New findings from NASA's Mars Reconnaissance Orbiter (MRO) provide the
strongest evidence yet that liquid water flows intermittently on
present-day Mars.
Using an imaging spectrometer on MRO,
researchers detected signatures of hydrated minerals on slopes where
mysterious streaks are seen on the Red Planet. These darkish
streaks appear to ebb and flow over time. They darken and appear
to flow down steep slopes during warm seasons, and then fade in cooler
seasons. They appear in several locations on Mars when
temperatures are above minus 10 degrees Fahrenheit (minus 23 Celsius),
and disappear at colder times.
"Our quest on Mars has been to
'follow the water' in our search for life in the universe, and now we
have convincing science that validates what we've long suspected", said
John Grunsfeld, astronaut and associate administrator of NASA's Science
Mission Directorate in Washington. "This is a significant
development, as it appears to confirm that water − albeit briny − is
flowing today on the surface of Mars".
These downhill flows, known as
recurring slope lineae (RSL), often have been described as possibly
related to liquid water. The new findings of hydrated salts on the
slopes point to what that relationship may be to these dark features.
The hydrated salts would lower the freezing point of a liquid brine,
just as salt on roads here on Earth causes ice and snow to melt more
rapidly. Scientists say it's likely a shallow subsurface flow, with
enough water wicking to the surface to explain the darkening.
"We found the hydrated salts only
when the seasonal features were widest, which suggests that either the
dark streaks themselves or a process that forms them is the source of
the hydration. In either case, the detection of hydrated salts on
these slopes means that water plays a vital role in the formation of
these streaks", said Lujendra Ojha of the Georgia Institute of
Technology (Georgia Tech) in Atlanta, lead author of a report on these
findings published by Nature Geoscience.
Ojha first noticed these puzzling
features as a University of Arizona undergraduate student in 2010, using
images from the MRO's High Resolution Imaging Science Experiment
(HiRISE). HiRISE observations now have documented RSL at dozens of
sites on Mars. The new study pairs HiRISE observations with
mineral mapping by MRO's Compact Reconnaissance Imaging Spectrometer for
Mars (CRISM).
The spectrometer observations show
signatures of hydrated salts at multiple RSL locations, but only when
the dark features were relatively wide. When the researchers
looked at the same locations and RSL weren't as extensive, they detected
no hydrated salt.
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Ojha and his co-authors interpret the spectral signatures as caused by
hydrated minerals called perchlorates. The hydrated salts most
consistent with the chemical signatures are likely a mixture of
magnesium perchlorate, magnesium chlorate and sodium perchlorate.
Some perchlorates have been shown to
keep liquids from freezing even when conditions are as cold as minus 94
degrees Fahrenheit (minus 70 Celsius). On Earth, naturally
produced perchlorates are concentrated in deserts, and some types of
perchlorates can be used as rocket propellant.
Perchlorates have previously been
seen on Mars. NASA's Phoenix lander and Curiosity rover both found them
in the planet's soil, and some scientists believe that the Viking
missions in the 1970s measured signatures of these salts. However,
this study of RSL detected perchlorates, now in hydrated form, in
different areas than those explored by the landers. This also is
the first time perchlorates have been identified from orbit.
MRO has been examining Mars since
2006 with its six science instruments.
"The ability of MRO to observe for
multiple Mars years with a payload able to see the fine detail of these
features has enabled findings such as these: first identifying the
puzzling seasonal streaks and now making a big step towards explaining
what they are", said Rich Zurek, MRO project scientist at NASA's Jet
Propulsion Laboratory in Pasadena, California.
For Ojha, the new findings are more
proof that the mysterious lines he first saw darkening Martian slopes
five years ago are, indeed, present-day water.
"When most people talk about water on
Mars, they're usually talking about ancient water or frozen water", he
said. "Now we know there's more to the story. This is the
first spectral detection that unambiguously supports our liquid
water-formation hypotheses for RSL".
The discovery is the latest of many
breakthroughs by NASA's Mars missions.
"It took multiple spacecraft over
several years to solve this mystery, and now we know there is liquid
water on the surface of this cold, desert planet", said Michael Meyer,
lead scientist for NASA's Mars Exploration Program at the agency's
headquarters in Washington. "It seems that the more we study Mars,
the more we learn how life could be supported and where there are
resources to support life in the future".
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Dark narrow streaks, called "recurring slope lineae", emanate from the
walls of Garni Crater
on Mars, in this view constructed from observations by the High Resolution
Imaging
Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter.
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