They were hailed as evidence of water on the red planet, but strange ‘streaks’ on the red planet’s surface could actually be sand, a NASA study has found.
The find was originally hailed as a ‘major scientific discovery’ in 2011 as results seemed to confirm that ‘dark fingers’ spotted in Mars Reconnaissance Orbiter (MRO) images were likely made by liquid moving across, or beneath, the planet’s surface.
However, now NASA believes they could be ‘granular flows’, where grains of sand and dust slip downhill to make dark streaks, rather than the ground being darkened by seeping water.
This inner slope of a Martian crater has several of the seasonal dark streaks called ‘recurrent slope lineae,’ or RSL, that a November 2017 report interprets as granular flows, rather than darkening due to flowing water. The image is from the HiRISE camera on NASA’s Mars Reconnaissance Orbiter.
Researchers carried out out a new analysis of the streaks with a powerful camera on NASA’s Mars Reconnaissance Orbiter (MRO) shows they exist only on slopes steep enough for dry grains to descend the way they do on faces of active dunes.
The findings published today in Nature Geoscience argue against the presence of enough liquid water for microbial life to thrive at these sites.
Many thousands of these Martian features, collectively called ‘recurring slope lineae’ or RSL, have been identified in more than 50 rocky-slope areas, from the equator to about halfway to the poles.
‘We’ve thought of RSL as possible liquid water flows, but the slopes are more like what we expect for dry sand,’ said Colin Dundas of the U.S. Geological Survey’s Astrogeology Science Center in Flagstaff, Arizona.
‘This new understanding of RSL supports other evidence that shows that Mars today is very dry.’
However, exactly how these numerous flows begin and gradually grow has still not yet been explained.
Authors of the report say it could include small amounts of water, indicated by detection of hydrated salts observed at some of the flow sites.
‘These features have evoked fascination and controversy since their 2011 discovery, as possible markers for unexpected liquid water or brine on an otherwise dry planet,’ said NASA.
They are dark streaks that extend gradually downhill in warm seasons, then fade away in winter and reappear the next year.
On Earth, only seeping water is known to have these behaviors, but how they form in the dry Martian environment remains unclear.
Spot the difference: The ‘recurring slope lineae’ (RSL) appear seasonally on Mars, resembling streaks of damp sand.These marks, which lengthen and darken as the seasons change, are the first evidence of their kind ever found on another planet
Dundas is lead author of the report, which is based on observations with the High Resolution Imaging Science Experiment (HiRISE) camera on MRO.
The data include 3-D models of slope steepness using pairs of images for stereo information.
Dundas and co-authors examined 151 RSL features at 10 sites.
‘The RSL don’t flow onto shallower slopes, and the lengths of these are so closely correlated with the dynamic angle of repose, it can’t be a coincidence,’ said HiRISE Principal Investigator Alfred McEwen at the University of Arizona, Tucson, a co-author of the new report.
The seasonal dark streaks have been thought of as possible evidence for biologically significant liquid water – sufficient water for microbial life – though explaining how so much liquid water could exist on the surface in Mars’ modern environment would be challenging.
A granular-flow explanation for RSL fits with the earlier understanding that the surface of modern Mars, exposed to a cold, thin atmosphere, lacks flowing water.
A 2016 report also cast doubt on possible sources of underground water at RSL sites.
Liquid water on today’s Mars may be limited to traces of dissolved moisture from the atmosphere and thin films, which are challenging environments for life as we know it.
However, RSL remain puzzling, the researchers admit.
Seasonal changes in hydration of salt-containing grains might result in some trigger mechanism for RSL grainflows, such as expansion, contraction, or release of some water.
Darkening and fading might result from changes in hydration. If atmospheric water vapor is a trigger, then a question is why the RSL appear on some slopes but not others.
‘RSL probably form by some mechanism that is unique to the environment of Mars,’ McEwen said, ‘so they represent an opportunity to learn about how Mars behaves, which is important for future surface exploration.’