tion that allows researchers to address increasingly focused questions that relate to astrobiology goals in particular.

Intellectual Approaches

Applying a broad approach to determining whether life ever existed on Mars will require focusing on the elements most relevant to life, especially carbon. It will be necessary to determine whether organic molecules are present on Mars and where, and which chemical characteristics will allow researchers to distinguish between meteoritic (nonbiological), prebiotic, and biological molecules. In addition, there is still much to be learned about the history and availability of water, and “following the water” is a strategy that researchers will also have to continue to apply.

Finding. The very successful intellectual approach of “follow the water” should be expanded to include “follow the carbon,” along with other key biologically relevant elements.

Planetary Protection Considerations

As researchers move toward increasingly detailed examination of the forms in which carbon exists within surface materials, characterization of the nature of possible organic molecules within soil or rock samples, and measurements to search for and identify possible biosignatures, it is clear that contamination by terrestrial materials is a significant concern. Obtaining the desired science results demands that issues related to terrestrial contamination be addressed by appropriate planetary protection approaches. It would be a mistake to have to avoid the sites identified as most promising for scientific discovery because of concerns over planetary protection. NASA must ensure that adequate measures are built into missions from the beginning, incorporating lessons learned from past analyses.

Finding. The desire to visit and sample the highest-priority astrobiological sites requires that future surface missions to Mars take the necessary and appropriate planetary protection measures.

The Importance of Sample Analysis

Useful science analysis of martian surface samples can be carried out either in situ on the martian surface or in terrestrial laboratories with samples returned from Mars to Earth. In situ missions have the advantage of being able to address questions appropriate to specific regions or to obtain measurements of characteristics that might be unstable (e.g., trace chemical species that might represent chemical disequilibrium). In addition, analysis in situ can be done more cheaply than sample-return missions. However, sample return offers the advantages of being able to carry out many more analyses on a sample than can be done in situ, of following up exciting measurements with additional measurements that had not previously been anticipated, and of being able to make measurements or observations using instruments that are not amenable to being accommodated on a lander or rover mission or that were not available at the time of mission development. Indeed, numerous previous studies have consistently pointed out the important contributions that sample-return missions from planetary bodies can make to virtually every area of solar system exploration in general, and to Mars exploration in particular.15 A Mars sample-return mission has thus been an essential component of the Mars exploration strategies advocated by the National Research Council for 30 years.611 Even from the narrower perspective of the search for life on Mars, “evidence” for martian life observed only in situ would create controversy, not conviction. That is, the discovery of past or present life on Mars would be of such importance that it is highly unlikely that the scientific community would be convinced by anything less than the power of laboratory analysis. Therefore, the anticipated astrobiology science results that would be obtained from a sample-return mission are much greater than those that would come from an in situ mission.

Finding. The greatest advance in understanding Mars, from both an astrobiology and a more general scientific perspective, will come about from laboratory studies conducted on samples of Mars returned to Earth.

The astrobiology science goals that have been put forward can be addressed appropriately via a series of robotic



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