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Thursday, August 4, 2011 / Duncan Forgan

Best Evidence Yet for Flowing Water on Mars


NASA’s Mars Reconnaissance Orbiter has discovered what amounts to the best evidence yet for liquid water on Mars.  Let’s be clear though, it’s not exactly a flowing spring, and you’re unlikely to be drinking this stuff fresh out of the ground, but the odds are now much better for extremophilic bacteria surviving on the Red Planet.

The results were reported in Science today (I was able to access through University subscriptions, but I am afraid that it’s behind a paywall).   H2O is somewhat old news on Mars, at least in ice form – we’ve been aware of subsurface ice there for some time now.  What the MRO has discovered is an evolving landscape:

Image Credit: NASA/JPL-Caltech/ASU/UA/LANL/MSSS

This graphic shows findings from the MRO (squares) mapped on top of previous findings of the Mars Odyssey probe (colour map), which traced the subsurface ice, concentrated at the poles (at the top and bottom of the image).  The white squares indicate recent impact craters which revealed water ice.  The red squares mark chlorite deposits – these are typically left behind by evaporated saltwater.  The blue squares are where the most exciting action is:

Image Credit: NASA/JPL-Caltech/Univ. of Arizona

This animation (click to play) shows how the landscape in one of these squares is changing with the seasons (between spring and midsummer of the following Martian year).  The narrow line markings are up to 5 metres wide (remember this image was taken from orbit – that’s how good the High Resolution Imaging Science Experiment (HiRISE) camera is). Water flowing down the slopes is the best hypothesis that fits the data (in comparison to alternatives such as CO2 – dry ice – sublimating).  The markings appear in channels, on relatively steep slopes that face the equator.   They grow in the warm seasons, and disappear in the cold seasons.

“Warm” is of course, a relative term.  The maximum temperature in these images is about -10 degrees C in summer, so it’s unlikely that this is freshwater we’re seeing.  It’s more likely that salty water (brine) is what’s causing these features.  Adding salt can reduce the freezing temperature by up to 70C, and it reduces the amount of evaporation, allowing these features to repeat every Martian year.  Not to mention, the blue squares tend to fit in with the red squares.

But how do these features appear? There are quite a few possible answers, but no-one is sure which is right yet.  To make seeping features like these, you need a lot of saltwater to come out through the porous ground, and these features are seen to appear in some areas topographically unlikely to have enough.  Even the reason why darkening occurs is unclear – it’s not simply that the ground is “wet” (this would be seen in spectra taken by other instruments, and hasn’t been).  It might be that the brine “sorts” the dust on the grain, sweeping away finer grains and leaving the coarser ones.  But then, why does the darkening fade?

In some ways, MRO has answered less questions than it’s thrown up with these results, but one thing is clear.  The case for arestrial life has received a massive boost.  Earth’s own halophilic bacteria (bacteria which thrive in very salty environments) might be able to survive in these features (at least for part of the year).  In fact, Professor Shiladitya DasSarma of the University of Maryland (not involved in the study), went on the record to say

Their results are consistent with the presence of large and extensive underground salty lakes on Mars.

All the more reason to keep visiting: The Curiosity Rover is slated to land in August 2012 inside Gale Crater.  Admittedly, this isn’t too close to these brine features (it’s roughly three grids to the right, and four from the bottom on the first graphic), but that won’t stop them looking for chlorites, brines and exposed water ice in the crater.  And the odds of Curiosity finding something more than just dust, rocks and saltwater are better than ever…

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