A US spacecraft orbiting Mars has provided tantalizing evidence of liquid water in the form of a series of images that show dark lines appearing to flow down steep ravines.
"The best explanation for these observations so far is the flow of briny water," said the University of Arizona's Alfred McEwen in a statement released by NASA. McEwen is the principal investigator for the orbiter's High Resolution Imaging Science Experiment (HiRISE), which captured the photos.
What causes these lines to appear each spring is unknown.
HiRISE is an experiment being conducted by the Mars Reconnaissance Orbiter (MRO), which entered into Martian orbit in March 2006 after having been launched on August 12, 2005, and began sending back its first pics just a few days later.
Originally, the MRO's mission was scheduled to end as last year came to a close, but – like those indefatigable Mars rovers Spirit and Opportunity – the orbiting eye in the sky has proven heartier than planned.
The HiRISE images clearly show dark, finger-like lines between 0.5 and 5 meters wide that appear during the red planet's warmest months, flowing down steep Martian slopes. The lines disappear in winter, then reappear the following spring.
And the lines aren't rare. In some locations, the HiRISE cameras caught over 1,000 individual flows.
The lines couldn't be made by the flow of pure water, which would freeze – Martian summer or not. Salt, however, lowers water's freezing temperature enough to allow it to flow even at the temperatures involved.
NASA notes that there are salt deposits over much of Mars that indicate that surface brines were common earlier in Martian history. "These recent observations," NASA reports, "suggest brines still may form near the surface today in limited times and places."
The observation that the brines may form near and not on the surface of Mars was supported by data from the MRO's Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), which checked for water in the lines, and found none.
"The flows are not dark because of being wet," McEwen said. "They are dark for some other reason."
Just what that reason may be is not yet known. The moisture causing the lines might evaporate immediately, the flows may be under the surface, or grains of sand may be disturbed in some way by transient flows and darken to the camera's eye – although, as NASA admits, why the lines would brighten again when temperatures drop is tough to understand.
"It's a mystery now," McEwen said, "but I think it's a solvable mystery with further observations and laboratory experiments."
Frozen water has been detected on Mars, and observations by the now deceased Phoenix Mars Lander raised the tantalizing possibility that liquid brine blobs splashed onto its legs upon landing. However, these HiRISE images give the best evidence yet that there's something liquid up there, and that it is, indeed, flowing.
Or, for that matter, we could be looking at armies of wee beasties on the march, lighting out in the summer months to feed on microscopic yummies living in subsurface brine, then burrowing down deeper into the Martian soil and tunneling their way back to their wintering grounds.
Or not. Download this 242MB ZIP file of animated GIFs from the HiRISE experiment, and see what your imagination comes up with. ®