THE WOODLANDS, Tex.—Today's Mars is a frigid desert, a place where water—the key to life as we know it—has gone into hiding. Whatever water may have once existed on Mars in rivers, lakes or even oceans is now frozen into ice caps, locked up in hydrated minerals or buried in debris-coated glaciers.
But last year compelling evidence emerged that when conditions are right, salty brines may persist to this day in liquid form at midlatitude regions on Mars. Alfred McEwen of the University of Arizona and his colleagues found tracks in high-resolution imagery that looked like liquid flowing downhill. The tracks appeared annually during the warmer Martian months on equator-facing slopes, extended downhill and then faded as temperatures dropped once again. One tantalizing interpretation was that the streaks were caused by briny water melting and seeping downhill through the soil.
At the annual Lunar and Planetary Science Conference being held here this week, the researchers provided an update on their search for and analysis of the mysterious features, which they call recurring slope lineae, or RSL. Not only have they more than doubled the count of known RSL but they have been unable to devise a good explanation that does not involve the presence of liquid water.
"No one has come up with alternative models that they believe," McEwen says. "Nor have we." Lujendra Ojha, an Arizona undergraduate student who has done much of the heavy lifting in identifying RSL in imagery from the High Resolution Imaging Science Experiment (HiRISE) camera on the Mars Reconnaissance Orbiter, announced in a talk at the conference that he and his colleagues have now confirmed RSL in 15 locations, up from seven when the features were first announced in a 2011 Science paper. Probable RSL have been identified on 23 additional slopes, but they have not yet been shown to recur year after year. "There's more there that we're going to find," McEwen says.
Other phenomena that have pointed to liquid water on modern-day Mars have plausible explanations involving only dry processes, McEwen notes. Streaky slopes closer to the equator, for instance, do not seem to display the seasonality that would be expected of melting and could simply be tracks from boulders rolling downhill. "In all these cases, you can explain the observations without liquid water," McEwen comments. "You have to favor the nonwater models if you can make it work. For the RSL, we can't make it work."
Support for the liquid-water explanation is coming from studies of arid, frigid regions here on Earth. In another talk at the conference, Joe Levy of Oregon State University compared RSL to similar-looking features caused by saline groundwater seeping downhill through the soil in Taylor Valley in Antarctica. Using satellite observations of Antarctic water tracks, Levy and a colleague found that they could estimate the soil's permeability from orbit by measuring how the tracks propagated downhill. And that estimate agreed reasonably well with the actual soil properties, which Levy and his colleagues have measured in field studies. "For flowing water features that darken the surface, it turns out you can do hydrogeology from orbit, which sure beats hiking," Levy said.
Applying the same calculations to Mars, he concluded that the RSL could be explained by brines if the slopes had the permeability of sand or silt. That matches the kind of soil expected to prevail at the sandy RSL sites. In other words, whatever is moving down the Martian slopes behaves as liquid would in that environment. "The RSL and the [Antarctic] water tracks are both flowing like water through sediment," Levy said. "If it moves like water, it may very well be water."