Posted on behalf of Adam Mann
Scientists no longer debate whether water exists on Mars. There is water ice at the planet’s poles and abundant evidence of frost and subsurface ice in other spots. What’s not settled is whether water currently flows openly on the surface at times, creating gullies in craters and dunes. Most of the time, Mars’ surface stays below the freezing point of water and its atmosphere is so thin and dry that any open water should quickly evaporate.
On 10 March, a couple of scientists presented new evidence of liquid water on the Martian surface. The findings were reported at the Lunar and Planetary Science Conference in The Woodlands, Texas.
In one presentation, Nilton Renno, an atmospheric scientist at the University of Michigan, Ann Arbor, showed spectral evidence for briny water existing as “puddles” up to 25 metres across. Mars’ rusty soil is very bright in the near-infrared whereas salt water deposits would absorb this wavelength and reflect in the blue-green portion of the spectrum. Using images taken from Mars Reconnaissance Orbiter (MRO), Renno analyzed spots of dark soil near the planet’s polar region and found that they looked the same as brine.
“There is nothing else on Mars that has this spectral signature,” he says. When observed three weeks later, the dark spots had disappeared, suggesting that the water had evaporated.
The report of puddles, however, met with a great deal of skepticism from other researchers at the meeting. “I don’t believe that color data; there’s no way those things are so extraordinarily blue,” says Alfred McEwen, a planetary scientist at the University of Arizona in Tucson. More likely, he says, there is a haze of carbon dioxide ice in the atmosphere blocking the true color reading of the surface.
During a separate talk, though, McEwen made his own case for water flowing on Mars using high-resolution orbital images. While the Martian “gullies” are hundreds of metres in size, McEwen observed very thin streaks of dark material–only a few metres wide–moving down 21 slopes during the summer. McEwen and his colleagues propose these are flows of briny water, which are able to remain liquid at such low temperatures because of their dissolved salts.
Others caution against such an interpretation. “There are many ways to create a feature that look like flowing water and we have to be sure not to jump to that conclusion,” says Steve Ruff, a planetary scientist from Arizona State University in Tempe, Arizona. Instead, dark sand and dust could be moving down slope like a miniature avalanche, he says. Wind could then blow this dust away, causing it to disappear in later images and mimicking evaporation.