultimately realized that they had enough data to tackle the bigger picture. “Our research started as kind of a joke,” says Di Achille. “We were working on this database of deltas and valleys, and we said: why don’t we try to check this ocean hypothesis?” [Nature News]

They found that 17 of 52 deltas were at the same height, which might imply that they fed the same body of water which could have once filled a basin on the Northern hemisphere of the planet. Given that basin covers about a third of planet’s surface, the paper’s author question if these deltas might have channeled water into an ancient Martian ocean.

“If Mars had an extensive hydrological cycle in the past, with rain, groundwater reservoirs, ice sheets and surface run-off towards lakes and possibly a northern hemisphere ocean, then there should be evidence of deltas ringing the margins of these lowlands at a common elevation,” said Mr Di Achille. “Likewise, river valleys draining into such an ocean should also flow down to the same elevation, and shouldn’t be found below this level.” [Australian Broadcasting Corporation]

The research compliments another study from Hynek, which also looks at these river deltas.

In a parallel study published in the Journal of Geophysical Research (Planets), Hynek and colleagues catalogued some 40,000 river valleys on Mars, four times the number previously suspected. “The abundance of these river valleys required a significant amount of precipitation,” Hynek said. “This effectively puts the nail in the coffin regarding the presence of past rainfall on Mars.” [AFP]

Still, many hope for more direct evidence before claiming that this solves the ocean debate.

Taylor Perron, a geologist at the Massachusetts Institute of Technology in Cambridge, says that the result “strengthens the argument in favour of oceans” but leaves some issues unresolved. He says that there is enough variability in the delta elevations to suggest that there is not just one level coastline, and that it is “hard to explain” why some valleys end at much higher elevations than the proposed ocean. “One possible explanation is a large-scale deformation of the planet, which warped the landscape, transforming what was once a level shoreline into one with more variable elevations,” he says. [Nature News]

Of course an ancient Martian ocean leads to other questions: For one, where did all that water go?

Related content:
80beats: Scientist Smackdown: Did Mars Phoenix Find Liquid Water?
Bad Astronomy: New evidence of (transient) liquid water on Mars!
Bad Astronomy: Unpeeling the history of water on Mars
Bad Astronomy: Are Martian gullies formed by water or not?

Image: NASA, Goddard Space Flight Center, Scientific Visualization Studio/ Greg Shirah