Planning for mars returned sample science: Final report of the MSR end-to-end international science analysis group (E2E-iSAG)

Returning samples from Mars to Earth for scientific analysis has been, and continues to be, among the highest-priority objectives of planetary science. Partly for this reason, the 2011 Planetary Science Decadal Survey placed high priority on a proposed 2018 rover mission that would conduct careful in situ science and use that scientific information to select and cache samples that could be returned to Earth by a potential future mission. To ensure that the potential contributions of the 2018 rover to the proposed Mars Sample Return (MSR) Campaign are properly planned, this study was undertaken to consider the science of the MSR Campaign concept from end to end. This white paper is the principal output of the MSR End-to-End International Science Analysis Group (E2E-iSAG): a group chartered by the Mars Exploration Program Analysis Group (MEPAG). We have built upon previous MEPAG and National Research Council (NRC) studies to consolidate and prioritize science objectives for a potential MSR campaign. Considering those objectives, we evaluated the implications for accessing, selecting, obtaining, and caching suitable samples on Mars during the proposed 2018 in situ science rover mission. Key issues addressed include the types of material needed (rock, regolith, gas), the number and character of samples and sample suites, the resulting sample mass, the in situ science measurements needed to establish the geological context of the samples, and the types of landing sites on Mars that could provide the diverse materials needed to meet the science objectives. As one of the key inputs to this analysis, we also evaluated the range of likely analytical investigations that would be carried out on the returned samples. In developing science objectives and priorities for MSR, the E2E-iSAG identified four overarching science themes or Aims: (A) Life and its organic chemical precursors, (B) Surface materials and the record of martian surface processes, (C) Planetary evolution of Mars and its atmosphere, and (D) Potential for future human exploration. Within these Aim categories, eight specific scientific objectives were defined that could be addressed through the analysis of returned materials. Using criteria based on the value of increased knowledge that could be gained by analyzing returned samples, we placed the eight objectives in priority order as follows: (1) Critically assess any evidence for past life or its chemical precursors, and place detailed constraints on the past habitability and the potential for preservation of the signs of life. (2) Quantitatively constrain the age, context, and processes of accretion; early differentiation; and magmatic and magnetic history of Mars. (3) Reconstruct the history of surface and near-surface processes involving water. (4) Constrain the magnitude, nature, timing, and origin of past planet-wide climate change. (5) Assess potential environmental hazards to future human exploration. (6) Assess the history and significance of surface modifying processes, including, but not limited to, impact, photochemical, volcanic, and aeolian processes. (7) Constrain the origin and evolution of the martian atmosphere, accounting for its elemental and isotopic composition with all inert species. (8) Evaluate potential critical resources for future human explorers. In addition, evaluating the possibility of extant life in all returned samples would be important, both to meet planetary protection requirements and because of the intrinsic scientific interest. However, we felt that there would be no logical way to implement the search for extant life as a primary mission objective that would be expected to be achieved. Returned sample types most likely to achieve the objectives described above are, in priority order; (1A) Suites of subaqueous or hydrothermal sediments (equal priority), (1B) Suites of hydrothermally altered rocks or lowtemperature fluid-altered rocks (equal priority), (2) Suite of unaltered igneous rocks, (3) At least one and preferably two or more samples of regolith, including airfall dust, obtained some distance from any landing site contamination and preferably including a subsurface sample, (4) At least one and preferably two aliquots of presentday atmosphere and samples of sedimentary-igneous rocks containing ancient trapped atmosphere. The E2E-iSAG found that the value of returned sample science is dependent on the quality of in situ science. Particularly to address the higher-priority science objectives, sample suites would need to be collected from a site that has been well characterized through a campaign of in situ field science. The goal of site characterization would be the establishment of geological context so that the relationship of samples to each other, and to their surroundings, could be understood. This information would ensure that only the best samples would be returned to Earth and that measurements made on Earth could be confidently interpreted and lead to the most significant discoveries. To obtain the required context, previous experience demonstrates the need for integrated observations ranging from macroscopic (i.e., regional, outcrop) down to microscopic (i.e., submillimeter) scales. Experience from terrestrial studies and the Mars Exploration Rovers (MERs) further demonstrates the need to evaluate many more rocks and soils than are eventually collected (by several orders of magnitude) and also the need to remove dust and weathering products from rock surfaces in order to interpret the rocks correctly. This characterization would require in situ measurements from outcrops and soils across the areas of interest as well as the precise locations of the samples selected; thus, a suite of scientific instruments and supporting capabilities on the sample-collecting rover would be needed. To achieve the proposed science objectives, the total number of rock samples should be *30. To prepare for new discoveries during surface operations, a capability to exchange ‡ 25% of earlier collected samples with later collected samples would add valuable scientific flexibility. The 2m ESA drill would provide unique sampling opportunities from the unexplored subsurface, with its enhanced likelihood of preservation of organics; accordingly, obtaining these samples is also highly desirable. To evaluate the size of individual samples needed to meet science objectives, the E2E-iSAG reviewed various analytical methods likely to be applied to returned samples by preliminary examination teams, for planetary protection (i.e., life detection, biohazard assessment), and by principal investigators. The E2E-iSAG concluded that samples should be sized so that all high-priority analyses could be done in triplicate and that at least 40% of each sample should be preserved for future scientific investigations, consistent with standard curatorial practice of extraterrestrial materials. Samples sized at 15-16 g would be optimal, and containers designed to accommodate sedimentary and igneous rocks of this mass would also be sufficient for regolith samples. Total mass of returned rocks, soils, blanks, and standards should be *500 g. To achieve all high-priority objectives related to an atmospheric gas sample, it should be sized at the equivalent of 50cm3 at Mars ambient atmospheric conditions (which is equivalent to 5 cm3 with 10 • compression). To preserve acceptable sample quality during storage on Mars-perhaps for many years and to transport the cache to Earth, the sample containers would need to be sealed. The critical volatile component to be considered in devising containment is structural and adsorbed water that may be present in some samples. Accordingly, individual sample tubes would require some level of sealing during storage on the martian surface. It would also be scientifically desirable to seal the entire sample canister before leaving Mars to avoid a significant pressure differential across sample tube seals during transit and thus minimize volatile mobility. Finding sites that would contain the desired samples and also be safe to land on is challenging. To overcome this challenge, it may be necessary to have sufficient mobility to explore outside landing ellipses and the capability to avoid or tolerate certain hazards during entry, descent, and landing (EDL) so that the ellipse may include rocky materials needed to address the science objectives. The E2E-iSAG formulated a reference landing site set to (1) demonstrate the ability to find sites that in principle could achieve the highest-priority science objectives and (2) provide environmental conditions to allow engineering planning to take place. The E2E-iSAG evaluated 85 sites previously proposed by the Mars science community. Threshold criteria, based on finding the materials that could address the science objectives and sampling priorities, were applied, and at least 10 sites that address most of the objectives were identified. Of these, seven were selected as ''reference sites'' because they have a range of properties that would help engineers define landing and roving capabilities and because they already have sufficient imaging to conduct terrain evaluation. In due course, a call for landing site proposals would be made to initiate a comprehensive site selection process similar to those employed for MER and Mars Science Laboratory (MSL).