[meteorite-list] Dusty, Acidic Glaciers Could Explain the Valles Marineris Interior Layered Deposits on Mars

[Ron Baalke]

http://www.psi.edu/news/press-releases#michalski

NEWS RELEASE FROM THE PLANETARY SCIENCE INSTITUTE

FROM:
Alan Fischer
Public Information Officer
Planetary Science Institute
520-382-0411
520-622-6300
fischer at psi.edu


Dusty, Acidic Glaciers Could Explain the Valles Marineris Interior
Layered Deposits on Mars

March 29, 2012, Tucson, Ariz. -- Researchers from the Planetary Science
Institute (PSI) and NASA Johnson Space Center (JSC) have proposed a new
hypothesis to explain a class of enigmatic geologic features on Mars
that have puzzled scientists for decades. The new results, published
recently in the journal Geology, suggest that large sedimentary deposits
in the Valles Marineris termed Interior Layered Deposits (ILDs) may have
formed in a cold, dry ancient Martian climate as the remnants of massive
dust-rich glaciers that may have once filled this canyon system.

"Icy weathering might be a major part of the geologic story on Mars,"
said PSI Research Scientist Joseph Michalski,  "The planet has been in a
cold, frozen state for a long time. In the distant past, it was also
cold, but volcanoes were much more active, periodically pumping huge
amounts of sulfur into the atmosphere, which could have ultimately ended
up trapped within ice alongside plentiful dust."

An atmospheric origin of Martian Interior Layered Deposits (ILDs): Links
to Climate Change and the global sulfur cycle was written by Michalski
and co-author Paul Niles of NASA’s JSC and was published online in
Geology
at http://geology.gsapubs.org/content/early/2012/03/23/G32971.1.full.pdf+html
on March 26.

Valles Marineris is a 3,000 kilometer-long tectonic trough system on
Mars, which reaches depths of approximately eight kilometers (five
miles) below the surrounding terrain. Inside the canyon are vast mounds
of layered sediments of enigmatic origins. Since their discovery by the
Mariner 9 spacecraft about 40 years ago, the ILD deposits found within
the Valles Marineris have escaped explanation. Their setting within the
trough and canyon system has prompted some previous researchers to
suggest that the ILDs formed from volcanic processes because the
faulting and rifting that formed the canyon could easily lead to
thinning of the crust, high heat flow, and ascent of magma. 

However, in the late 1990s, the Thermal Emission Spectrometer instrument
showed that the deposits contain gray hematite, similar to deposits
explored at Meridiani Planum by the Mars Exploration Rover Opportunity.
And shortly after, a French team of researchers produced intriguing new
results that further complicated the interpretations of ILDs on Mars.
They showed that the deposits contain sulfate minerals – which are
typically found in desert playa or shallow sea environments on Earth and
are not dominant phases in volcanic terrain. 

One big part of the problem is the size of the altered, layered
sediments that rise several kilometers from the canyon floor in places.
Proposed ideas have included the suggestion that the canyon once housed
a vast system of deep lakes. But, the canyon is not topographically
constrained on all sides, so it is difficult to imagine how a lake could
have existed there without spilling into the low topography to the north. 

A number of recent publications have suggested that the sulfate-bearing
ILDs formed by groundwater upwelling, where subsurface water breaches
the surface during occasional upwelling events. This process has been
invoked to explain most of the sulfate-bearing deposits on Mars.

However, Michalski and Niles point out that this process cannot explain
the origin of the ILDs because they occur in deposits spanning all
elevations of the canyon, and seemingly have draping sedimentary
relationships against the canyon walls. If groundwater formed the ILDs,
it would require that essentially all of they canyon was at one time
filled with sulfur rich sediments. And, that scenario is simply not
possible because it would require more sulfur than would have ever been
available on Mars. 

As an alternative, Michalski and Niles suggest that the ILDs formed from
massive, dusty ice deposits that once filled much of the canyon. Such a
scenario is consistent with the size, topographic configuration of the
deposits, and with their mineralogy, which suggests weathering in an
acidic, water-limited environment. 

"Icy weathering might be a major part of the geologic story on Mars and
should be much more important than the Earth because Mars lacks liquid
surface water to buffer chemical processes" Michalski said. " As was
pointed out by the great mineralogist Roger Burns more than 20 years
ago, icy systems should become very acidic on Mars."   In an article in
Nature Geoscience in 2009, Niles led a paper proposing that these
ice-dominated systems could also explain the origin of the deposits at
Meridiani Planum.

"We are of the opinion that Mars has been cold and dry for quite a long
time, and that during that time, the various cycles of dust deposition,
ice precipitation and sublimation, and sulfur outgassing can explain
quite a bit of the surface geology. Even so, the hypotheses are not
without certain complications and uncertainties. It is our hope that
more scientists will be begin to test these ideas."


CONTACT:
Joseph Michalski
Research Scientist
+44 7432619949