Little Green Martian Mineral
Pasadena, Sol 17
The first use of the tools on the arm of NASA’s Mars Exploration Rover Spirit reveals puzzles about the soil it examined and raises anticipation about what the tool will find during its studies of a martian rock, called Adirondack.
|Astrobiology Magazine had the opportunity to review some of the martian olivine mystery with planetary scientist Dr. Bill Hartmann, a Mars Global Surveyor team member.|
Astrobiology Magazine: There seems to be a brewing mystery centered around the geology of Mars, in that it has water-formed minerals like hematite, but also has water-reactive minerals like olivine. This seems to indicate that flowing water can’t be there, particularly if olivine remains. Can you comment, and do you think these kinds of issues can be resolved with the current generation of experiments?
Bill Hartmann: The lack of spectral detection of lakebed salts and carbonates does not prove that lakes never formed (as widely reported in the press) but only that if they did form, say 3 billion years ago, they are now covered and hidden by sediments and dust drifts.
Olivine has been detected spectrally in a few regions, and part of the dominant basaltic rock type, and it’s true geologically that prolonged water exposure weathers basalt to other forms. So it’s been argued that Mars was never very wet.
But on the other hand, it’s just not true that this rules out water activity. Most Mars meteorites, studied in labs on earth, have clear evidence of having been exposed to moisture and salty water. One (named Lafayette) has enough weathered minerals that they could be dated by two labs (California and Arizona) and the water exposure was found to have happened 670 million years ago.
It’s not a question of "never any flowing water on Mars," but rather a question of dates of water, duration of exposure of the rock and soils to water, replacement by fresh unweathered rocks such as lavas. After all, earth has lots of basalt rich lava flows and even whole beaches of olivine rich sand with wave lapping on them (I’ve walked on them!). And no one is going to characterize Earth as a planet devoid of flowing water!
"We’re really happy with the way the spacecraft continues to work for us," said Jennifer Trosper, mission manager at NASA’s Jet Propulsion Laboratory. The large amount of data — nearly 100 megabits — transmitted from Spirit in a single relay session through NASA’s Mars Odyssey spacecraft today "is like getting an upgrade to our Internet connection."
Scientists today reported initial impressions from using Spirit’s alpha particle X-ray spectrometer, Mossbauer spectrometer and microscopic imager on a patch of soil that was directly in front of the rover after Spirit drove off its lander Jan. 15.
"We’re starting to put together a picture of what the soil at this particular place in Gusev Crater is like. There are some puzzles and there are surprises," said Dr. Steve Squyres of Cornell University, Ithaca, N.Y., principal investigator for the suite of instruments on Spirit and on Spirit’s twin, Opportunity. The rover’s tool-carrying arm is called the Instrument Deployment Device (IDD) because it houses four diagnostic instruments used to answer question about Mars’ water history. The arm has a rotating head or ‘turret’, that can place various instruments into action against a rock, soil, or boulder: the Microscopic Imager, Mossbauer, Alpha Proton X-ray Spectrometer (APXS), and the Rock Abrasion Tool (RAT).
The Olivine Mystery
One unexpected finding was the Mossbauer spectrometer’s detection of a mineral called olivine, which does not survive weathering well. Olivine is a shiny green rock commonly found in lava on Earth. This spectrometer identifies different types of iron-containing minerals; scientists believe many of the minerals on Mars contain iron. "This soil contains a mixture of minerals, and each mineral has its own distinctive Mossbauer pattern, like a fingerprint," said Dr. Goestar Klingelhoefer of Johannes Gutenberg University, Mainz, Germany, lead scientist for this instrument.
|First Mossbauer data from Spirit, Click image to enlarge. |
Image Credit: NASA/JPL
Scientists were puzzled by the discovery of olivine because it implies the soil consists at least partially of ground up rocks that have not been weathered or chemically altered. The lack of weathering suggested by the presence of olivine might be evidence that the soil particles are finely ground volcanic material, Squyres said. Another possible explanation is that the soil layer where the measurements were taken is extremely thin, and the olivine is actually in a rock under the soil. The other two iron-bearing minerals have yet to be pinned down.
A team of researchers from USGS,Arizona State University and NASA, that found abundant quantities of olivine on Mars, announced earlier this year that such a finding from orbital pictures may give clues to martian water history.
Olivine is significant because it decomposes rapidly in the presence of water. Finding olivine on the surface may therefore be a good indicator of a dry Martian surface. Olivine, a transparent green-colored mineral found in many volcanic regions, is susceptible to chemical weathering and readily alters to other minerals such as iddingsite, goethite, serpentine, chlorite, smectite, maghemite and hematite in the presence of water. Except for trace amounts of hematite, which gives Mars its red color, none of these other weathering products have been found. The olivine occurs in many eroded areas, for example in canyons, so was emplaced when many of the erosional events occurred on Mars.
About three percent of the martian surface mapped so far by the Mars Global Surveyor contains abundant olivine, and another three percent contains coarse-grained hematite, consistent with Mars’ red color. The fact that so much olivine is exposed at the surface indicates that there has been little to no weathering due to water, thus no liquid water-mineral chemical reactions. The age of the surface is somewhat uncertain but is probably over 3 billion years old.
Soil That Binds
|First APXS spectra, Click image to enlarge |
Image Credit: NASA/JPL
Scientists were also surprised by how little the soil was disturbed when Spirit’s robotic arm pressed the Mossbauer spectrometer’s contact plate directly onto the patch being examined. Scientists initially thought that the soil was dust-like and therefore would collapse as the instrument pressed down on it with approximately 4 ounces (113 grams) of force. Microscopic images from before and after that pressing showed almost no change. "I thought it would scrunch down the soil particles," Squyres said. "Nothing collapsed. What is holding these grains together?"
Information from another instrument on the arm, an alpha particle X-ray spectrometer (APXS), may point to an answer. This instrument "measures X-ray radiation emitted by Mars samples, and from this data we can derive the elemental composition of martian soils and rocks," said Dr. Johannes Brueckner, rover science team member from the Max Planck Institute for Chemistry, Mainz, Germany. The instrument found the most prevalent elements in the soil patch were silicon and iron. It also found significant levels of chlorine and sulfur, characteristic of soils at previous martian landing sites but unlike soil composition on Earth.
Squyres said, "There may be sulfates and chlorides binding the little particles together." Those types of salts could be left behind by evaporating water, or could come from volcanic eruptions, he said. The soil may not have even originated anywhere near Spirit’s landing site, because Mars has dust storms that redistribute fine particles around the planet. The next target for use of the rover’s full set of instruments is a rock, which is more likely to have originated nearby.
Spirit landed in the Connecticut-sized Gusev Crater on Jan. 3 (EST and PST; Jan. 4 Universal Time). Spirit’s twin Mars Exploration Rover, Opportunity, will reach Mars on Jan. 25 (EST and Universal Time; 9:05 p.m., Jan. 24, PST) to begin a similar examination of a site on the opposite side of the planet, at a place called Meridiani Planum.
When NASA announced site selections for the two mission in April 2003, Squyres described the contrast expected: "They’re the two sites that practically everybody in the Mars science business has been hoping we’d get: Meridiani Planum and Gusev Crater. Meridiani is a place where there’s a large concentration of a mineral called coarse crystalline hematite… stuff that on Earth usually forms in the presence of liquid water. And Gusev is a large impact crater with an big dried-up riverbed flowing into it. Long ago there was a lake in Gusev Crater, and the crater must still be full of sediments. They’re both great sites… in fact, they’re the two best sites you could possibly find for a mission like this. We’re thrilled with them."