Jack D. Farmer
NASA-Ames Research Center
During the coming decade, robotic field science will play a fundamental role in exploring Mars for evidence of past life and/or prebiotic chemistry. To create a context for such exploration, we especially need to understand the mineralogy and chemistry of the Martian surface. We have learned that the preservation of biological signatures in rocks on Earth is favored by rapid mineralization processes that are restricted to a comparatively small number of geological settings. Thus, a detailed knowledge of surface mineralogy will provide valuable clues about past Martian environments as a necessary context for future exobiological exploration.
Information about past climate and volatile history resides in mineralogy, and specifically the mineralogy associated with aqueous sedimentary processes. The targets of choice in exploring for past life on Mars are therefore aqueous sedimentary deposits, particularly those in ancient terranes that date to the early clement period in Mars' history when liquid water was present at the surface. Site selection for landed missions will be critical because we cannot expect to land just anywhere on Mars and find the kinds of deposits that are likely to preserve evidence of past Martian life or climatic history. Even if life never developed on Mars, the same kinds of target deposits are likely to harbor a prebiotic chemical record that would be crucial in understanding the origin of terrestrial life. Achieving a scientific consensus on the question of past Martian life is likely to require multiple sample returns, followed by extensive interdisciplinary studies carried out in labs on Earth.
To find the best places to explore, we will need to optimize site selection by carrying out high spatial resolution mineralogical mapping from orbit, with ground truthing of surface mineralogy using well-placed and appropriately instrumented robotic rovers. Remote sensing studies of geological terranes on Earth suggest that spatial resolutions of < 30 m/pixel for visible range imaging and < 100 m/pixel for multispectral imaging may be required to accurately identify high-priority targets on Mars for ancient climate and life studies.
Increased landing precision and highly mobile rovers are technological priorities that will ensure we reach high-priority targets during nominal mission times. Precise mobility requirements will vary with the science goals of each mission, reflecting a balance between landing precision and the size of target deposits. Landing targets will need to be pre-selected from orbital imaging, and the higher the spatial resolution the better for precisely locating the best landing sites. To optimize for a high science return, it is likely that rovers will need to be able to traverse distances exceeding their own landing errors (perhaps multiple kilometers or more during nominal mission times). In order to target samples for remote analysis by Earth-based investigators and for pre-planning traverses using virtual terrane models, rovers will also require instrumentation for analyzing the composition of rocks at a distance (e.g., "spot" spectrometers).
Considering the small amount of material that will be brought back to Earth, in situ mineralogical analysis will be crucial for selecting the best materials for sample return. Rock surfaces are likely to be covered by dust or weathering rinds, and accurate compositional analysis will require access to freshly exposed interior rock surfaces. Microscopic imaging of rock surfaces will provide valuable microtextural and mineralogical information to assist in targeting smaller areas on rock surfaces for more detailed compositional analyses. Given the minimal sample preparation required, spectral analysis (e.g., by infrared or laser Raman spectroscopy), especially in combination with reflectance techniques for elemental analysis (e.g., alpha proton X-ray spectroscopy or X-ray fluorescence), provides an especially favorable approach to in situ mineralogical and organic analysis for early missions.
The exploration for extant Martian life will require a fundamentally different approach from that used for ancient climate studies and Exopaleontology. The Viking missions revealed the surface of Mars to be inhospitable for life as we know it, owing primarily to the absence of liquid water. However, it has also been suggested that life could exist in the subsurface of Mars (perhaps tens to hundreds of kilometers depth), where an extensive hydrosphere could be present. It is also possible that areas of rising ground water may provide shallow subsurface oases capable of sustaining life. We know that on Earth, subsurface environments are a haven for a wide variety of heterotrophic microorganisms that do not necessarily require a direct connection to the surface environment for their survival. During the upcoming decade of exploration, we could initiate systematic orbital searches for such "oases" using high spatial resolution multispectral remote sensing to explore for anomalous concentrations of water vapor, methane, or other reduced gases, as well as thermal anomalies suggestive of near-surface hydrothermal systems or fumeroles. This kind of exploration would be especially interesting if targeted at low elevation areas where atmospheric density is higher and where crustal thinning is likely to have occurred, increasing the heat flow to the surface (e.g., the floors of chasmata).
Given the present technological challenges of deep drilling, robotic platforms are likely to provide very limited access to the Martian subsurface. As presently envisioned, rover-based drilling systems are unlikely to penetrate much deeper than a few meters. The implementation of a subsurface exploration program to search for extant Martian life could require drilling to kilometer depths. Drilling to such depths will quite likely require a human presence. It follows that extensive exploration of the Martian subsurface to search for deep subsurface water and an extant microbiota may provide the most compelling reasons for carrying out human missions to Mars.
A fundamental issue facing the scientific community and public at large is planetary protection. We are embarking upon a new decade of Mars exploration with a clearly identified objective of sample return. Aside from the concerns associated with forward contamination of Mars, particularly in connection with missions aimed at detecting extant life, the threat of back-contamination raises issues of broader concern. These issues have yet to be fully addressed, and concerns over such things as the reliability of sample containment technologies, the effects of sterilization on the science return of missions, and the added costs to missions must be clearly understood and transformed into an effective policy. Planetary protection issues could be a sleeping giant that, once awakened, could dictate the future of Mars exploration.
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Last updated Feb-10-1997
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