State-of-the-Art Mineralogy for Mars
|Artist conception of Mars long-range science laboratory.|
Planetary exploration proceeds by steps. Because life on Earth requires liquid water, the MER mission aimed to search for signs that liquid water existed on Mars in the past. MER has found such signs. The aptly named Mars Science Laboratory scheduled for launch in 2009 will take a step further. It will search for detailed evidence that the area where it lands could have supported life in the past. That seemingly small step requires more precise characterization of the minerals in the MSL rover’s surroundings.
On Earth, scientists use three general methods to identify minerals. In the field, they first use their eyes (with and without magnification). On MSL, this job goes to the MastCam, the imaging part of the ChemCam, and the Mars Hand Lens Imager. These three packages are mounted on the mast or the robotic arm of the rover.
In the laboratory, earthbound scientists try to analyze the chemical composition of a mineral sample. They can grind a sample and heat it, breaking down the mineral into gasses such as carbon dioxide or sulfur dioxide. Each mineral yields its gasses in a characteristic pattern as the temperature rises.
|Opportunity explores dunes in the base of Endurance Crater.|
Image Credit: NASA/JPL
To achieve the most precise mineral identification, though, mineralogists beam x-rays at the sample, measuring the radiation that returns to a detector. The internal chemical and mineralogical laboratory on the MSL rover will contain two instruments that for the first time on Mars will be capable of performing the all of these laboratory tests.
"We are trying to really bring sophisticated laboratory capabilities to Mars to some extent. With this mission, we have the resources to be able to push in that direction," says planetary scientist Paul Mahaffy, of the Goddard Space Flight Center.
Sniffing Martian Air
Mahaffy is the principal investigator for the instrument package called Sample Analysis at Mars, or SAM. SAM’s two main instruments deal with gasses, both organic molecules and inorganic molecules important to biological activities. SAM analyzes gasses in the atmosphere and those produced by heating dust, sand or crushed rock. The goal, Mahaffy says, is "to do a general survey for the range of organic compounds that might be there, and then really to focus in on molecules that we know are relevant to terrestrial life, such as amino acids, nucleobases, the types of things that proteins and DNA and life on Earth are made of."
|Opportunity found ‘blueberry-like’ concretions scattered across the Meridiani plains. Click image for larger view.|
Image Credit: NASA/JPL
SAM can analyze atmospheric gasses directly, first passing them through an instrument called a gas chromatograph-mass spectrometer (GCMS). Because such GCMS systems are widely used in laboratories on Earth separating out molecules based on their masses results obtained on Mars can be compared easily with earthly libraries to identify specific molecules. "The gas chromatograph system basically serves to separate out the organics into individual components that can be individually detected and uniquely identified by comparing the mass spectrometer fragmentation pattern with libraries," Mahaffy says.
A second SAM instrument, called a tunable laser spectrometer, specializes in detecting particular isotopes, Mahaffy says. It "measures the isotope ratios in carbon, hydrogen and oxygen and sulfur" in gas molecules such as water, methane, carbon monoxide, carbon dioxide and hydrogen peroxide. For example, the instrument could separate carbon 12, the usual form of the element, from carbon 13, a rarer form. An elevated ratio of carbon 13 to carbon 12 could indicate that life was once present.
Sniffing Martian Rocks
Using the same gas-analysis system to analyze rocks requires some preparatory steps. The sample must be picked up and ground to a fine sand before being presented to the instruments. The rover will contain a complete subsystem for this step.
|Characteristic landforms that accompany glacial fracturing terrestrially.Credit: Mars Express|
"SAM will basically accept samples delivered by the MSL sample processing system into little cups– we have 84 cups planned –and move one cup at a time into an oven, basically, where the sample is heated to about 1,100 degrees" Celsius (about 2000 degrees Fahrenheit), Mahaffy explains. As the temperature rises, molecules escape from the sample as gas. Both the mass spectrometer and the tunable laser spectrometer analyze these gasses. In a second step, the gasses are passed to the gas chromatograph. For both instruments, the scientists link the data with the slowly increasing temperature.
"We do our heating of the samples in a very controlled manner –instead of the rapid heating to a final temperature that Viking used," Mahaffy says. Among other advances on Viking’s instruments are SAM’s high-capacity, high-throughput gas pumps.
