Science Priorities for Mars Sample Return
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II.INTRODUCTIONSince the dawn of the modern era of Mars exploration, the return of Martian samples to Earth has been recognized as an essential component of a cycle of exploration that began with orbital reconnaissance and in situ surface investigations (see, for example, the discussion of sample return in three decades of reports by the National Research Council: e.g. NRC, 1978; 1990a, 1990b, 1994; 1996; 2001; 2007). Global reconnaissance and surface observations have “followed the water” and revealed a geologically diverse Martian crust that could have sustained near-surface habitable environments in the distant past. However, major questions about life, climate, and geology remain, and many of these require answers that only Earth-based state-of-the-art analyses of samples could provide. The stems from the fact that flight instruments cannot match the adaptability, array of sample preparation procedures, and micro-analytical capability of Earth-based laboratories (Gooding et al., 1989). For example, analyses conducted at the submicron scale were crucial for investigating the ALH84001 meteorite, and they would be essential for interpreting the returned samples. Furthermore, spacecraft instrumentation simply cannot perform certain critical measurements, such as, precise radiometric age dating, sophisticated stable isotopic analyses, and comprehensive life-detection experiments. If returned samples yield unexpected findings, subsequent investigations could be adapted accordingly. Moreover, potions of returned samples could be archived for study by future generations of investigators using ever more powerful instrumentation. Some samples from Mars are available for research on Earth in the form of the Martian meteorites. The Martian meteorites, while indeed valuable, provide a limited view of Martian geologic processes. These samples are all igneous in nature, and minimally altered and thus do not record the history of low temperature water based processes. These samples certainly do not represent the most promising habitable environments (Gooding et al., 1989), and it is possible that the most extensively water-altered materials might be too fragile to survive an interplanetary journey. Most meteorites have young crystallization ages less than 1.3 billion years indicating that they represent only the most recent igneous activity on Mars (Borg and Drake, 2005). Their geochemical characteristics suggest that they are closely related to one another and are consequently not representative of all of the lithologic and geochemical diversity that is likely to be present in igneous Martian rock suite (Borg and Draper, 2003; Borg et al., 2003; Symes et al., 2008). Because the meteorites arrived by natural processes, and lack geologic context, it is extremely difficult to extrapolate the results from geologic studies of these samples to rocks observed from space or on the Martian surface by landed spacecraft. In contrast, returned samples could be obtained from sites within a known geologic context and be selected in order to achieve the goals and objectives of the Mars exploration community. Nevertheless, sample return missions must surmount key challenges such as, engineering complexity, cost, and planetary protection concerns, before their enormous potential could be recognized. This document is intended to define this critical step forward toward realizing the enormous potential of Mars sample return. On July 10, 2007, Dr. Alan Stern, Associate Administrator for the Science Mission Directorate (SMD), described to the participants in the 7th International Conference on Mars his vision of achieving Mars Sample Return (MSR) no later than the 2020 launch opportunity. He requested that the financial attributes, scientific options/issues/concerns, and technology development planning/budgeting details of this vision be analyzed over the next year. The Mars Exploration Program Analysis Group (MEPAG) is contributing to this effort by preparing this analysis of the science components of MSR and its programmatic context. To this end, MEPAG chartered the Next Decade MSR Science Analysis Group (ND-MSR-SAG) to complete four specific tasks: (1) Analyze what critical Mars science could be accomplished in conjunction with, and complementary to, a next decade MSR mission. (2) Evaluate the science priorities associated with guiding the makeup of the sample collection to be returned by MSR. (3) Determine the dependencies of mobility and surface lifetime of MSR on the scientific objectives, sample acquisition capability, diagnostic instrument complement, and number and type of samples. (4) Support MSR science planning as requested by the International Mars Exploration Working Group (IMEWG) MSR study. The charter is presented in Appendix I. The return of any reasonable sample mass from Mars would significantly increase our understanding of atmospheric, biologic, and geologic processes occurring there, as well as permit evaluation of the hazards to humans on the surface. This is largely independent of how the samples are selected, collected, and packaged for return, and stems from the fact that there are no analogous samples on Earth. Thus, a mission architecture in which a limited number of surface samples are collected in a minimum amount of geologic context has been recommended in the past and has huge scientific merit (e.g., MacPherson et al., 2005). It is also important to realize that a significantly greater scientific yield would result from samples that are more carefully selected. Analytical results from samples that are screened, placed in detailed geologic context, collected from numerous locations and environments, and are packaged and transported under conditions that more closely approximate those encountered on the Martian surface, would dramatically clarify the picture of Mars derived from the mission, as well as allow analytical results to be more rigorously extrapolated to the planet as a whole. As a consequence of these facts, this document outlines a sampling strategy that is necessary to maximize scientific yield. The inability to complete all of the surface operations associated with this sampling strategy by no means negates the usefulness of these samples. Rather, it results in a proportional loss of science yield of the mission. Thus, this study is expected to constitute input to a Mars program architecture trade analysis between scientific yield and cost. |