interiors (MER rovers utilized a rock abrasion tool) are required. It is also possible that weathering could produce chemical gradients at rock surfaces that might actually be exploited by microorganisms, and so examination before and after grinding or drilling may be desirable.
Global-scale observations by orbiting spacecraft also have a role in the geological exploration of local sites. As an example, orbital imagery of channels was instrumental in interpretations of the geological history of the Mars Pathfinder landing site. As another, chemical biomarkers must be detected in the context of the global environment. The 13C biomarker is a good indicator of life, but only if nonbiological fractionations in 13C in the carbon dioxide background are known (see Chapter 3).
Determining the geological and environmental history of a local site provides a critical filter for assessing the plausibility of life at that site. Remote sensing by instruments on rovers and orbiters can provide adequate characterization in most cases, although advances in the identification of minerals and measurement of trace elements and isotopes (which constrain environments) and in geochronology (which constrains geological history) are needed.
From the lessons learned from ALH 84001 and the early life on Earth debate (see Chapter 2), a multi-instrument, multi-measurement strategy must be used in robotic exploration and analyses to be able to make statements about the presence of biomarkers with any confidence. Although the focus of this section is in situ measurements, the same capabilities apply to the collection and analysis of samples returned from Mars. In situ astrobiological instruments must provide for the following tasks:
Acquire appropriate samples,
Understand the context,
Identify the best place on the sample for further analysis, and
Perform a number of mutually confirming independent measurements.
Accomplishing these tasks requires that any mission be able to identify suitable samples from a distance; perform contact instrument analysis to confirm suitability of the sample for further investigation (sometimes called sample triage); and utilize a suite of instruments to analyze a selected portion of the sample, either in situ on Mars or in terrestrial laboratories on returned samples. These instruments should be capable of making the following observations and measurements:
Comprehensive imaging. Image each investigation scene to assess the variety of local environments expressed in surface features such as outcrops.
Definitive mineralogy and chemistry. Determine mineralogical and chemical (elemental) composition at all scales of investigation: site/scene surface reconnaissance scale (range: infinity/horizon to meter; resolution: kilometer to centimeter); hand-sample scale (range: meter to centimeter; resolution: centimeter to millimeter); and acquired subsample scale (bulk measurement of a few grams or milligrams of subsample with high accuracy).
Redox potential. Assess the redox potential and oxidation chemistry of materials in the near-surface environment.
Fine-scale surface analyses. Investigate the surfaces of selected exposed or acquired samples at fine scales for morphological, chemical, and molecular signatures suggesting preservation of prebiotic or biotic organic compounds. This may include directly detected compositional markers, evidence of minerals formed in or altered by liquid water, or particular sample textures. Color optical (microscopic) imaging with a high resolution should also be used to provide context for any co-focused spectroscopic tools such as ultraviolet-excitation fluorescence, laser Raman, or other fine-scale techniques to perform chemical signature detection.