"At the very lowest temperatures, where we’ll start our thermal processing, we will be driving off gasses that are just on the surface of porous materials. And then, as we go up in temperature, we will start driving off the water that is chemically bonded, and then when we get up to very high temperatures in the 500- to 800-degree [Celsius, or 900- to 1500-degree Fahrenheit] range, we really start breaking minerals apart." Minerals release specific molecules at characteristic temperatures, so the pattern of gasses released over time as the oven heats up will tell scientists what minerals are present in a sample.
|"If the methane observation is borne out, maybe there is something living there under the ice." -David Grinspoon|
Image Credit: NASA
"We know at what temperatures we expect various minerals to break up and release simple gasses like carbon dioxide, sulfur dioxide, nitrogen compounds and so on," Mahaffy says. Computer analysis of the raw data can tease apart the characteristic signatures even from mixtures of minerals."
The Gold Standard for Minerals
Analyzing gasses produced by heating minerals provides important evidence. But on Earth, the mineralogical gold standard is x-ray diffraction, says astrobiologist David F. Blake, of NASA Ames Research Center. Blake is the principal investigator for the second half of the MSL rover’s chemical and mineralogical laboratory, called CheMin.
"X-ray diffraction is the only definitive way to characterize minerals absolutely," Blake says. "This is really the first fully definitive measurement of mineralogy that will be done on the Mars surface." CheMin also includes an x-ray fluorescence detector, functionally similar to instruments on MER and Viking.
"There’s an x-ray tube that produces a small pencil-like beam of x-rays that goes through a sample of material, crushed rock, or dust, and there’s a CCD detector that receives all of the x-rays that come from the sample." Beaming x-rays on a sample can cause two different results. The radiation can be diffracted and it can be absorbed and re-emitted as fluorescence.
|The Viking 2 Lander.|
Diffraction occurs when radiation, such as visible light or x-rays, changes direction as it passes an opaque edge. X-ray diffraction uniquely identifies a mineral. But diffraction is a rare event, Blake says. "If you have a single crystal and you shine a beam of x-rays through it, it’s actually unlikely that you’ll get any diffracted beam." Diffraction requires that the x-ray fall on the crystal at a particular angle. To deal with this problem, Blake says, earthbound mineralogists grind minerals to a fine powder, creating, "essentially an infinite number of little, tiny, tiny crystals in the beam, so that there’s all possible orientations of those crystals to produce all possible diffraction." CheMin will use a few mechanical tricks to come close to duplicating this effect. "The imperfect powder that’s produced by the MSL’s rock crusher can be made to look like a very nice, fine grained powder."
Fluorescence occurs when an atom absorbs electromagnetic radiation and then emits radiation at a different frequency. Where x-ray diffraction identifies a mineral, x-ray fluorescence identifies particular atoms within that mineral.
|Erosion channel on Mars. The detailed structure of the channel’s walls and floor looks much like erosion patterns on Earth.|
Another Pair of Twins?
Because the double-rover strategy NASA used for the MER mission was so successful, the agency is considering "the possibility of doing two rovers in 2011 instead of just one rover in 2009," Mahaffy says. "You kind of double the places that you are able to access and you give the entire mission a degree of scientific robustness. Of course then you wait two years longer to get your data, so that’s the downside."
Mahaffy has proposed some liquid chemistry experiments for SAM, but NASA has not yet decided if the rover can accommodate those. Liquid chemistry would give SAM the ability to preprocess certain chemical compounds that are too fragile to survive a gas chromatograph. The less sophisticated GCMS on Viking might have missed such compounds.
But however advanced MSL’s scientific suite, Blake sees some limits to what MSL will be able to do. "It would be wonderful to be able to age-date samples," he says. "That’s just not possible right now in a spacecraft instrument. It’s actually a difficult thing to do on the Earth. Presently, that’s something that you only could do with a sample return."
Related Web Pages
Malin Space Sciences
Giving Mars Back its Heartbeat
Looking for Martian Life
Should We Terraform?
Walking Naked on the Red Planet
United Nations of Mars
Living on Mars
The Martian Future