Science priorities for Mars sample return

THE RETURN OF MARTIAN SAMPLES TO EARTH has long been recognized as 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). (1) Determine the chemical, mineralogical, and isotopic composition of the crustal reservoirs of carbon, nitrogen, sulfur, and other elements with which they have interacted and characterize carbon-, nitrogen-, and sulfur-bearing phases down to submicron spatial scales in order to document processes that could sustain habitable environments on Mars both today and in the past. (2) Assess the evidence for prebiotic processes, past life, and extant life on Mars by characterizing the signatures of these phenomena in the form of structure/morphology, biominerals, organic molecular and isotopic compositions, and other evidence within their geologic contexts. (3) Interpret the conditions of martian water-rock interactions through the study of their mineral products. (4) Constrain the absolute ages of major martian crustal geologic processes, including sedimentation, diagenesis, volcanism/plutonism, regolith formation, hydrothermal alteration, weathering, and cratering. (5) Understand paleoenvironments and the history of nearsurface water on Mars by characterizing the clastic and chemical components, depositional processes, and postdepositional histories of sedimentary sequences. (6) Constrain the mechanism and timing of planetary accretion, differentiation, and the subsequent evolution of the martian crust, mantle, and core. (7) Determine how the martian regolith was formed and modified and how and why it differs from place to place. (8) Characterize the risks to future human explorers in the areas of biohazards, material toxicity, and dust/granular materials and contribute to the assessment of potential in situ resources to aid in establishing a human presence on Mars. (9) For the present-day martian surface and accessible shallow subsurface environments, determine the preservation potential for the chemical signatures of extant life and prebiotic chemistry by evaluating the state of oxidation as a function of depth, permeability, and other factors. (10) Interpret the initial composition of the martian atmosphere, the rates and processes of atmospheric loss/gain over geologic time, and the rates and processes of atmospheric exchange with surface condensed species. (11) For martian climate-modulated polar deposits, determine their age, geochemistry, conditions of formation, and evolution through the detailed examination of the composition of water, CO₂, and dust constituents, as well as isotopic ratios and detailed stratigraphy of the upper layers of the surface. 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. They 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 that occurs at nearsurface ambient conditions (typically < ∼20°C) creates 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 that involve 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 (PLD) 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 that contain 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 that range from cm to several m within regolith or from rock outcrop 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 scientific objectives: (1) Sample size. A full program of scientific 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 2 independent landing sites would be much more valuable. (5) In situ measurements for sample selection and documentation of field context. Relatively few samples could be returned from the vast array of materials the MSR rover would encounter; thus we must be able to choose wisely. At least 3 kinds of in situ observations are needed (color imaging, microscopic imaging, and mineralogy measurement) and possibly as many as 5 (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.