Unusual dark lines in images from the Mars Reconnaissance Orbiter could… (NASA )
Salty water may flow on Mars in the form of strange, dark lines on the terrain that grow and fade with the seasons, according to recent images. The findings, reported in the journal Science, provide a new line of evidence that life could exist on the Red Planet.
The findings, released Thursday, describe images taken by NASA's Mars Reconnaissance Orbiter, currently circling the planet. The otherwise unremarkable lines on the planet's slopes grow more prominent during the warm season, proliferating from late Martian spring into early fall. This suggests they were made by volatile chemicals that can boil at relatively low temperatures, such as water and carbon dioxide, the authors wrote.
They look rather like flow lines that would be left by running water, ending in light-colored patches that could be material deposited by the flow, the authors added.
If these are indeed signs of liquid water on the surface, the possibility that life exists on Mars "looks more likely," said lead author Alfred McEwen, a planetary geologist at the University of Arizona in Tucson who is in charge of a high-resolution camera on the Mars Reconnaissance Orbiter.
The dark streaks were observed in the mid-latitudes of the southern hemisphere — far enough south for ground ice to be present. They are present on equator-facing slopes that could get warm enough to melt ice.
The apparent flow marks varied in size, from 0.5 meters to 5 meters wide. While they're very rare — the researchers have identified just seven, with 20 other possible candidates — certain areas have thousands of such flow marks that could potentially have held thousands of gallons of water.
University of Arizona undergraduate student Lujendra Ojha first noticed the seasonal changes after comparing two images of the same location taken at different points in time.
"I was very surprised," Ojha said. "It wasn't something I was looking for, and when I first saw it I didn't know what to make of it."
Team members were hesitant to leap to the conclusion they were seeing seasonal water. Although water exists in the form of ice in Mars' polar caps and beneath the planet's surface, thus far no instruments have directly detected it in liquid form, McEwen said. But there could be a reason for that, he added. If water were present in these flow lines, the liquid would probably evaporate into Mars' thin atmosphere within hours. And such relatively small amounts of water would be difficult to pick up with current instruments.
At the end of the day, no other possibility seemed as likely as water, McEwen said. Temperatures there were too high for carbon dioxide to exist in the form of frost. And they were too low for freshwater ice to melt and rise to the surface.
But for salt water, which stays liquid at lower temperatures than pure water does, the conditions could be just right.
The dark marks were not caused by wetness, McEwen emphasized. Rather, the dark marks would be caused as water swept lighter mineral grains down a slope's surface, leaving behind larger, rougher grains that would cast greater shadows on the planet's surface.
The scientists did not rule out the possibility that other forces could have caused the lines.
Perhaps, for example, the streaks were the result of dust devils kicking material downward. But there would be no reason for dust devils to appear solely on steep slopes facing the equator.
Or maybe they were caused by material flaking off the surface as the ground expanded and contracted with the rising and falling temperatures. But this would be a slow process that should not present such dramatic seasonal changes.
Other recent pieces of evidence have also supported the idea that liquid salt water can exist on Mars. Salt deposits have been detected in abundance on the planet. In addition, signs of possible saltwater droplets were detected on the struts of the Phoenix Mars Lander, a robotic spacecraft sent to Mars in 2008 to look for environments that could host microbial life.
Finally, certain gullies on the planet's surface look relatively new — possibly carved by water.
"I've been something of a skeptic of the possibility of liquid water right at the surface under present conditions," said Michael Carr, a retired U.S. Geological Survey geologist in Menlo Park, Calif., who was not involved with the study, "and I've been irritated in the past by people immediately jumping to yell, 'Water on Mars!' "
But in this study, he added, "I think they make a very good case. … The story hangs together."
It also whetted researchers' appetites for closer study.
"The real thing we need to do is go to places where we think there was liquid water … and bring them back to Earth [to look for] any presence of life," said Bruce Jakosky, a planetary scientist at the University of Colorado in Boulder.