NASA scientists on Monday provided the "strongest evidence yet" that liquid water now exists on Mars, findings that make it more likely that there could be life on the fourth planet from the sun.
"Life requires liquid water, and so this takes us a step closer," said Richard Zurek, chief scientist of the Mars program at NASA's Jet Propulsion Laboratory in Pasadena, Calif., and a member of the team overseeing equipment on the orbiting spacecraft that found the signs of water. "But we don't know what life on Mars could have been able to adapt to."
While others have reported signs of water on Mars in the past — scientists believe that several billion years ago Mars was covered with rivers, lakes, and possibly an ocean — the new research offers the best evidence so far that liquid water exists on the planet now.
J. Taylor Perron, an associate professor of geology in the Massachusetts Institute of Technology's earth, atmospheric, and planetary sciences department, called the research "tantalizing."
"It gives us a glimpse of what might be happening on the shallow surface of Mars," said Perron, who wasn't involved in the research.
In a paper published in the journal Nature Geoscience, the scientists said equipment aboard the orbiter was able to detect salty compounds called perchlorates that can keep liquids from freezing at temperatures as low as 94 degrees below zero. On earth, perchlorates are found in deserts.
Mars has long been considered inhospitable to life, with its thin atmosphere, frigid temperatures, and vast amounts of red-hued dust.
The findings are based on nearly a decade of data collected by the orbiter. In 2010, the same team of scientists said they believed liquid water played a role in the dark streaks descending along the slopes of craters, canyons, and mountains that appeared in photographs of the planet.
The streaks — called recurrent slope linae — appeared during the spring, lengthened during the summer, and then faded when temperatures decreased.
It's unclear, however, where the water is coming from. It could be that it's the result of a fine rain or snow from the variable humidity in the planet's carbon-dioxide filled atmosphere. It also could be seeping from something similar to aquifers, shallow subsurface reservoirs of water.
The streaks are up to 15 feet wide and some are longer than 300 feet, scientists said.
The compounds detected probably reflect that the water would be too salty for life, though some kinds of bacteria have been known to survive in such environments.
"As far as life goes, we still have to resort to speculation," Perron said. "But it does check off one box that we think life requires."
Even if the water on Mars is not suitable to support native life there, it does make it more likely that visiting humans could survive for a prolonged time on the planet. NASA has said it hopes to send a manned mission to Mars in the 2030s.
Visitors to the red planet would probably require some kind of indigenous water source, as it would be too expensive to bring enough water to support a long-term presence.
"This is a home run for a resource," said Jack Mustard, a professor of earth, environmental, and planetary sciences at Brown University and a member of the team analyzing data from the orbiter.
The water could be distilled for drinking and oxygen. The perchlorates could also be used to help make rocket fuel.
"These findings give us more confidence in our models that there's water — and that is very accessible," Mustard said.
The next step, he and others said, is to collect rocks and have them analyzed on earth. A US spacecraft is scheduled to depart in 2020 to do just that.
Overall, he said, the findings represent "the first definite evidence of liquid water flowing on the surface . . . though you certainly couldn't canoe down it." Mustard wasn't involved in the analysis that went into the paper's findings, but he said they "hold together nicely."
"We've been fooled many times before, but this evidence has been observed for several years now," Mustard said. Still, he said, "I have low expectations of extant life," he said. "I don't think this changes that equation."
Other scientists noted the relatively low resolution of the images being examined and that it's possible the data could be compromised by the distance of the orbiter from the planet.
The Mars Reconnaissance Orbiter relies on six instruments and has been surveying the planet since 2006. Its telescopic camera found the streaks, while an on-board instrument called a spectrometer detected the chemical signature of the perchlorates.
Dino Bellugi, a postdoctoral associate who specializes in changing landscapes at MIT, concurred that the findings are believable, noting the perchlorates had been detected before on Mars.
NASA's Phoenix lander and Curiosity rover found them in the planet's soil, and some scientists believe that the Viking missions in the 1970s measured signatures of these salts in different areas of the planet.
But Bellugi said there can't be complete certainty. "The orbiter could have detected other compounds in a similar spectrum," he said.
If there really is water oozing from the surface of the planet, he said, it would mean "there could definitely be microbial life."
"It's definitely makes it more likely," Bellugi said.