Science Priorities for Mars Sample Return
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I.EXECUTIVE SUMMARYThe return of Martian samples to Earth has long been recognized to be an essential component of a cycle of exploration that begins with orbital reconnaissance and in situ surface investigations. Major questions about life, climate and geology require answers from state-of-the-art laboratories on Earth. Spacecraft instrumentation cannot perform critical measurements such as precise radiometric age dating, sophisticated stable isotopic analyses and definitive life-detection assays. Returned sample studies could respond radically to unexpected findings, and returned materials could be archived for study by future investigators with even more capable laboratories. Unlike Martian meteorites, returned samples could be acquired with known context from selected sites on Mars according to the prioritized exploration goals and objectives. The ND-MSR-SAG formulated the following 11 high-level scientific objectives that indicate how a balanced program of ongoing MSR missions could help to achieve the objectives and investigations described by MEPAG (2006).
MSR would attain its greatest value if samples are collected as sample suites that represent the diversity of the products of various planetary processes. Sedimentary materials likely contain complex mixtures of chemical precipitates, volcaniclastics, impact glass, igneous rock fragments, and phyllosilicates. Aqueous sedimentary deposits are important for performing measurements of life detection, observations of critical mineralogy and geochemical patterns and trapped gases. On Earth, hydrothermally altered rocks can preserve a record of hydrothermal systems that provided water, nutrients and chemical energy necessary to sustain microorganisms and also might have preserved fossils in their mineral deposits. Hydrothermal processes alter the mineralogy of crustal rocks and inject CO2 and reduced gases into the atmosphere. Chemical alteration occurring at near-surface ambient conditions (typically < ~20°C) create low temperature altered rocks and includes, among other things, aqueous weathering and various nonaqueous oxidation reactions. Understanding the conditions under which alteration proceeds at low temperatures would provide important insight into the near-surface hydrological cycle, including fluid/rock ratios, fluid compositions (chemical and isotopic, as well as redox conditions), and mass fluxes of volatile compounds. Igneous rocks are expected to be primarily lavas and shallow intrusive rocks of basaltic composition. They are critical for investigations of the geologic evolution of the Martian surface and interior because their geochemical and isotopic compositions constrain both the composition of mantle sources and the processes that affected magmas during generation, ascent, and emplacement. Regolith samples (unconsolidated surface materials) record interactions between crust and atmosphere, the nature of rock fragments, fine particles that have been moved over the surface, exchange of H2O and CO2 between near-surface solid materials and the atmosphere, and processes involving fluids and sublimation. Regolith studies would help facilitate future human exploration by assessing toxicity and potential resources. Polar ices would constrain present and past climatic conditions and help elucidate water cycling. Surface ice samples from the Polar Layered Deposits or seasonal frost deposits would help to quantify surface/atmosphere interactions. Short cores could help to resolve recent climate variability. Atmospheric gas samples would constrain the composition of the atmosphere and processes that influenced its origin and evolution. Trace organic gases (e.g., methane and ethane) could be analyzed for abundances, distribution, and relationships to a potential Martian biosphere. Returned atmospheric samples containing Ne, Kr, CO2, CH4 and C2H6 would confer major scientific benefits. Chemical and mineralogical analyses of Martian dust would help to elucidate the weathering and alteration history of Mars. Given the global homogeneity of Martian dust, a single sample is likely to be representative of the planet. A depth-resolved suite of samples should be obtained from depths ranging from cm to several m within regolith or from rock outcrop in order to investigate trends in the abundance of oxidants (e.g., OH, HO2, H2O2 and peroxy radicals) the effects of radiation, and the preservation of organic matter. Other sample suites include impact breccias that might sample rock types that are otherwise not available locally, tephra consisting of fine-grained regolith material or layers and beds possibly delivered from beyond the landing site, and meteorites whose alteration history could provide insights into Martian climatic history. The following factors would affect our ability to achieve MSR’s science objectives. 