Scientific Method / Science & Exploration

Orbiter spots possible water seepage on surface of Mars

Repeated imaging using a high resolution camera has spotted some slopes on …

Some slopes on Mars darken in the summer (left), but return to a lighter color by the following spring.
Image: Science/AAAS


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.


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.



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.


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