Mars may be the second smallest planet in our solar system, but contrary to
popular belief, it is filled with mystery and a whole lot of personality. It
has been reported that this red planet has surface features reminiscent of
the impact craters of the moon, as well as the volcanoes, valleys, deserts
and polar ice caps of Earth. Most recently, scientists have discovered signs
of flowing water - meaning that there is a high chance that the planet could
have supported life.
However, dark features on Mars previously considered evidence for subsurface
flowing of water are interpreted by new
NASA 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.
While we may be familiar with the more natural red-looking sights and scenes
of Mars, these pictures portray a fascinating, yet baffling side to our
neighboring planet.
Layered Craters - 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.
Mineral Deposits - 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.
Water Ice in Crater? - "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."
Storm Clouds over the Planet - 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.
What Earth looks like from Mars - "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.
Mariner Valley (the Grand Canyon of Mars) - 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.
An Avalanche on Mars - RSL remain puzzling. Traits with uncertain explanations
include their gradual growth, their seasonal reappearance, their rapid fading
when inactive, and the presence of hydrated salts, which have water molecules
bound into their crystal stucture.
Craters of Dry Ice? - 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.
Possible Evidence of Salty Water? - "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."
Mars's South Pole - 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.
Dust Devil in Action - "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."
Amazing Layered Dunes - "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."
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