1. Sample size. A full program of science investigations would likely require samples of >8 g for bedrock, loose rocks and finer-grained regolith. To support required biohazard testing, each sample requires an additional 2 g, leading to an optimal size of 10 g. Textural studies of some rock types might require one or more larger samples of ~20 g. Material should remain to be archived for future investigations. 2. Number of samples. Studies of differences between samples could provide more information than detailed studies of a single sample. The number of samples needed to address MSR scientific objectives effectively is 35 (28 rock, 4 regolith, 1 dust, 2 gas), If the MSR mission recovers the MSL cache, it should also collect 26 additional samples (20 rock, 3 regolith, 1 dust and 2 atmospheric gas). The total mass of these samples is expected to be about 345 g (or 380 g with the MSL cache). The total returned mass with sample packaging would be about 700 g. 3. Sample encapsulation. To retain scientific value, returned samples must not commingle, each sample must be linked uniquely to its documented field context, and rocks should be protected against fragmentation during transport. A smaller number or mass of carefully managed samples is far more valuable than a larger number or mass of poorly managed samples. The encapsulation of at least some samples must retain any released volatile components. 4. Diversity of the returned collection. The diversity of returned samples must be commensurate with the diversity of rocks and regolith encountered. This guideline substantially influences landing site selection and rover operation protocols. It is scientifically acceptable for MSR to visit only a single site, but visiting two independent landing sites would be much more valuable. 5. In situ measurements for sample selection and documentation of field context. Relatively few samples can be returned from the vast array materials that the MSR rover will encounter, thus we must be able to choose wisely. At least three kinds of in situ observations are needed (color imaging, microscopic imaging, and mineralogy measurement), and possibly as many as five (also elemental analysis and reduced carbon analysis). No significant difference exists in the observations needed for sample selection vs. sample documentation. Revisiting a previously occupied site might result in a reduction in the number of instruments. 6. Surface operations. To collect the samples required by MSR objectives, the lander must have significant surface mobility and the capability to assess and sample the full diversity of materials. Depending on the geology of the site, at least 6 to 12 months of surface operation will be required in order to explore a site and to assess and collect a set of samples. 7. Sample acquisition system. This system must sample weathered exteriors and unweathered interiors of rocks, sample continuous stratigraphic sequences of outcrops that might vary in their hardness, relate the orientation of sample structures and textures to those in outcrop surfaces, bedding planes, stratigraphic sequences, and regional-scale structures, and maintain the structural integrity of samples. A mini-corer and a scoop are the most important collection tools. A gas compressor and a drill have lower priority but are needed for certain samples. 8. Sample temperature. Some key species (e.g., organics, sulfates, chlorides, clays, ice, and liquid water) are sensitive to temperatures above surface temperatures. Objectives could most confidently be met if samples are kept below -20oC, and with less confidence if they are below +20oC. Significant loss, particularly to biological studies, occurs if samples reach +50oC for 3 hours. Temperature monitoring during return would allow any changes to be evaluated. 9. Planning considerations involving the MSL/ExoMars caches. Retrieving the MSL or ExoMars cache might alter other aspects of the MSR mission. However, given the limitations of the MSL cache, differences in planetary protection requirements for MSL and MSR, the possibility that the cache might not be retrievable, and the potential for MSR to make its own discoveries, the MSR rover should be able to characterize and collect at least some of returned samples. 10. Planetary protection. A scientifically compelling first MSR mission does not require the capability to access and sample a special region, defined as a region within which terrestrial organisms may propagate. Unless MSR could land pole-ward of 30° latitude, access rough terrain, or achieve significant subsurface penetration (>5 m), MSR is unlikely to be able to use incremental special regions capabilities. Planetary protection draft test protocols should be updated to incorporate advances in biohazard analytical methods. Statistical principles governing mass requirements for sub-sampling returned samples for these analyses should be re-assessed. 11. Contamination control. Inorganic and organic contamination must be minimized in order to achieve MSR science objectives. A study is needed to specify sample cleanliness thresholds that must be attained during sample acquisition and processing.
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