Over the last several decades, evidence has piled up that Mars once played host to liquid water on its surface. But in its current geological era, the red planet is too cold and has too little atmosphere to allow liquid to survive for long. Even at the peak of Martian summer, water would evaporate off quickly during the day, or freeze solid as soon as night hit. But that doesn't mean it couldn't exist beneath the surface, where pressures and temperatures might be quite different, so researchers have been looking for signs that some subterranean liquid might bubble to the surface. Now, scientists are reporting some changes on the Martian surface that seem to be best explained by a watery seep.

The information comes courtesy of the finest resolution camera we've ever put in orbit there, the High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter. The MRO has been circulating Mars for long enough that it's been able to image certain areas multiple times over a Martian year or more, which has enabled the authors of a new paper to identify seasonal changes on the planet's surface.

One of the changes that the authors spotted was a "recurring slope lineae," or RSL (their name, not ours). These were regions of steep downward slopes (25 degrees or more) that darkened during the Martian summer and reverted to a lightened color once summer ended. These were mostly concentrated in the southern hemisphere, but tended to be on northward-facing slopes, suggesting they required elevated temperatures to form. They were also associated with channels that are visible year-round, and they appeared to flow around obstacles, rather than simply barreling down-slope. Typically, they appeared at the base of a bedrock outcrop; in a few locations, hundreds of these RSLs were present.

Overall, the authors have confirmed 7 sites with RSLs, and have a strong indication of another dozen—the latter just haven't been imaged long enough to confirm their annual reappearance. Another 20 sites appear to be worth following up on. Each site hosts anywhere from a dozen to thousands of these features on different areas of the slope, each less than five meters across.

All of that, to the authors' mind, implies that the features are caused by seeping water. There are a number of other potential sources, such as debris flows or seasonal coverings of dust, but they don't fit the observations nearly so well. Debris flows can easily end up covering obstacles in their path rather than flowing around them, and there's no reason they should be limited to equator-facing slopes. Dust in the Martian atmosphere should cover all features with the same general time course, but dust at other sites doesn't seem to track with the RSLs over the course of a Martian year. And there's no reason for any simple geological processes like these to be limited to so few sites on the surface of the entire planet.

Liquid water, however, would be expected to be very limited in scope. And the association of the RSLs with exposed bedrock would seem to suggest a source inside the Martian crust.

There are two potential problems with this explanation identified by the authors. The first is that many of the slopes are expected to have temperatures well below the freezing point of water. This, they contend, isn't really an issue; a salty brine can depress the freezing point of water by as much as 70K, meaning that all of the sites should be expected to have liquid brines during the Martian summer, when the RSLs appear.

The second issue is much more serious: the MRO also has a spectrometer on board, which should be able to pick up a clear indication of water in the area. It doesn't. The authors suggest that the brine might be flowing below the surface, which would keep it from wetting material that can be imaged by the spectrometer, but that seems like a weak explanation—if it's not at the surface, why is the surface so obviously changing color?

This isn't the first case where a feature on Mars has been interpreted as indicating the presence of liquid water, but many of the past findings haven't shown the sort of seasonal activity and bias toward the warmer slopes of a feature that the authors have found here. The lack of a clear signal from water is definitely troubling, but the paper can be viewed as inviting the wider community to think a bit about the results, and either come up with a better explanation for these features, or suggest why the spectrometer wouldn't be seeing any water.

Science, 2011. DOI: 10.1126/science.1204816 (About DOIs).