The following HTML text is provided to enhance online
readability. Many aspects of typography translate only awkwardly to HTML.
Please use the page image
as the authoritative form to ensure accuracy.
The Quarantine and Certification of Martian Samples
The Study of Extracts
Organic molecules in rocks and sediments are commonly studied after they have been physically separated from their host inorganic matrices. Typically, the rocks or sediments are washed or “extracted” with organic solvents that dissolve the organic molecules. Recovery and evaporation of the solvent leaves the organic molecules as a residue. Further separations of that residue, followed by spectroscopic analyses, can reveal the types and amounts of organic molecules that are present.4 However, these techniques are not appropriate for preliminary analyses of the martian samples. There is a chance that the solvents will react with the inorganic matrix, especially if strong oxidants are present. In addition, the purification of solvents is notoriously difficult, the efficiency with which materials can be recovered is imperfect, and, for precious materials, the process is unacceptably wasteful. Nevertheless, obtaining the best possible information about any organic molecules that are present is clearly required as part of the preliminary analyses.
A significant portion of the organic molecules that would be recovered by extraction with organic solvents can instead be released from milligram-sized aliquots of martian samples by thermal desorption. Samples are placed in a small chamber swept with helium and heated, stepwise, to (for example) 150, 250, and 350 ºC. Compounds volatilized in this way can be readily analyzed by gas chromatography-mass spectrometry (GC-MS; see Box 3.1 in Chapter 3). This procedure would recapitulate the GC-MS experiment used by the Viking landers on Mars. On Earth, 30 years later, the experiment will be far easier and still exceptionally informative. Samples are likely to be sterilized by this analytical procedure but not highly altered. The inorganic residues can be released for geochemical analyses.
More-polar organic compounds, those typically soluble in water and resembling simple cellular metabolites, proteins, and nucleic acids, would not be well characterized by the thermal-desorption plus GC-MS treatment. They can be extracted from samples in aqueous solution, but extremely pure water would be required to rule out very-low-level contamination, and water extraction cannot be expected to sterilize the material leached. Polar organic compounds should be extractable using supercritical fluids, such as liquid carbon dioxide or halocarbons. Such media can be evaporated far more readily than water, and supercritical fluid extraction is probably miniaturized more easily than water extraction. It is even possible that supercritical fluid extracts are demonstrably sterile, and research needs to be done to systematically and critically analyze the effectiveness of these solvents as sterilizing agents. Study of these techniques should be given a high priority in preparation for the preliminary analyses.
A protocol for these procedures is needed that defines how Mars samples are to be ground, prepared, and extracted. With such a protocol in place it will be possible to remove extracted organic compounds from quarantine, which will permit a wide variety of analytical studies of any organic compounds found. These studies can test for terrestrial contamination, differentiate between abiotic and biotic signatures in sets of compounds, search for biomarkers, and provide feedback to life detection experiments. In addition, isotopic studies of bulk extracts, as well as individual organic compounds, will provide evidence to test whether the organic compounds are indigenous and whether they contain evidence of possible biological isotopic fractionation.
Recommendation. It is important that a program of research be conducted to determine the efficacy of supercritical fluids and commonly used organic solvents in killing organisms. It is highly desirable to be able to remove solvent extracts from quarantine without the damage to dissolved biomarker compounds that would be caused by heat or ionizing radiation. Sterilization probably is systematically achieved by the supercritical fluids used in making extracts, but this needs to be verified before extracts can be removed from the Mars Quarantine Facility.
Studies to Be Conducted in the Quarantine Facility
Certain studies of the Mars samples, including biohazard testing, cannot be carried out on sterilized samples. These studies will thus have to be performed within the quarantine facility (category c in Figure 4.1), and it will be
Moldowan, J.W., Albrecht, P., and Philp, R.P. 1992. Biological Markers in Sediments and Petroleum. Prentice-Hall, Englewood Cliffs, N.J. 411 pp.