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Mars Sample Return: Issues and Recommendations 6 Evaluation and Characterization of Samples Returned from Mars The initial evaluation of samples returned from Mars will focus on whether they pose any threat to Earth's biosphere. The only potential threat posed by returned samples is the possibility of introducing a replicating biological entity of nonterrestrial origin into the biosphere. Therefore, the initial evaluation of potential hazard should focus exclusively on whether the samples contain any evidence of organisms or biological activity. The scientific and technical knowledge available to address this task has improved enormously since the days of the lunar sample-return missions. SEARCHING FOR SIGNS OF LIFE Based on our current understanding of biology, it must be assumed that life elsewhere, if it exists at all, is composed of the same chemical elements and compounds that make up living organisms here on Earth. Extremely sensitive methods of chemical analysis are available, and these techniques will no doubt improve considerably by the time returned samples are ready to be examined. For example, methods for detecting biogenic compounds and resolving isotopic signatures soon may be capable of identifying a single cell in an otherwise sterile matrix of sample material. Specialized staining techniques are available that allow the identification of nucleic acids, proteins, lipids, and other biomolecules. With some additional research specifically directed toward developing improved techniques for detecting life at very low limits, this capability could be greatly extended.
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Mars Sample Return: Issues and Recommendations Initial investigations of martian samples will include optical and scanning electron microscopy to search for possible microbial bodies. There are no known living systems that are not associated with structures. Even the simplest organisms require membranes to establish charge separation and to separate cell components from the external environment. The technology for viewing surfaces or preparations and searching for features in the size range1 of biological entities has undergone rapid development, and, if features are identified that are reminiscent of cells or cell components, the technology soon will be available to determine whether such features are of biological origin. If a community of only a few organisms occurs in a portion of sample material to be analyzed, the techniques of life detection are expected to be sufficiently advanced by the time a martian sample actually is returned to Earth that there is confidence that those organisms will be detected. The chief difficulty will be preparing a representative portion for analysis. Any returned sample is likely to be heterogeneous, containing rock fragments of various types as well as soil. Great care will be required to select a representative portion that includes all of the potential habitats included in the overall sample. Choosing the portions for detailed analysis will be a critical task for the science team associated with the sample-receiving facility. AVOIDING FALSE POSITIVES Although forward contamination of Mars by terrestrial organisms conveyed aboard outbound spacecraft will be stringently avoided, it will remain possible that organisms from Earth could be transported to Mars and, in turn, contaminate the returned sample. It should be possible, however, to distinguish organisms that evolved on Earth from those that evolved on Mars through the use of molecular sequence comparisons. This technique should be effective even if putative martian organisms share a common ancestry with Earth organisms through the exchange of meteoritic debris (see Chapter 3), as long as the organism has had sufficient time on Mars to evolve away from its terrestrial ancestor. EVALUATING SAMPLE MATERIAL FOR POTENTIAL HAZARDS It is conceivable that returned samples could contain compounds that would be toxic to the researchers handling the sample material. This is not a planetary protection issue. The amount of material to be returned is quite small, and there is simply no danger to the public posed by a small, well-contained portion of material, even if it does happen to contain toxic compounds. Any potential dan- 1 Biological features should be greater than 10 nanometers in any dimension; the thinnest membranes are approximately 8 nanometers in width.
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Mars Sample Return: Issues and Recommendations ger to the researchers analyzing the samples would be obviated by standard laboratory control procedures. The only risk posed by a sample returned from Mars is the potential for including a replicating organism that could possibly grow and multiply on Earth. The possibility of such an occurrence is remote (see Chapters 2 and 4), but it is not zero. Therefore, adequate precautions must be taken. In Chapter 4 the task group recommends that martian sample material be contained and treated as though potentially hazardous until proven otherwise. Evaluation of the sample for potential hazards should focus exclusively, then, on searching for evidence of living organisms, their resting states (e.g., spores or cysts), or their remains in the sample. Attempts to cultivate putative organisms, or to challenge plant and animal species or tissues, are not likely to be productive. Moreover, if viable exogenous biological entities are discovered in the sample material, prudence would indicate that they remain segregated from Earth's biosphere (i.e., they should remain in containment or be made nonviable through sterilization). In keeping with the task group's recommendation in Chapter 4, if viable biological entities are discovered in sample material returned from Mars, and those entities cannot be accounted for by terrestrial organisms conveyed on the outbound spacecraft, then the sample material should be deemed hazardous and no portion should be removed from containment without first being sterilized.
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