A vast ocean chock-full of microbes may have once covered more than a third of Mars's surface, scientists say.
The new evidence, from an analysis of dried-up Mars river deltas, adds to growing signs the red planet was once wet.
On Earth, river deltas all lie at more or less the same elevation and reflect the current sea level. In fact, by estimating the elevations of ancient deltas, scientists can reconstruct how sea levels have changed over time.
Brian Hynek and Gaetano Di Achille of the University of Colorado at Boulder applied this thinking to Mars.
By determining the elevation of 52 desiccated deltas on Mars, the scientists found that 17 of the deltas lie at approximately the same elevation.
"If there was an ocean on Mars, then the deltas should be at a constant elevation—or at least the majority of them, and that is what we found," Hynek said.
The deltas would have ringed an ocean in Mars's northern hemisphere that existed about 3.5 to 3.7 billion years ago. (See Mars pictures.)
The ocean lasted for at least several hundred million years — about the time it would have taken for all the deltas to form, Hynek said.
Ancient Mars Had Shallow Ocean
Scientists first proposed the idea of an ancient Martian ocean in the 1980s after spotting potential beach ridges and other features in images taken by NASA's Viking probes.
Some, but not all, of that early evidence was later discounted with more precise mapping data from other Mars missions, and the debate has continued.
If it existed, Mars's ancient ocean was not very big or deep by Earth standards. Its volume was only about one-tenth that of Earth's oceans, and it would have only been about 5.5 kilometers (3.4 miles) deep on average.
By contrast, "the bottom of the Atlantic [Ocean] is roughly 5 kilometers [3.1 miles] deep across a lot of it, but in the Pacific you can get some very deep trenches that are a quite a bit deeper," said Hynek, whose research appeared in the June 13 issue of the journal Nature Geoscience.
Mars Had Earth-like Atmosphere?
An ancient ocean on Mars would mean the planet once had a thick atmosphere and a hydrological cycle similar to that of Earth.
"If Mars had an ocean, it must have had clouds and rain, snow in the highlands, ice caps forming, a groundwater system, and all of these things that we have here on Earth," Hynek said.
And where water flowed, life could have arisen. (Read more about the search for life on Mars.)
"Life on Earth started in the oceans and stayed in the oceans for billions of years," Hynek said.
"If Mars had oceans for even a couple hundred million years, it gives me more hope that we'll find evidence of past microbial life on Mars."
No "Final Word" on Mars Ocean
The new study strengthens the argument for an ancient Martian ocean, but it is "unlikely to give the final word on the [Mars] ocean debate, since it is such indirect reasoning," said James Head III, a planetary scientist at Brown University in Rhode Island who was not involved in the study.
In addition, Taylor Perron, a geologist at the Massachusetts Institute of Technology, said the new study still leaves some issues unresolved.
For example, why were the elevations of all the deltas studied not at the same level?
"The variability in the elevations of the deltas they mapped allows for the possibility that the deltas do not record a single, level sea surface," Perron wrote in an email.
(Related: "Lava, Not Water, Made Mars 'Riverbed.'")
But, he added, "one possible explanation is a large-scale deformation of the planet that warped the landscape, transforming what was once a level shoreline into one with more variable elevations."