Characteristics Of Potential Target Sites

What are the characteristics of potential targets for Mars exploration that are particularly suited for elucidating the prebiotic and possibly biotic (and postbiotic) history of Mars, and what methods can be used to identify these targets? Current understanding of the environmental requirements for supporting an origin or the continued evolution of life and the nature of the interplay between organisms and their geological and planetary environment is based on our terrestrial example. This understanding informs researchers' views of the types of martian environments that might be fruitful for detailed investigation and the types of individual materials at those sites that might contain pertinent evidence.

Life has the potential to exist in a broad range of environments. Although any place on the surface might provide pertinent information, the most likely places should be explored first. These include places where liquid water might be present today or might have been present for extended periods at some time in the past.

Such sites are pertinent both for searching for evidence of present or past life and for understanding the nature of martian habitability. For present-day or geologically recent water, these sites include the surface, interior, and margins of the polar caps; cold, warm, or hot springs or underground hydrothermal systems; and the source region or outflow region associated with near-surface aquifers that might be responsible for the "gullies" that have been observed. For geologically ancient water, pertinent sites include the source or outflow regions for the catastrophic flood channels, the ancient highlands that formed at a time when surface water might have been widespread (e.g., in the Noachian), and deposits of minerals that are associated with surface or subsurface water or with ancient hydrothermal systems or cold, warm, or hot springs.

Recommendation. Sites that NASA should target with highest priority to advance astrobiology science objectives are those places where liquid water might exist today or might have existed in the past and where organic carbon might be present or might have been preserved.

Site Selection

Although the committee anticipates that new measurements are likely to include major discoveries relevant to astrobiology that will point researchers to specific sites, a foundation of data is already available to identify exciting and appropriate sites for either in situ analysis or sample return.

There is no single site that will provide all of the answers pertinent to the astrobiology science goals for Mars. Further, the Mars Reconnaissance Orbiter (MRO) spacecraft is just beginning its mission at this writing, and it will be several years before those results are fully integrated into the (anticipated) revision of our understanding of Mars. Thus, it is premature either to identify a specific landing site for future detailed investigation (either in situ on the surface or as a source of samples for return to Earth) or to limit the range of places that can be visited by spacecraft.

An argument has been made that if only a single sample-return mission is programmatically feasible in the foreseeable future, then it must return the "right" sample from the "right" site, the "right" sample being one that has the best chance for uniquely providing astrobiologically significant information (such as information bearing on the detection of life). The committee disagrees with this viewpoint and argues that there is no such thing as the "right" sample. Any well-selected samples—i.e., samples selected carefully from a thoughtfully chosen site—would provide information that would be incredibly valuable for addressing the scientific goals for the exploration of Mars, in general, and astrobiological goals, in particular. Furthermore, the information necessary to choose an appropriate sampling site is available within existing data sets or data sets whose acquisition is imminent (e.g., those from MRO). Although they cannot be known to identify specific sites as having life, the relevant data do 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.

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