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

[Ron Baalke]

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