Mars water? Scientists say the pictures don't prove it, but they fit with other evidence from other probes.
The spacecraft, Mars Reconnaissance Orbiter, has been circling Mars since 2006, and during that time, Mars -- which has seasons like ours -- has gone around the sun three times. Each year, MRO photographed brown streaks in the Martian spring and summer. In the colder seasons, they disappeared.
"The best explanation for these observations so far is the flow of briny water," said Alfred McEwen of the University of Arizona, the principal investigator using the ship's high-resolution camera system.
Mars, located at least 35 million miles farther from the sun than us, is far colder than Earth. But if water is thick with salt and other minerals, its freezing point would be lower than it is for clear water here.
"We expect water on Mars to be briny, to be salty, because we know that the surface is salty from all of the past landers and rovers," said McEwen. "Furthermore, the salt serves to depress the freezing point of the water, so in places where it's below freezing, we see this activity, it is still plausible for that to be salty water."
McEwen and his team published their findings in Friday's issue of the journal Science. They reported that the streaks appeared only on steep slopes. They could be hundreds of yards long, and often resembled gullies on Earth.
Other readings showed no chemical signal on the Martian surface, leading the scientists to suggest it may dry very quickly, or be just below the upper layer of Martian dust.
"These dark lineations are different from other types of features on Martian slopes," said Richard Zurek, the project scientist for MRO, in a statement. "Repeated observations show they extend ever farther downhill with time during the warm season."
'Follow the Water'
This is not the first suggestion of Martian water. For the last 15 years, NASA has been looking for it, because water would be a very good sign for past -- or even present -- microbial life there.
In 2004, the Mars rover Opportunity found chemical compounds that scientists said most likely would have formed if there had once been standing water on the Martian surface. Steven Squyres, the rovers' chief scientist, said at the time he could imagine Mars, eons ago, with pools of ruby-red brackish water where today there is only dust.
In 2006 scientists said images from an orbiter, Mars Global Surveyor, showed streaks that seemed to come and go from the walls of craters; the theory at the time was that underground water may be spurting through the surface. Doubters said the "gullies" did not resemble ones formed by liquid in Earth.
In 2008, another probe, called Phoenix, landed near Mars' north pole, and produced some pictures showing strange blobs on the struts of its landing legs. Briny ice? Scientists argued for months.
But NASA has made no secret of its hope to find water on Mars.
"NASA's Mars Exploration Program keeps bringing us closer to determining whether the Red Planet could harbor life in some form," NASA Administrator Charles Bolden said today, "and it reaffirms Mars as an important future destination for human exploration."
(The Associated Press contributed to this story.)