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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