[meteorite-list] Rocks Rich in Silica Present Puzzles for Mars Curiosity Rover Team

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 17 Dec 2015 17:29:37 -0800 (PST)
Message-ID: <201512180129.tBI1TbE3024128_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/news/news.php?feature=4799

Rocks Rich in Silica Present Puzzles for Mars Rover Team
Jet Propulsion Laboratory
December 17, 2015

In detective stories, as the plot thickens, an unexpected clue often delivers
more questions than answers. In this case, the scene is a mountain on
Mars. The clue: the chemical compound silica. Lots of silica. The sleuths:
a savvy band of Earthbound researchers whose agent on Mars is NASA's laser-flashing,
one-armed mobile laboratory, Curiosity.

NASA's Curiosity rover has found much higher concentrations of silica
at some sites it has investigated in the past seven months than anywhere
else it has visited since landing on Mars 40 months ago. Silica makes
up nine-tenths of the composition of some of the rocks. It is a rock-forming
chemical combining the elements silicon and oxygen, commonly seen on Earth
as quartz, but also in many other minerals.

"These high-silica compositions are a puzzle. You can boost the concentration
of silica either by leaching away other ingredients while leaving the
silica behind, or by bringing in silica from somewhere else," said Albert
Yen, a Curiosity science team member at NASA's Jet Propulsion Laboratory,
Pasadena, California. "Either of those processes involve water. If we
can determine which happened, we'll learn more about other conditions
in those ancient wet environments."

Water that is acidic would tend to carry other ingredients away and leave
silica behind. Alkaline or neutral water could bring in dissolved silica
that would be deposited from the solution. Apart from presenting a puzzle
about the history of the region where Curiosity is working, the recent
findings on Mount Sharp have intriguing threads linked to what an earlier
NASA rover, Spirit, found halfway around Mars. There, signs of sulfuric
acidity were observed, but Curiosity's science team is still considering
both scenarios -- and others -- to explain the findings on Mount Sharp.

Adding to the puzzle, some silica at one rock Curiosity drilled, called
"Buckskin," is in a mineral named tridymite, rare on Earth and never seen
before on Mars. The usual origin of tridymite on Earth involves high temperatures
in igneous or metamorphic rocks, but the finely layered sedimentary rocks
examined by Curiosity have been interpreted as lakebed deposits. Furthermore,
tridymite is found in volcanic deposits with high silica content. Rocks
on Mars' surface generally have less silica, like basalts in Hawaii, though
some silica-rich (silicic) rocks have been found by Mars rovers and orbiters.
Magma, the molten source material of volcanoes, can evolve on Earth to
become silicic. Tridymite found at Buckskin may be evidence for magmatic
evolution on Mars.

Curiosity has been studying geological layers of Mount Sharp, going uphill,
since 2014, after two years of productive work on the plains surrounding
the mountain. The mission delivered evidence in its first year that lakes
in the area billions of years ago offered favorable conditions for life,
if microbes ever lived on Mars. As Curiosity reaches successively younger
layers up Mount Sharp's slopes, the mission is investigating how ancient
environmental conditions evolved from lakes, rivers and deltas to the
harsh aridity of today's Mars.

Seven months ago, Curiosity approached "Marias Pass," where two geological
layers are exposed in contact with each other. The rover's laser-firing
instrument for examining compositions from a distance, Chemistry and Camera
(ChemCam), detected bountiful silica in some targets the rover passed
on its way to the contact zone. The rover's Dynamic Albedo of Neutrons
instrument simultaneously detected that the rock composition was unique
in this area.

"The high silica was a surprise -- so interesting that we backtracked
to investigate it with more of Curiosity's instruments," said Jens Frydenvang
of Los Alamos National Laboratory in New Mexico and the University of
Copenhagen, Denmark.

Gathering clues about silica was a major emphasis in rover operations
over a span of four months and a distance of about one-third of a mile
(half a kilometer).

The investigations included many more readings from ChemCam, plus elemental
composition measurements by the Alpha Particle X-ray Spectrometer (APXS)
on the rover's arm and mineral identification of rock-powder samples by
the Chemistry and Mineralogy (CheMin) instrument inside the rover.

Buckskin was the first of three rocks where drilled samples were collected
during that period. The CheMin identification of tridymite prompted the
team to look at possible explanations: "We could solve this by determining
whether trydymite in the sediment comes from a volcanic source or has
another origin," said Liz Rampe, of Aerodyne Industries at NASA's Johnson
Space Center, Houston. "A lot of us are in our labs trying to see if there's
a way to make tridymite without such a high temperature."

Beyond Marias Pass, ChemCam and APXS found a pattern of high silica in
pale zones along fractures in the bedrock, linking the silica enrichment
there to alteration by fluids that flowed through the fractures and permeated
into bedrock. CheMin analyzed drilled material from a target called "Big
Sky" in bedrock away from a fracture and from a fracture-zone target called
"Greenhorn." Greenhorn indeed has much more silica, but not any in the
form of tridymite. Much of it is in the form of noncrystalline opal, which
can form in many types of environments, including soils, sediments, hot
spring deposits and acid-leached rocks.

"What we're seeing on Mount Sharp is dramatically different from what
we saw in the first two years of the mission," said Curiosity Project
Scientist Ashwin Vasavada of JPL. "There's so much variability within
relatively short distances. The silica is one indicator of how the chemistry
changed. It's such a multifaceted and curious discovery, we're going to
take a while figuring it out."

For more about Curiosity, which is examining sand dunes this month, visit:

http://mars.jpl.nasa.gov/msl/


Media Contact

Guy Webster
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6278
guy.webster at jpl.nasa.gov

Dwayne Brown / Laurie Cantillo
NASA Headquarters, Washington
202-358-1726 / 202-358-1077
dwayne.c.brown at nasa.gov / laura.l.cantillo at nasa.gov
Received on Thu 17 Dec 2015 08:29:37 PM PST