Water was common across a vast region of ancient Mars, creating habitable conditions for long stretches of time billions of years ago, scientists said Thursday.
New data reveal water in the Meridiani Planum region of Mars extended across hundreds of thousands of square miles, at least as groundwater and possibly as shallow lakes or seas.
The work significantly expands the amount of surface area on Mars known to have once been water-laden, and it extends the period of time that the water was present.
Rocks that clearly formed in water extend throughout 980 feet (300 meters) of layered material in several locations across the plains, said Ray Arvidson, an Earth and planetary sciences expert at Washington University in St. Louis, Missouri. The layers were built up over time, which means water was present, at least off and on, for extended periods of the early martian history.
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The findings mean that the conditions for life were present.
"Everything that we're finding makes the probability [of a habitable environment] go up," Arvidson told SPACE.com.
The signature of water is a scrawl of various minerals known as evaporates, which are left behind when water turns to vapor. They are the same aqueous signatures detected last year in two craters by NASA's Opportunity Rover.
"We've been able to show that the evaporates that Opportunity sees in the two craters extend over a much larger area," Arvidson said in a telephone interview.
Liquid water is the key ingredient for life as we know it, but the fact that early Mars was loaded with water does not necessarily mean life ever got going. Mars is comparatively dry today, though it harbors significant amounts of water ice.
Some scientists believe liquid water may lurk in pockets under the surface, and it may even support life now.
The new research is based on data from the European Space Agency's orbiting Mars Express spacecraft. It is presented in five separate papers in the Feb. 18 issue of the journal Science.
Opportunity found minerals, including jarosite, that can only form in the presence of water.
Other evidence in layered rock called "etched terrain" revealed that the water existed in large quantities, such as shallow lakes or underground aquifers, and that it persisted for long stretches. But the rover could only examine limited sections of they layers -- typically a few inches or feet that poked above the surface.
Over the past few months, scientists have worked to calibrate what they observe from the orbiting Mars Express craft to ground-based observations. The signatures of water are now evident across the landscape, but in much thicker sections that represent longer stretches of time during which rock formed in the presence of water.
The signs of past water exist throughout the entire 980 feet of the etched terrain's thickness.
"The water may have formed in shallow seas or lakes," said Arvidson, who is also deputy principal investigator for the Mars Exploration Rover instruments. "We don't know yet."
However, there was at least temporary standing water that possibly dried out, with the same area being affected by wind-blown debris before getting wet again, he said. The layered terrain is from the end of the Noachian period and maybe into early Hesperian period, more than 3 billion years ago.
Small areas of similar mineral deposits have been found in Valles Marineris, the huge scar on Mars bigger than the Grand Canyon. Pictures of Valles Marineris suggest it was carved by water.
"These minerals contain water, and had to be formed in the presence of water," said Brown University geologist John Mustard, author of another of the new papers.
Other research has turned up clay minerals that also require water to form.
Importantly, Mustard told SPACE.com, these clay minerals can form in more neutral pH waters, unlike the very acidic environments that are thought to have created the minerals in the etched terrains. "This could be very important in the identification of regions that supported habitable conditions," Mustard said.
"These results strongly support the view that Mars had habitable environments," Mustard said. "There are now hundreds of places beyond the Opportunity landing site that need to be investigated."
He said the mineral signatures identified by OMEGA, an instrument on Mars Express, "open up a much more complex and diverse palette of possible habitats to consider, including hydrothermal."
There's a catch, however.
"All the locations where we find these signatures are ancient, probably formed in the first billion years of Mars' history," Mustard said.
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