Rocks Rich in Silica Present Puzzles for Mars Rover Team

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Rocks Rich in Silica Are a Mystery

'Marias Pass,' Contact Zone of Two Martian Rock Units.

'Marias Pass,' Contact Zone of Two Martian Rock Units This May 22, 2015, view from the Mast Camera (Mastcam) in NASA's Curiosity Mars rover shows the "Marias Pass" area where a lower and older geological unit of mudstone -- the pale zone in the center of the image -- lies in contact with an overlying geological unit of sandstone. Credit: NASA/JPL-Caltech/MSSS

It's a puzzle...

Nearly 40 months after landing on Mars, NASA's Curiosity rover still surprises us.

The latest discovery -- the highest concentration of silica-laden rock​s found anywhere on the planet - up to 90% in some rocks.

Note: Silica is a rock-forming chemical that combines silicon and oxygen, commonly seen on Earth as quartz, although it's also present in many other minerals.

Rocks on Mars' surface generally have much less silica -- like the basalts you might find in Hawaii. Some silica-rich (silicic) rocks have been found by Mars rovers and orbiters, but never at this level of concentration.

While that might only be exciting to geologists​, it could actually be a key to revealing Mars past geological history. As Albert Yen, a Curiosity team member at NASA's Jet Propulsion Laboratory in Pasadena, California said:

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. Either of those processes involve water. If we can determine which happened, we'll learn more about other conditions in those ancient wet environments.
Rocks rich in silica on Mount Sharp

'Big Sky' and 'Greenhorn' Drilling Area on Mount Sharp This view from the Mast Camera (Mastcam) on NASA's Curiosity Mars rover covers an area in "Bridger Basin" that includes the locations where the rover drilled a target called "Big Sky" on the mission's Sol 1119 (Sept. 29, 2015) and a target called "Greenhorn" on Sol 1137 (Oct. 18, 2015).

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.

But it's clear that water must have been involved.

Add in the findings of the Spirit rover, on the other side of the planet, and you get some interesting clues as to Mars early history.​

Spirit found signs of sulfuric acidity -- an acidic water vapor, released as part of a volcanic eruption, that clearly changed the composition of rocks. Acid fog, in other words.

Could that have happened at Mount Sharp, too? Curiosity's science team is still considering both scenarios -- and others -- to explain the findings from Curiosity.

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.

RELATED: Lakes on Mars

On Earth, Tridymite is typically found in volcanic deposits with high silica content. It's a result magma cooling and aging. So, the Tridymite found at Buckskin may be evidence for magmatic evolution on Mars.

Marias Pass

Curiosity rover mission path

Curiosity's Path During 2015 Studies of Silica-Rich Rocks This map shows the route on lower Mount Sharp that NASA's Curiosity followed between April 19, 2015, and Nov. 5, 2015. During this period the mission investigated silica-rich rock targets including "Buckskin," in the "Maria Pass" area, and "Greenhorn," in the "Bridger Basin" area. Credit: NASA

Curiosity has been steadily moving up the slopes of Mount Sharp since 2104. As the rover reaches successively younger layers, scientists are 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.

Jens Frydenvang of Los Alamos National Laboratory in New Mexico and the University of Copenhagen, Denmark, commented:

The high silica was a surprise -- so interesting that we backtracked to investigate it with more of Curiosity's instruments.

In fact, gathering clues about silica became a major emphasis -- for four months.

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. As Liz Rampe of Aerodyne Industries at NASA's Johnson Space Center, Houston said:

We could solve this by determining whether trydymite in the sediment comes from a volcanic source or has another origin. A lot of us are in our labs trying to see if there's a way to make tridymite without such a high temperature.
rocks rich in silica clearly evident in fractured zones

Discolored Fracture Zones in Martian Sandstone This view from NASA's Curiosity Mars rover shows an example of discoloration closely linked to fractures in the Stimson formation sandstone on lower Mount Sharp. The pattern is evident along two perpendicular fractures. Credit: NASA

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.

As Curiosity Project Scientist Ashwin Vasavada of JPL added:

What we're seeing on Mount Sharp is dramatically different from what we saw in the first two years of the mission.  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/​

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