Mars appears to be flowing with rivulets of salty water, at least in the summer, scientists reported Monday in a finding that boosts the odds of life on the red planet.
"It suggests that it would be possible for there to be life today on Mars," NASA's science mission chief, John Grunsfeld, said at a news conference.
Scientists in 2008 confirmed the existence of frozen water on Mars. Now instruments aboard NASA's Mars Reconnaissance Orbiter have yielded the strongest evidence yet that salt water in liquid form trickles down certain Martian slopes each summer, according to the researchers.
"Mars is not the dry, arid planet that we thought of in the past," said Jim Green, director of planetary science for NASA. "Under certain circumstances, liquid water has been found on Mars."
The rivulets -- if that's what they are, since the evidence for their existence is indirect -- are about 12 to 15 feet wide and 300 feet or more long, scientists said.
"What we're dealing with is wet soil, thin layers of wet soil, not standing water," said Alfred McEwen of the University of Arizona at Tucson, the principal scientist for the Mars Reconnaissance Orbiter's high-resolution imaging experiment.
Because liquid water is essential to life, the findings could have major implications for the possibility of Martian life. The researchers said in the journal Nature Geoscience that further exploration is warranted to determine whether microscopic life exists on the planet.
McEwen said he, for one, believes the possibility of life on Mars to be "very high," though it would be microbial and somewhere in the Martian crust.
The presence of liquid water could also make life easier for astronauts visiting or living on Mars. Water could be used for drinking and for creating oxygen and rocket fuel. NASA's goal is to send humans there in the 2030s.
The evidence of flowing water consists largely of dark, narrow streaks on the surface that tend to appear and grow during the warmest Martian months and fade the rest of the year.
Mars is extremely cold even in summer, and the streaks are in places where the temperature is as low as minus-10 degrees Fahrenheit. But salt can lower the freezing point of water and melt ice.
The source of the water is a mystery. Scientists noted it could be melting ice, an underground aquifer, water vapor from the thin Martian atmosphere, or some combination.
McEwen said that there appears to be a "significant volume" of water, speculating it could fill many Olympic swimming pools, but that it is spread thin.
The streaks were spotted by the orbiter's high-resolution, telescopic camera. Another on-board instrument detected the chemical signature of salt compounds combined with water.
Michael Meyer, lead scientist for NASA's Mars exploration program, said the only definitive way for now to determine whether there's life on Mars is to collect rocks and soil for analysis on Earth -- something a U.S. lander set to lift off in 2020 will do.
Now that scientists know what they're looking for, a better, more methodical search can be carried out, Green said.
"Water is one of the most precious resources necessary for a human mission to the red planet," said Rep. Lamar Smith, R-Texas, chairman of the House science, space and technology committee. "The more evidence we find of it, the more encouraged I am for future Mars missions."
Present-day Mars is nothing like ancient Mars. Three billion years ago and more, our most Earthlike neighbor had a huge ocean, but something radical happened, and exactly what remains a mystery.
The idea of water -- and life -- on Mars has been irresistible to earthlings for generations.
In 1877, Italian astronomer Giovanni Schiaparelli spied what he called "canali" on Mars -- Italian for "channels" -- but the word was mistranslated to "canals" in English, causing imaginations to run wild. In the early 1900s, amateur astronomer Percival Lowell claimed to have spotted irrigation canals and theorized they were built by Martians.
In 2008, NASA's Phoenix spacecraft landed on Mars and confirmed the long-suspected presence of ice in the soil. The Mars Reconnaissance Orbiter has been circling the planet since 2006.
The lead author of the research paper, Lujendra Ojha, is a Ph.D. candidate at Georgia Institute of Technology.
For NASA, at least, the timing couldn't be better. This Friday, the NASA-approved movie "The Martian" has its premiere.