identify sites that have had liquid water or chemical alteration typically associated with liquid water and that have morphologies indicative of the long-term presence of liquid water.

Finding. Identification of appropriate landing sites for detailed analysis (whether in situ or by sample return) can be done with the data sets now available or imminently available from currently active missions.

The potential target sites listed above that would important for in situ investigation pertinent to ancient or recent life are at high elevations or at polar latitudes. These include most of the ancient Noachian terrain that would tell researchers about the potential earliest life and polar regions where melting of ice (e.g., at relatively recent epochs of high obliquity) could provide liquid water to sustain life. Accessing these sites will require an increased capability to land at a wider range of latitudes and elevations than are accessible by the Mars Science Laboratory (MSL), for example. Among the requirements are advances in landing site selection and in entry, descent, and landing technologies, and the provision of more capable power systems to ensure spacecraft survival during extended missions in the polar regions. In addition, given the importance of mobility as demonstrated by the Mars Exploration Rover (MER) mission, future rovers should have adequate capability to visit a wide range of geographical and geological terrains on a single mission.

Recommendation. Future surface missions must have the capability to visit most of the martian surface, including Noachian terrains and polar and high-latitude areas, and to access the subsurface.

Exposure to strongly oxidizing environments or to high fluxes of radiation is not conducive to the preservation of biologically diagnostic carbon compounds; this knowledge should be factored into decisions about where to collect samples. Terrestrial-based knowledge suggests that fine-grained sedimentary rocks, evaporites, and hydrothermal deposits are examples of rock types that can preserve biosignatures. Inadvertent processing of samples by heating or shock during sampling, or processing prior to in situ analysis or return to Earth, should be avoided.

Recommendation. Selection of samples for analysis (either in situ or of samples returned from Mars to Earth) should emphasize those having the best chance of retaining biosignatures.

BIOSIGNATURES

What biosignatures reflect fundamental and universal characteristics of life? Unfortunately, there is no single comprehensive or unique biosignature whose presence would indicate life and whose absence would uniquely indicate the absence of life. Experience from examination of the ALH 84001 meteorite and analysis of evidence relating to claims of the earliest life on Earth have demonstrated that the potential interplay between putative organisms and their geological environment is so complex that researchers may never be able to identify a unique biosignature that would work in all environments and at all times. Rather, the sum total of all measurements on a sample, in the context of understanding of the origin and evolution of the martian environment, will be required. What has been learned from the study of Earth’s earliest life and of the interplay between organisms and their planet, as well as from modern biology, provides the most appropriate guide to selecting targets on Mars and searching for biosignatures.

Of all the various life-detection techniques available, analysis of carbon chemistry is the first among equals. Organic analysis is likely to provide a more robust way to detect life than imaging technologies, mineral assemblages, isotopic measurements, or any one other single technique. This is the case because, on Earth, the patterns of biogenic carbon compounds reflect organized polymerization of smaller subunits, or precursors, and comprise mixtures with a limited range of atomic spatial arrangements very different from those made by abiological processes. However, organic analysis alone is insufficient to detect life. An ensemble of all of the relevant methodologies, combined with analysis of geological and environmental plausibility, will likely provide the best evidence for the presence or absence of life in a sample; there is no single, unique characteristic that would allow researchers to identify a region that might now have, or might once have had, life, or to determine whether life is, indeed, indicated.

Recommendation. The lack of a comprehensive understanding of all of the potential biosignatures for Mars exploration means that NASA should employ a combination of techniques that utilize both Earth-centric and non-Earth-centric approaches that focus on the basic concepts in carbon chemistry, imaging, mineral assemblages, and isotopic measurements.



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