Curiosity's soil sample analysis reveals no surprises
By Brian Dodson
December 4, 2012
NASA's Mars Curiosity rover has used its Sample Analysis at Mars (SAM) and Chemistry and Mineralogy (CheMin) instruments to analyze its first scoop of Martian soil. These instruments allowed Curiosity to perform a wide range of chemical and structural tests which found signs of a complex and active soil chemistry – but no sign of life.
Soil samples were taken from a sand drift called "Rocknest," whose composition was earlier found by the Alpha Particle X-ray Spectrometer (APXS) and the Laser Induced Breakdown Spectroscope (ChemCam) to be similar to volcanic rocks on Earth.
A Rocknest soil sample was examined using the CheMin powder X-ray diffraction camera, which sends a beam of X-rays through a soil sample, then takes a picture of the X-rays after diffract from the atoms in the sample. The results showed that the soil sample is about half common volcanic minerals and half non-crystalline materials such as volcanic glass. These results are consistent with the APXS and ChemCam results.
A different Rocknest soil sample was studied by SAM, which is essentially a laboratory designed to analyze gases for their chemical and isotopic composition. In studying a soil sample, SAM starts by slowly heating the sample to drive off gases, absorbed moisture, and other volatiles. These gases are then passed through a gas chromatograph, an instrument that separates mixed gases into their various components based on their chemical activity. This allows the number of components and information about their chemical characteristics to be measured. The oven requires nearly the entire output of Curiosity's radioisotope thermal generator.
As the gaseous components emerge from the chromatograph, they are ionized and then directed into SAM's quadrupole mass spectrometer. This device uses electric fields to separate charged particles having distinct ratios of molecular mass to electric charge. SAM's mass spectrometer is capable of detecting and determining the mass (but not the composition) of, for example, organic molecules having as many as 40 carbon atoms.
The Tunable Laser Spectrometer (TLS) is the last stop for gas samples being analyzed. Unlike the previous SAM instruments, the TLS is only sensitive to methane, carbon dioxide, and water, but it can detect these gases at the parts per billion level, while also measuring the relative abundances of hydrogen, carbon, and oxygen isotopes in those gases. Ratios of stable isotopes are important signatures for numerous geophysical and astrobiological processes.
SAM's analysis of the Rocknest soil sample found water, carbon dioxide, oxygen, hydrogen sulfide, sulfur dioxide, and a variety of chlorinated hydrocarbons. Deuterium was present in the soil's water at five times the concentration as found on Earth, but in agreement with atmospheric water vapor. When Mars lost most of its atmosphere, it is conjectured that more of the lighter hydrogen was lost, driving an increase in the relative amount of deuterium.
The Martian environment contains all of these elements, which can react to form these compounds triggered by the lightning known to be present on Mars. NASA is offering none of these results as evidence for life on Mars, especially when combined with SAM's earlier non-detection of methane in the atmosphere.
"We have no definitive detection of Martian organics at this point, but we will keep looking in the diverse environments of Gale Crater," said SAM Principal Investigator Paul Mahaffy of NASA's Goddard Space Flight Center in Greenbelt, Maryland.
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