Based on the geology of Mars's northern plains, the new study suggests that bodies of water formed as groundwater slowly seeped through cracks in the crust. This process would have made oceans and lakes quickly—within just a few years—but also could have sustained the bodies of water over millennia.
However, even when Mars was supposedly wet, the planet likely didn't have a very thick atmosphere. Many scientists therefore think that if life as we know it evolved on Mars, the best places to look for it would be where liquid water would have been protected from extreme temperature changes and damaging ultraviolet radiation from the sun.
To find life, "we have to look for regions on the planet where water would have been stable. In the case of Mars, this is in the subsurface," said study leader J. Alexis Palmero Rodriguez of the Planetary Science Institute in Arizona.
This presents a dilemma for fossil hunters, since digging deep to find potential traces of Martian life would involve time and equipment not available to the robotic rovers sent to explore the planet's surface.
But according to the new study, "the water upwelling [in northern Mars] would have been very ancient water trapped in the subsurface for billions of years. That's a very stable environment for organisms to form and evolve," Rodriguez said.
And that means some sediments left by those ancient seas—in surface deposits that would be accessible to rovers—may be hotbeds for Martian fossils.
Mars Water Pooled From Slow Seeps?
Mineral evidence on Mars suggests that surface water must have been present at some point in the past. In fact, several huge sediment deposits in the northern plains remind observers of the bottoms of Earth's oceans.
Previous theories had suggested Mars oceans formed due to massive, abrupt discharges of groundwater—but there's a hitch: "The channels thought to have been produced by this type of discharge are rare and only occur in a few regions of the planet," Rodriguez said.
What's more, "there are no obvious widespread channel systems extending from the highlands into the large Martian basins where oceans are thought to have once existed," he said. "What's the mechanism for the formation of these water bodies without widespread channels that can account for the amount of water required to form these lakes or oceans?"
In a region of northern Mars south of a scarp called Gemini Scopuli, sediments sit atop a basin that's highly fractured by tectonic processes and impact craters. Based on the rocks and minerals present—as seen via spectroscopy data from orbiting probes—groundwater appears to have come to the surface in this region for about two billion years, the study authors say.
The overall landscape suggests that, rather than abrupt gushes, pressurized groundwater could have escaped through the crust fractures in slow, long-lasting seeps, according to the new paper, published in this month's issue of the journal Icarus.
The study team thinks the water must have come from an extensive underground aquifer that reached from the plains to higher elevations.
A raised water table in the plains stopped surface water from sinking back into the soil, the team says. The upwelling water therefore pooled to form shallow oceans or systems of small lakes, similar to those seen when ground ice melts each spring on Alaska's northern slopes.
The new theory hints that oceans and lakes could have remained stable on Mars for perhaps thousands of years, undergoing seasonal cycles of freezing and thawing—as the Martian ice caps do today.
"The bodies of water would have remained stable for as long as groundwater emergence continued," Rodriguez said. "Highly saline lakes can remain liquid at freezing temperatures, and on Earth they have been observed to contain living organisms. The stability of ponded water would also increase if covered by ice," he said.
And stable lakes might have allowed any underground creatures brought to the surface to survive the difficult transition to an environment bombarded with UV radiation, Rodriguez said.
"We know that evolution and successful adaptations of life-forms to new environments are more likely to occur when there are geologically long periods of time available," he said. "So gradual and long-lived groundwater emergence would have increased the chances of successful adaptations to the surface and near-surface environments."
Don't Expect Mars Fossils to Be Familiar
Based on this theory, it's possible future robotic landers could find Martian fossils in deposits along exposed crater walls or surface fractures in the northern plains.
Even more tantalizing, the fractured basins, such as those seen near the north pole, are widespread across Mars, opening up a variety of sites where past slow-growing oceans—and potential fossils—may exist.
For example, NASA has yet to pick a landing site for the next big mission to the red planet, the Mars Science Laboratory. But one of the candidate sites, Mawrth Vallis, fits with the new study's ocean-formation model.
Overall, Rodriguez and colleagues "have put together an idea that ties together numerous diverse, otherwise anomalous phenomena—and one that's certainly a worthy idea to bring into the mix," said Victor Baker, a planetary scientist and geoscientist at the University of Arizona in Tucson who was not involved with the research.
Baker also agrees that Martian groundwater had the potential to support life.
"There are subsurface environments on Mars, even today, which are undoubtedly not much different chemically, or [in terms of] temperatures and pressures, to subsurface environments on Earth that have life in them," he said.
But when dreaming of Martian fossils, Baker cautioned, don't expect to find the kinds most familiar to us on Earth.
"Whatever is an indication of previous activities of living organisms can be a fossil. It doesn't have to be bones. It can be traces. It could be evidence of chemistry that one can tie back to a biological process," he said.
"To expect that Mars would have achieved something like the Cambrian explosion"—Earth's most intense burst of evolution—"would really be stretching it," he added.
"But to expect that Mars might have [microorganisms] similar to what was characteristic life for most of Earth's very early history is not too great of a stretch."