and might later cause false-positives during subsequent biological examination in a terrestrial laboratory. Yet the necessary bioload-reduction procedures are likely to be similar to those that would be required if the same sample acquisition and handling system was processing materials for an in situ detection experiment.
NASA’s 1995 report An Exobiological Strategy for Mars Exploration advocated sample return only after Mars had been thoroughly studied at global and regional scales, and following detailed local investigations that would ensure that any returned samples would have astrobiological relevance. NASA has implemented those recommendations in its exploration planning, with the unfortunate result that Mars sample return has been repeatedly pushed to (or beyond) the end of any planning cycle. The committee suggests that the strategy advocated by the 1995 report should not delay sample caching at multiple sites; NASA should cache samples at every opportunity, and return the most interesting collection as expeditiously as possible. The scientific advantages for astrobiology of a diverse collection of rocks and soils returned to Earth from any promising site outweigh the promise of just the “right” sample at some time in the indefinite future.
This is not to say that continued characterization of the global environment and of regional and local environments is not critical to the selection of astrobiologically relevant sites. An understanding of the regional geological context of the samples at any particular site is essential for interpreting their formation histories and thus the martian paleoenvironments represented by these samples. The synergy resulting from ground-based observations and measurements, coupled with orbital data (sometimes obtained synchronously), has proven critical in investigations by the twin rovers of the MER mission at Meridiani Planum and the Gusev crater. The current suite of orbiting spacecraft at Mars (NASA’s Mars Odyssey and Mars Reconnaissance Orbiter and ESA’s Mars Express) have already provided and will continue to supply global and regional mapping data for Mars at unprecedented resolution. As is evident from the ongoing site selection process for the 2009 Mars Science Laboratory, there are numerous promising candidates for sites of astrobiological interest on Mars based on the available mapping.
The search for extraterrestrial life begins here on Earth, with studies of both ancient rocks containing the earliest traces of terran life, and modern systems that serve as analogs for conditions and environments present on modern-day and ancient Mars. Studies of these Earth analog systems and also other Earth environments characterized by conditions of extreme temperature, pH, radiation, salinity, water activity, and so on, have greatly expanded the known range of environments that harbor life and the metabolic pathways and conditions under which life can thrive. Earth analog studies are primary drivers of astrobiological research and are also crucial for defining zones of habitability and targets for astrobiological exploration in extraterrestrial systems. However, it must be emphasized that terrestrial locations can only be used as analogs of various aspects of Mars, not as perfect analogs of an entire martian environment. Thus, when using a terrestrial analog, it is important to understand what might be similar and what might be different, and to ensure that the differences do not affect the analysis for which the analog is being used.
Studies of Earth analog sites should continue to be a fundamental aspect of Mars astrobiological research, because they provide several critical functions:
Provide Mars-like environments for testing and development of instrumentation (e.g., via investigations supported by NASA’s Astrobiology Science and Technology for Exploring Planets and Astrobiology Science and Technology Instrument Development programs) to be used on missions and a testbed for crucial real-time problem solving during missions (e.g., recent MER mobility issues).
Allow testing of sample-collection and sample-handling protocols under Mars-like conditions, an often overlooked but critical aspect of Mars astrobiology exploration.
Provide environments for development and validation of biosignature techniques, including establishing baseline abiotic signatures and assessment of biomarker preservation potential and alteration.
Allow discovery of novel organisms and metabolisms and the chemical/isotopic imprints of these metabolisms on Mars-like environments. Recently identified metabolisms—not all necessarily from Earth analog sites—include nitrate-Fe respiration, H2O radiolysis, and ammoxidation-PO3– respiration.