Recurring martian streaks: flowing sand, not water?

Recurring martian streaks: flowing sand, not 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. Credit: NASA/JPL-Caltech/UA/USGS

Dark features on Mars previously considered evidence for subsurface flowing of water are interpreted by new research as granular flows, where grains of sand and dust slip downhill to make dark streaks, rather than the ground being darkened by seeping water.

Continuing examination of these still-perplexing seasonal dark 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 for microbial life to thrive at these sites. However, exactly how these numerous flows begin and gradually grow has not yet been explained. Authors of the report propose possibilities that include involvement of 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. 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.

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."

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 are almost all restricted to slopes steeper than 27 degrees. Each flow ends on a slope that matches the dynamic "angle of repose" seen in the slumping dry sand of dunes on Mars and Earth. A flow due to liquid water should readily extend to 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," 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. Traits with uncertain explanations include their gradual growth, their seasonal reappearance, their rapid fading when inactive, and the presence of , which have water molecules bound into their crystal stucture.

The new report describes possible connections between these traits and how RSL form. For example, salts can become hydrated by pulling water vapor from the atmosphere, and this process can form drops of salty water. 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 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."

"Full understanding of RSL is likely to depend upon on-site investigation of these features," said 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 further

What could explain the mystery of how land formed on Mars without much water

More information: Granular flows at recurring slope lineae on Mars indicate a limited role for liquid water, Nature Geoscience (2017).
Journal information: Nature Geoscience

Citation: Recurring martian streaks: flowing sand, not water? (2017, November 20) retrieved 25 August 2019 from
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Nov 20, 2017
This illustrates the continuing problem of quasi-religious pathoskeptics in scientific groups' no acceptance of reality while continuing to cling to Earthcentric mantras of 'uniqueness'. To continually deny the existence of water on Mars in so many ways is to actually commit crimes against humanity. This is because, like Steven Hawking said, we NEED new homes off planet in event of an planet wide catastrophe. Possibilities abound for 'wiping us out' or at least doing that to many of us, and we all soooo know that the holy rollers will have a ready answer for that one too......'tis the will of (their) gawd!! Well God had his people bioengineer long ago, as in turn those folks had been bioengineered millennia before, and so forth. NO! Our God wants us to live, prosper, and reach out from our birth world...since we fast using it up anyway like the young chick uses up its yolk and moves on to hatch. We NEED those minerals, water, and shelter in our birth system. We WILL survive!

Nov 20, 2017
^^^^^ There has been plenty of scientific evidence presented for current/ past water on Mars. The authors are merely saying that this particular case may not be a correct interpretation.

Nov 20, 2017
Interesting "new savvy", post-published articles:
https://www.acade...nd_Earth and
More interesting is where the article was published.
"They are dark streaks that extend gradually downhill in warm seasons, then fade away in winter and reappear the next year."
On Earth is H2O, on Mars is CO2. In the winter, freezes and covers the stripes, in the summer it sublimates.

Nov 20, 2017
Well, I have no trouble with all the comicbook fantasists who want to scurry off and escape the oncoming Matricide, fleeing to the Moon or Mars. Deluding themselves they can find another planet to infest like a metastasizing cancer.

Lacking the courage to stand and fight the profitmongering polluters. Too lazy and self-centered to step up to the hard work that is needed to salvage the only known Biosphere and rebuild a future for Humanity.

It will amuse me to hear their radio broadcasts pleading for everybody else to expend a fortune rescuing them from their own hubris.

But gosh, you frontier-toddlers claim to be self-sufficient, manly men of impeccable survival skills. Humm, why can't you fund, invent and build your own spaceships? Stop leaching off the Public teat? Follow your cult prophetess Ayn Rand into drunken suicide.

Nov 22, 2017
Water Cycle on Dry as a Bone Mars.
Water bound in Perchlorate crystals is the answer to all the questions.
1. What rain falls on Mars?
2. What lakes fill up with on Mars?
3. What do you fill your two buckets with on Mars?
4. What gives 1 bucket of water when boiled at 24C on Mars?
5. What yields water dew twice a day on Mars?
6. Why build a habitat next to a lake on Mars?
7. What makes up 1% of the regolith on Mars?
8. Where did the oceans go for conservation when Mars dried up?
9. Where does the ice ablation of the poles go in summer on Mars?
10. What flows across the surface of Mars faster than sand?
Tony Lance Dip.Math(Open) 22nd November 2017

Nov 22, 2017
TL, your first three questions are easy to answer. "Dust"

Q.4 results in the question "What is the source of energy and the material/human resources needed for collecting the crystals in the quantities required to output a sufficient quantity of water for a colony?"

Q.5 TL, What physical evidence do you have that there is specifically potable water dew fall occurring with any regularity on Mars?

Q.6 Location. location, location?

Q.7 rev.Q. What makes up the estimated 1% regolith of the few patches of barely scraped Mars surface that have been physically sampled?

This article is an example of how often Mars researchers change their minds.

Q.8 What oceans? And whatever liquidity was there? Evaporated to gases that in 1/3 e-gravity and a weak-to-none magneticfield, the thin atmosphere got blown away into space.

Q.9 see Q.8

Q. 10 Not a simple answer but a correct one would be "gossip, innuendo, rumor, lies, damn lies and drunken lies!"

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