Test for Damp Ground at Mars' Seasonal Streaks Finds None
Jet Propulsion Laboratory
August 23, 2016
Seasonal dark streaks on Mars that have become one of the hottest topics
in interplanetary research don't hold much water, according to the latest
findings from a NASA spacecraft orbiting Mars.
The new results from NASA's Mars Odyssey mission rely on ground temperature,
measured by infrared imaging using the spacecraft's Thermal Emission Imaging
System (THEMIS). They do not contradict last year's identification of
hydrated salt at these flows, which since their 2011 discovery have been
regarded as possible markers for the presence of liquid water on modern
Mars. However, the temperature measurements now identify an upper limit
on how much water is present at these darkened streaks: about as much
as in the driest desert sands on Earth.
When water is present in the spaces between particles of soil or grains
of sand, it affects how quickly a patch of ground heats up during the
day and cools off at night.
"We used a very sensitive technique to quantify the amount of water associated
with these features," said Christopher Edwards of Northern Arizona University,
Flagstaff. "The results are consistent with no moisture at all and set
an upper limit at three percent water."
The features, called recurring slope lineae or RSL, have been identified
at dozens of sites on Mars. A darkening of the ground extends downhill
in fingerlike flows during spring or summer, fades away in fall and winter,
then repeats the pattern in another year at the same location. The process
that causes the streaks to appear is still a puzzle.
"Some type of water-related activity at the uphill end still might be
a factor in triggering RSL, but the darkness of the ground is not associated
with large amounts of water, either liquid or frozen," Edwards said. "Totally
dry mechanisms for explaining RSL should not be ruled out."
He and Sylvain Piqueux of NASA's Jet Propulsion Laboratory, Pasadena,
California, analyzed several years of THEMIS infrared observations of
a crater-wall region within the large Valles Marineris canyon system on
Mars. Numerous RSL features sit close together in some parts of the study
region. Edwards and Piqueux compared nighttime temperatures of patches
of ground averaging about 44 percent RSL features, in the area, to temperatures
of nearby slopes with no RSL. They found no detectable difference, even
during seasons when RSL were actively growing.
The report of these findings by Edwards and Piqueux has been accepted
by the peer-reviewed Geophysical Research Letters and is available online.
There is some margin of error in assessing ground temperatures with the
multiple THEMIS observations used in this study, enough to leave the
that the RSL sites differed undetectably from non-RSL sites by as much
as 1.8 degrees Fahrenheit (1 Celsius degree). The researchers used that
largest possible difference to calculate the maximum possible amount of
water -- either liquid or frozen -- in the surface material.
How deeply moisture reaches beneath the surface, as well as the amount
of water present right at the surface, affects how quickly the surface
loses heat. The new study calculates that if RSL have only a wafer-thin
layer of water-containing soil, that layer contains no more than about
an ounce of water per two pounds of soil (30 grams water per kilogram
of soil). That is about the same concentration of water as in the surface
material of the Atacama Desert and Antarctic Dry Valleys, the driest places
on Earth. If the water-containing layer at RSL is thicker, the amount
of water per pound or kilogram of soil would need to be even less, to
stay consistent with the temperature measurements.
Research published last year identified hydrated salts in the surface
composition of RSL sites, with an increase during the season when streaks
are active. Hydrated salts hold water molecules affecting the crystalline
structure of the salt.
"Our findings are consistent with the presence of hydrated salts, because
you can have hydrated salt without having enough for the water to start
filling pore spaces between particles," Edwards said. "Salts can become
hydrated by pulling water vapor from the atmosphere, with no need for
an underground source of the water."
"Through additional data and studies, we are learning more about these
puzzling seasonal features -- narrowing the range of possible explanations,"
said Michael Meyer. "It just shows us that we still have much to learn
about Mars and its potential as a habitat for life."
The new study touches on additional factors that add to understanding
-- If RSL were seasonal flows of briny water followed by evaporation,
annual buildup of crust-forming salt should affect temperature properties.
So the lack of a temperature difference between RSL and non-RSL sites
is evidence against evaporating brines.
-- Lack of a temperature difference is also evidence against RSL being
cascades of dry material with different thermal properties than the
slope material, such as would be the case with annual avalanching of powdery
dust that accumulates from dusty air.
Arizona State University, Tempe, provided and operates the THEMIS camera,
which records observations in both infrared and visible-light wavelengths.
JPL, a division of Caltech, manages the Mars Odyssey project for NASA.
Lockheed Martin Space Systems, Denver, built the orbiter and collaborates
with JPL to operate it.
Updated (Aug. 26 at 11:00 a.m.) to correct the conversion to 30 grams
of water per kilogram of soil as the upper limit on amount of water in
News Media Contact
Jet Propulsion Laboratory, Pasadena, Calif.
Dwayne Brown / Laurie Cantillo
NASA Headquarters, Washington
202-358-1726 / 202-358-1077
dwayne.c.br...@nasa.gov / laura.l.canti...@nasa.gov
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