Scientists find 'best evidence' yet for liquid water now on Mars

Liquid water – a necessary ingredient for organic life – may finally have revealed its presence on today's Mars.

Images unveiled today from NASA's Mars Reconnaissance Orbiter have revealed dark, narrow streaks that appear and slowly snake down walls of craters that populate the planet's southern hemisphere. The process occurs from late spring to early fall. The streaks vanish with the return of late fall and winter, only to reappear the following spring.

The streaks originate at rock outcroppings high on steep crater walls, but only on walls facing in the general direction of the equator, which receive more sunlight than poleward-facing slopes.

The features are too narrow to be formed by silt sliding downhill, which has been seen elsewhere on the planet, according to the team of scientists formally reporting the observations in the Aug. 5 issue of the journal Science. Moreover, these streaks are far more rare than the silt slides, suggesting a far less common process at work.

Sudden thaws of carbon-dioxide frost probably triggered those other, silty slides, scientists have theorized. These newly-discovered features, by contrast, appear – and grow – when surface temperatures are too warm for carbon-dioxide frost to exist. Those temperatures range from about 9 degrees below zero Fahrenheit to as high as 80 degrees F during a summer's mid-afternoon. Nine degrees below zero may be too warm for CO2 frost, but not for liquid water heavily laced with salts, says Alfred McEwen, a planetary scientist at the University of Arizona at Tucson and leader of the team reporting the results.

He cautions that the evidence for liquid water as the driver behind these ephemeral stripes is circumstantial. So far, the orbiter's spectrometer has not detected the chemical fingerprints of water in these features, although the team has plans to use the spectrometer's high-resolution option to hunt for them in greater detail than the initial, low-resolution measurements could achieve.

Circumstantial? Yes, others say, but important.

"This is the best evidence we have to date of liquid water occurring today on Mars," says Philip Christiansen, a researcher at Arizona State University at Tempe and a lead scientist on an earlier mission, the Mars Odyssey orbiter.

For 15 years, NASA has geared its Mars exploration program to "follow the water," in order to determine how hospitable the red planet was to at least simple forms of life early in its history, and perhaps today as well.

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