Despite its reputation as a forebodingly dusty, desolate and lifeless place, Mars seems to be a little bit wet even today.
Scientists reported Monday definitive signs of liquid water on the surface of present-day Mars, a finding that will fuel speculation that life, if it ever arose there, could persist to now.
"This, I think, gives a focus of where we should look more closely," said Alfred S. McEwen, a professor of planetary geology at the University of Arizona and the principal investigator of images from a high-resolution camera on NASA's Mars Reconnaissance Orbiter.
In a paper published in the journal Nature Geoscience, McEwen and other scientists identified waterlogged molecules -- salts of a type known as perchlorates -- in readings from orbit.
"That's a direct detection of water in the form of hydration of salts," McEwen said. "There pretty much has to have been liquid water recently present to produce the hydrated salt."
By "recently," McEwen said he meant "days, something of that order."
Although young Mars was inundated by rivers, lakes and maybe even an ocean a few billion years ago, the modern moisture is modest. Scientists have long known that large amounts of water remain -- but frozen solid in the polar ice caps. There have been fleeting hints of recent liquid water, like fresh-looking gullies, but none have proved convincing.
In 2011, McEwen and colleagues discovered in photographs from the Mars Reconnaissance Orbiter dark streaks descending along slopes of craters, canyons and mountains. The streaks lengthened during summer, faded as temperatures cooled, then reappeared the next year.
They named the streaks recurrent slope linae, or RSLs, and many thousands of them have now been spotted. "It's really surprisingly extensive," McEwen said.
Scientists suspected that water played a critical role in the phenomenon, perhaps similar to the way concrete darkens when wet, with no change in the shape of the surface, and returns to its original color when dry.
But that was just an educated guess.
Lujendra Ojha, a graduate student at the Georgia Institute of Technology, turned to another instrument on the orbiter that identifies types of molecules by which colors of light they absorb. But this instrument, a spectrometer, is not as sharp as the camera, making it hard to zoom in on readings from the narrow RSLs.
"We had to come up with new techniques and novel ways to do analysis of the chemical signature," said Ojha, the lead author of the Nature Geoscience article.
The researchers were able to identify the telltale sign of a hydrated salt at four locations. In addition, the signs of the salt disappeared when the streaks faded. "It's very definitive there is some sort of liquid water," Ojha said.
The perchlorate salts lower the freezing temperature, and the water remains liquid. The average temperature of Mars is about minus 70 degrees Fahrenheit, but summer days near the Equator can reach an almost balmy 70.
Many mysteries remain. For one, scientists do not know where the water is coming from.
"There are two basic origins for the water: from above or from below," McEwen said. The perchlorates could be acting like a sponge, absorbing moisture out of the air, but measurements indicate very low humidity on Mars -- only enough for 10 microns, or about 1/2,500th of an inch, of rain across the planet if all of the wetness were wrung out of the air.
That idea cannot be entirely ruled out if the lower part of the atmosphere turns out more humid than currently thought.
"We have very poor measurements of relative humidity near the surface," McEwen said.
The other possibility is underground aquifers, frozen solid during winter, melting during summer and seeping to the surface.
Liquid water is considered one of the essential ingredients for life, and its presence raises the question of whether Mars, which appears so dry and barren, could possess niches of habitability for microbial Martians.
Christopher P. McKay, an astrobiologist at NASA's Ames Research Center in Mountain View, California, does not think the RSLs are a very promising place to look. For the water to be liquid, it must be so salty that nothing could live there, he said. "The short answer for habitability is it means nothing," he said.
He pointed to Don Juan Pond in Antarctica, which remains liquid year round in subzero temperatures because of high concentrations of calcium chloride salt. "You fly over it, and it looks like a beautiful swimming pool," McKay said. "But the water has got nothing."
Earthly life adapts to many hostile environments, but Don Juan Pond is lifeless.
Others are not so certain. David E. Stillman, a scientist at the Southwest Research Institute's space studies department in Boulder, Colorado, said water for the streaks might be different in different regions. In some, they form only during the warmest times, suggesting that those waters might not be too salty for microbes.
"If it was too salty, they would be flowing year round," Stillman said. "We might be in that Goldilocks zone."
Even though RSLs appear to be some of the most intriguing features on Mars, no one is likely to get a close-up look any time soon.
RSLs are treated as special regions that NASA's current robotic explorers are barred from because the rovers were not thoroughly sterilized, and NASA worries that they might be carrying microbial hitchhikers from Earth that could contaminate Mars.
Of the spacecraft NASA has sent to Mars, only the two Viking landers in 1976 were baked to temperatures hot enough to kill Earth microbes. NASA's next Mars rover, scheduled to launch in 2020, will be no cleaner. Sterilizing spacecraft, which requires electronics and systems that can withstand the heat of baking, adds to the cost and complicates the design.
In selecting the landing site for the 2020 rover, the space agency is ruling out places that might be habitable, including those with RSLs.
That prohibition may continue even though two candidate RSLs have been identified on the mountain in Gale Crater that NASA's Curiosity rover is now exploring, not very far from its current planned path.
NASA and the Curiosity team could decide to approach the streaks without driving onto them, or to simply observe from a distance. The rover still has at least several months of driving before it would pass them.
"These are ongoing conversations," said Catharine A. Conley, NASA's planetary protection officer, who coordinates the efforts to minimize the chances of life inadvertently crossing the solar system.
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