SCIENCE -- August 4, 2011 at 4:38 PM ET
Mysterious Markings May Indicate Water on Mars
During its warmer seasons, dark fingerlike streaks that look like rivers, streams and small channels appear along the hills and slopes of Mars. The markings are seasonal: They swell during the planet's warm season and fade as it gets cold, indicating that liquid water might be lurking at or just under the surface of the Red planet.
Scientists have found no direct evidence of water and no explanation of its origins, just behavior at the planet's southern hemisphere that's surprisingly waterlike.
"We've identified a new class of changing features on Mars," says James Wray, an author of the study and assistant professor of earth and planetary science at Georgia Institute of Technology. "It's a new way in which Mars is changing, probably day by day, at least week by week or month by month. ... And all the independent pieces of evidence add up to the interpretation that there's flowing water today on the surface of Mars, and the strongest case of it to date."
It is believed from earlier research that ancient Mars had significant liquid water. Evidence of this is seen in ancient river channels, water vapor in the Martian atmosphere, water-forming minerals and abundant water ice in polar areas. But an outstanding question remains: Does liquid water exist on modern-day Mars? Finding water is important, because it indicates the potential for biological processes -- for life.
"The story has been growing," said Philip Christensen, a geophysicist from Arizona State University and longtime Mars researcher. "We've found water in an increasing number of places, in increasing abundances. ... But I think this is the best evidence we have to date of liquid water occurring today on Mars."
The images were taken by the High Resolution Imaging Science Experiment, known as HiRISE on the Mars Reconnaissance Orbiter spacecraft, which was launched in 2005 and now beams back a handful of images from the planet daily.
The markings look like narrow fingers that extend down Martian slopes, spreading their way around obstacles. "If there are boulders, it will go around them as a fluid flowing down the slope would," Wray said.
They have been found only in the mid-latitudes of the Southern hemisphere, where it's hottest. They grow from late Martian Spring to early Fall and either fade or disappear during other seasons.
"They form and grow, they darken and some of them start fading while new lineaments are fading and growing," said Alfred McEwen of the University of Arizona's Lunar and Planetary Laboratory, the study's lead author.
It's not an iron-clad case. The team has been unable to detect evidence in the infrared spectrum with the MRO's Compact Reconnaissance Imaging Spectrometer for water or salt left behind after the water melted. And still a mystery is where the water might have originated.
But that doesn't factor water out, Wray said. Determining the composition of the dark features from orbit has been challenging since they are smaller than the objects the spacecraft's spectrometer is designed to detect.
Scientists say water could be coming from subsurface seeps or atmospheric water vapor, absorbed by salt. And if it is water, it's most likely salty water or brine, since most of the features were found in places where temperatures rarely climb above freezing. Salt would bring the water's freezing level down, allowing it to stay liquid at lower levels.
One other explanation is that temperature changes are triggering dry avalanches of soil, rock or other debris down the slopes.
"Dry granite or materials can flow like fluid," McEwen said. "But there's the fact that they only occur in a certain latitude and under certain temperatures and that they darken and fade." Plus, he added, there isn't another volatile that's geochemically plausible at these temperatures.
"I think it's going to be laboratory experiments on Earth that give us the best confirmation," he said. "If we can show a mechanism matching these computations, that would be very powerful."
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