Recurring martian streaks: flowing sand, not water?

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This interior slope of a Martian crater has a number of of the seasonal darkish streaks referred to as “recurrent slope lineae,” or RSL, November 2017 report interprets as granular flows, relatively than darkening as a consequence of flowing water. The picture is from the HiRISE digital camera on NASA’s Mars Reconnaissance Orbiter. Credit: NASA/JPL-Caltech/UA/USGS

Dark options on Mars beforehand thought-about proof for subsurface flowing of water are interpreted by new badysis as granular flows, the place grains of sand and dirt slip downhill to make darkish streaks, relatively than the bottom being darkened by seeping water.


Continuing examination of those still-perplexing seasonal darkish streaks with a robust digital camera on NASA’s Mars Reconnaissance Orbiter (MRO) reveals they exist solely on slopes steep sufficient for dry grains to descend the way in which they do on faces of energetic dunes.

The findings revealed immediately in Nature Geoscience argue in opposition to the presence of sufficient liquid water for microbial life to thrive at these websites. However, precisely how these quite a few flows start and steadily develop has not but been defined. Authors of the report suggest prospects that embrace involvement of small quantities of water, indicated by detection of hydrated salts noticed at a few of the move websites.

These options have evoked fascination and controversy since their 2011 discovery, as doable markers for sudden liquid water or brine on an in any other case dry planet. They are darkish streaks that stretch steadily downhill in heat seasons, then fade away in winter and reappear the subsequent yr. On Earth, solely seeping water is understood to have these behaviors, however how they kind within the dry Martian setting stays unclear.

Many hundreds of those Martian options, collectively referred to as “recurring slope lineae” or RSL, have been recognized in additional than 50 rocky-slope areas, from the equator to about midway 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,” mentioned 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.”

Dundas is lead creator of the report, which is predicated on observations with the High Resolution Imaging Science Experiment (HiRISE) digital camera on MRO. The knowledge embrace Three-D fashions of slope steepness utilizing pairs of pictures for stereo data. Dundas and co-authors examined 151 RSL options at 10 websites.

The RSL are virtually all restricted to slopes steeper than 27 levels. Each move ends on a slope that matches the dynamic “angle of repose” seen within the slumping dry sand of dunes on Mars and Earth. A move as a consequence of liquid water ought to readily lengthen to much less steep slopes.

“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,” mentioned HiRISE Principal Investigator Alfred McEwen on the University of Arizona, Tucson, a co-author of the brand new report.

The seasonal darkish streaks have been regarded as doable proof for biologically important liquid water—ample water for microbial life—although explaining how a lot liquid water might exist on the floor in Mars’ fashionable setting could be difficult. A granular-flow clarification for RSL suits with the sooner understanding that the floor of recent Mars, uncovered to a chilly, skinny environment, lacks flowing water. A 2016 report additionally solid doubt on doable sources of underground water at RSL websites. Liquid water on immediately’s Mars could also be restricted to traces of dissolved moisture from the environment and skinny movies, that are difficult environments for all times as we all know it.

However, RSL stay puzzling. Traits with unsure explanations embrace their gradual development, their seasonal reappearance, their speedy fading when inactive, and the presence of hydrated salts, which have water molecules certain into their crystal stucture.

The new report describes doable connections between these traits and the way RSL kind. For instance, salts can turn out to be hydrated by pulling water vapor from the environment, and this course of can kind drops of salty water. Seasonal adjustments in hydration of salt-containing grains may lead to some set off mechanism for RSL grainflows, resembling growth, contraction, or launch of some water. Darkening and fading may outcome from adjustments in hydration. If atmospheric water vapor is a set off, then a query is why the RSL seem on some slopes however not others.

“RSL probably form by some mechanism that is unique to the environment of Mars,” McEwen mentioned, “so they represent an opportunity to learn about how Mars behaves, which is important for future surface exploration.”

“Full understanding of RSL is likely to depend upon on-site investigation of these features,” mentioned MRO Project Scientist Rich Zurek of NASA’s Jet Propulsion Laboratory, Pasadena, California. “While the new report suggests that RSL are not wet enough to favor microbial life, it is likely that on-site investigation of these sites will still require special procedures to guard against introducing microbes from Earth, at least until they are definitively characterized. In particular, a full explanation of how these enigmatic features darken and fade still eludes us. Remote sensing at different times of day could provide important clues.”


Explore additional:
What might clarify the thriller of how land fashioned on Mars with out a lot water

More data:
Granular flows at recurring slope lineae on Mars point out a restricted position for liquid water, Nature Geoscience (2017). nature.com/articles/doi:10.1038/s41561-017-0012-5

Journal reference:
Nature Geoscience

Provided by:
Jet Propulsion Laboratory

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