Mars used to have oceans, but began drying out about 4.2 billion years ago.However, oasis-like pools of water may have sheltered microscopic alien life.New maps show shallow pools were common on Mars for hundreds of millions of years.Today Mars is a freeze-dried rock with so little air, you couldn't even gasp if you tried.The scene was much different about 4.5 billion years ago: Water covered about 20% of the planet's surface, and 400-foot-tall megatsunamis raced across ancient oceans.Alien life might have emerged and thrived on this warm, wet, and dynamic world ' that is, until solar storms began to blow away the air and water some 300 million years later.However, new maps of the planet created by US Geological Survey (USGS) scientists reveal that habitable oases once littered the bottom of Valles Marineris ' a canyon system the size of the continental United States ' hundreds of millions of years after Mars' oceans evaporated.If groundwater consistently fed these lakes, pools, and puddles, as rock formations hint, it's possible that microbial life took shelter there."There was a huge potential for a habitable environment here, and there's a lot of promise for [microscopic] fossils, perhaps, or some biological signals," Chris Okubo, an astrogeologist at USGS and an author of the new maps, told Business Insider.The oases of ancient MarsOn Earth, the typical oasis is fed by underground sources of water, such as a leaky aquifer.Okubo and other scientists think the process was similar on Mars."[G]roundwater was abundant and occasionally seeped onto the ground surface, forming pools," Okubo said in a USGS press release. "These pools would have been habitable for life, just as they are on Earth."Okubo told Business Insider that he first saw signs of these ancient oases in a small part of Valles Marineris.The feature is often called the "Grand Canyon" of Mars ' but is about four times longer, 20 times wider, and nearly five times deeper than Earth's famous canyon. Okubo had seenthin layers of sediment in the basin that had piled up, one atop another, over about 500 million years.Such layers form when groundwater gurgles to the surface, forming a shallow pool ' and a potentially habitable oasis, says Okubo. Over time, grit blows into the water, which responds by pushing more upward to form another pool. More grit then blows in. This cycle repeats itself over eons to form a flat terrain of finely layered sediments.Fast-forward a few billion years, and wind erosion has carved into and exposed these layers for scientists like Okubo to detect from space.Evidence of such shallow pools on Mars isn't new. However, their newly discovered extent should raise some eyebrows, says Okubo.The new work by Okubo and his USGS colleagues zoomed out for a wider view of the canyons, yet used incredibly detailed images from the Mars Reconnaissance Orbiter's HiRISE camera to look for signs of ancient shallow pools. That analysis revealed telltale sediment layers over much of the gigantic canyon's basin.What's more, the maps hint that shallow pools lasted from about 3.5 billion to 3 billion years ago; in an older yet similar blink of a geological eye, life on Earth may have appeared."We're only seeing the top of the pile," Okubo said. "We don't know what's going on further back in time because we can't see it ' it's buried."If these shallow pools existed for at least 700 million years earlier ' or when the oceans of Mars began to evaporate ' they may have bridged a crucial gap for microbial life on the planet.To find out for certain, Okubo says we'll need to send robots or people to dig into the soil or take readings with ground-penetrating radar."This work argues soundly for a future landing site, whether it's a sample return or a rover," he said. "Anything would be great."NASA now has a legal mandate to get people to Mars by 2033, though the first crew of four astronauts may just orbit the world. However, the space agency's Mars 2020 rover is expected to launch in July 2020, meaning it should arrive more than a decade earlier and gather samples for a future return mission.Researchers are now studying possible landing sites for the nuclear-powered robot. NASA has narrowed the list to three candidates, though none of them within Valles Marineris.Does Mars still have water'Scientists today think Mars still has a water table, though one that must be hidden deep below its surface."Although [satellites] in orbit around Mars haven't found any subterranean aquifers, there's a sneaking suspicion we're only seeing the upper kilometer or so, because radar's a challenge," Michael Meyer, a lead scientist at NASA's Astrobiology Institute, told Charles Q. Choi in an Oct. 2016 story for the space agency.Over the past decade scientists believed they'd found a smoking gun for water on Mars: dark streaks in sand on the slopes of dry gullies, called "recurring slope lineae" (RSL), which show up around the same time each season.However, closer looks at more than 100 of these RSL sites revealed no presence of groundwater or its byproducts. (The water might come from salts absorbing a little water in the air, then melting any ice that's slowly built up when the climate temporarily warms up.)That's not to say Mars is bone-dry today, and that astronauts won't be able to recover waterthrough drilling or other means, like burning methane captured from Martian air with hydrogen fuel.The planet's apparent desolation also doesn't make scientists like Okubo give up hope: Detecting fossils that are even billions of years old would verify the existence of extraterrestrial life."I wouldn't be surprised. I suspect there to be life elsewhere in the universe," Okubo said. "This would be an exciting finding, but it wouldn't change my perspective. I don't think life as we know it on Earth is a singular event."SEE ALSO:The first Mars astronauts may be trapped inside of a tube for 3 years with no chance of escapeDON'T MISS:NASA just quietly rolled out a new Mars rover concept that looks like a BatmobileJoin the conversation about this storyNOW WATCH: Stephen Hawking warned us about contacting aliens, but this astronomer says it's 'too late
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