forward by NASA in response to the Vision focused on astrobiology. For Mars, the objective was to “[c]onduct robotic exploration of Mars to search for evidence of life, to understand the history of the solar system, and to prepare for future human exploration,” and the goals across the solar system all had astrobiology objectives at their center.6 Irrespective of the central roles that astrobiology, in general, and the search for life on Mars, in particular, appear to play in NASA’s enunciated goals, budgetary decisions made in 2006 seemed to undercut this commitment.7 Fortunately, events in 2007 suggest that the downward trend in NASA’s astrobiology spending may have been reversed.

At approximately the same time as these policies were being developed, the National Research Council’s (NRC’s) decadal survey for solar system exploration emphasized that astrobiology goals, related to understanding the habitability of the planets and satellites and determining the distribution of life in our solar system, should be considered central to the underlying rationale for solar system exploration.8 The missions identified by the decadal survey as addressing high-priority science goals, although not identified as astrobiology missions per se, all addressed key astrobiology goals.

The closest preceding document related to a Mars astrobiology strategy was the 1995 NASA report titled An Exobiological Strategy for Mars Exploration.9 That report laid out the scientific objectives for martian astrobiology and identified the then-key points for implementing a successful astrobiology strategy. The basic approach outlined in that document was to carry out the exploration of Mars in discrete phases, each providing an increasingly detailed look at the planet. The phases described were the following:

  1. Global reconnaissance in order to understand global processes and to identify sites for detailed in situ investigations;

  2. Exploration of particular sites in detail using landed packages in order to understand their geology and history;

  3. Deployment of astrobiologically relevant instruments onto the surface that focus on prebiotic chemistry, past life, and/or present life;

  4. Robotic return of samples of the surface back to Earth for detailed analysis; and

  5. Human exploration of the martian surface.

Remarkably, the Mars Exploration Program as actually implemented in the last decade closely follows the first three exploration phases recommended in the 1995 NASA report. Global reconnaissance has been carried out by Mars Global Surveyor, Mars Odyssey, and, now, Mars Reconnaissance Orbiter. The combination of high-resolution imaging; multispectral mapping in the visible, near-infrared, and thermal infrared; compositional mapping using gamma-ray and neutron techniques; and radar mapping of the subsurface has provided detailed information on the geological history of Mars and on processes that pertain to the potential for liquid water and for life. In situ analysis by the Mars Exploration Rovers Spirit and Opportunity investigated two sites for which there was evidence for significant water-related activity and confirmed that liquid water played an important role at both. Two missions planned for the rest of this decade, the 2007 Phoenix lander that will investigate the geology and chemistry of high-latitude ground ice and the 2009 Mars Science Laboratory that will investigate astrobiologically relevant chemistry and climate behavior, are both missions that address fundamental astrobiology science objectives.

It is in this context that the present scientific strategy for the astrobiological exploration of Mars has been formulated. Although the science issues had been discussed previously as components of prior NRC reports,1012 this is the first time that an integrated astrobiology strategy has been constructed for Mars by the NRC. The construction of such a strategy recognizes the increased scientific importance of astrobiology, and of astrobiology for Mars in particular, within the overall space science community. Not only does Mars arguably have the best chance in the solar system (other than Earth) of having or having had life, but it is also the most accessible of the bodies that are important to the astrobiological study of the solar system.

All of these considerations have informed the committee’s effort to describe the science objectives for the subsequent astrobiological investigation of Mars. Although science objectives are emphasized in the committee’s strategy, they cannot be developed in isolation from knowledge of what instruments are or might be available to

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