Science Priorities for Mars Sample Return

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III.EVALUATION PROCESS

Prior to beginning this study, the ND-SAG was briefed on the conclusions of the NASA Mars Sample Return Science Steering Group II (MacPherson et al., 2005; Appendix III) and the NRC Committee on an Astrobiology Strategy for the Exploration of Mars. These reports document the importance of sample return in a complete strategy for the exploration of Mars, and many of their conclusions are reiterated here. However, the current analysis has benefited from discoveries made in the interval since these reports were written, such as phyllosilicates, silica, and the distribution and context of poly-hydrated sulfates on the surface of Mars. It is expected that some of the conclusions of this report will be further elucidated and/or strengthened as results from Phoenix, MSL, and ExoMars become available. This may be particularly true of the results from analyses of organic matter and ices.

Assumptions used in this study are:

(1) The sample return mission would begin in either 2018 or 2020.

(2) MSL will launch in 2009, and will prepare a rudimentary cache of samples that would be recoverable by the MSR mission. ExoMars would carry a similar cache.

(3) The functionality of sample acquisition associated with MSR would be independent of MSL. This functionality may either be landed at the same time as the sample return element of MSR, or it may be separated into a precursor mission.

(4) The Mars Exploration Program would maintain a stable program budget of about $625M/year that grows at 2%/year.

In order to complete these tasks and to link strongly the report of the ND-SAG to the MEPAG Goals document, the ND-SAG was divided into four subteams corresponding to each of the four main MEPAG goals. The goals, as outlined in the Goals document, are: determine if life ever arose on Mars, understand the processes and history of climate on Mars, determine the evolution of the surface and interior of Mars, and prepare for human exploration. Each group examined the individual investigations outlined in the MEPAG Goals Document and considered the following:

Whether sample return would facilitate the investigation.
The type, mass, number, and diversity of samples that would be required to complete the investigation.
The physical condition of the samples (rock, pulverized rock, etc.).
The vulnerability of specific sample types to degradation effects during sample collection, encapsulation, and transport, as well as the impact of this degradation on individual investigations.
The measurements required at the time of sample collection in order to select appropriate samples and place them in the necessary geologic context.
The mobility necessary to obtain required samples.
The packaging and handling priorities necessary to preserve the characteristics of interest in the samples.

The results of this analysis are presented in detail in Appendix II. Below we summarize the consensus of the ND-SAG that was derived from this analysis.

IV.SCIENTIFIC OBJECTIVES OF MSR

IV.A.History, Current Context of MSR’s scientific objectives

The 2003/2005 Mars Sample Return mission (which was cancelled in 2000, prior to launch) was the most recent effort that formulated scientific objectives for MSR. The way this mission chose to frame its scientific objectives are listed in Table 1. Since 2000, there have been numerous scientific advances that have greatly increased our understanding of the red planet. It is critical to take these into consideration in setting the new scientific objectives for MSR. In particular, it is important to incorporate actual or anticipated results from the following:

Recent and on-going flight missions

Since the last MSR analysis in 2000, the Mars Global Surveyor (1999-2006), Mars Odyssey (2002-present), Mars Exploration Rovers (2004-present), Mars Express (mapping from 2004-present), and the Mars Reconnaissance Orbiter (mapping from 2006-present) have made important discoveries. These investigations have greatly improved our understanding of Mars and have resulted in progressive refinement of key Martian scientific objectives, as documented by the evolution of the MEPAG Goals Document (MEPAG, 2001; MEPAG, 2004; MEPAG, 2005; MEPAG, 2006).

Future (but pre-MSR) flight missions

Two major missions to the Martian surface are scheduled during the next six years - the Mars Science Laboratory (MSL; scheduled for launch in 2009), and ExoMars (scheduled for launch in 2013). Both missions will analyze rock samples on the surface of Mars using in-situ methods. It is therefore necessary to consider the scientific objectives of these missions when planning the objectives of the first MSR mission, and to build upon their expected accomplishments. The scientific objectives of the MSL and ExoMars missions, as of 2007, are listed in Table 1.

Meteorite studies

More than 35 Martian meteorites have been found in Antarctica and desert environments by meteorite recovery programs, including private and government-sponsored efforts. The number of recovered meteorites continually increases. As a consequence, MSR science objectives and sample selection strategy must respond to scientific advances derived from meteorite studies and also strive to complement the existing meteorite collections.

Table 1 Scientific Objectives, ‘03/’05 MSR, 2009 MSL, and 2013 ExoMars (order listed as in the originals)

Sources of information: MSR: O’Neil and Cazaux (2000); MSL: http://mepag.jpl.nasa.gov/MSL_Science_Objectives.html (as of Jan. 7, 2008); ExoMars: Vago and Kiminek (2007)

IV.B.Possible Scientific Objectives for a Next Decade MSR

To translate the general statements about the possible value of MSR into specifics, in Appendix II, the ND-SAG analyzed how returned samples might contribute to each of the scientific objectives and investigations described by MEPAG (2006). The investigations listed in MEPAG (2006) do not have equal scientific priority, nor do they benefit equally from returned sample analyses. By considering the most important potential uses of returned samples, the ND-SAG has formulated eleven relatively high-level scientific objectives for MSR. However, we note that no single landing site could address all of these objectives. Those objectives that any single MSR mission could achieve would reflect the capabilities of its architecture/hardware and the geologic terrain and local climate of the site. Even though all of these objectives could not be achieved on the first MSR mission, it is ND-SAG’s hope that by making this analysis as complete as possible, it will set the scene for future MSR missions beyond the first one.

Prioritization of the science objectives

The ND-SAG team considered the relative priority of the possible objectives listed below, using the following prioritization criteria:

The investigation priority in the Goals Document (MEPAG, 2006). The analysis in Appendix II finds that returned samples could significantly advance 34 of the investigations identified by MEPAG (2006), and each of these investigations has been assigned a priority by MEPAG. The way in which these 34 investigations are consolidated into the 11 objective statements below is shown graphically in Appendix IV.
The impact of MSR on investigation(s) associated with these objectives. For thirteen of the 34 investigations, MSR would not only be expected to advance them more substantially than for the others, in some cases MSR is essential (shown by the color coding in Appendix IV).

However, the achievable degree of progress towards these scientific questions would also depend on the choice of landing site (and the kinds of samples that are available to be collected there), the capability of the engineering system (e.g. number and quality of samples), the degree of complexity of the geologic process under study (and how many samples it might take to evaluate it), and other factors. For example, Objective #5 involves processes that are very complex, and a quantum jump in our understanding may be difficult with only a few samples. However, the objective is clearly important, and we should let it help guide the engineering. For these reasons, the ND-SAG team felt it appropriate to list the scientific objectives below in only two general priority groups: the first five are considered high priority, and the last six are considered medium priority. These priorities will clearly need to be reconsidered as the specifics of MSR are refined.

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.

Discussion. A critical assessment of the habitability of past and present Martian environments must determine how the elemental building blocks of life have interacted with crustal and atmospheric processes (Des Marais et al., 2003). On Earth, such interactions have determined the bioavailability of these elements, the potential sources of biochemical energy, and the chemistry of aqueous environments (e.g., Konhauser, 2007). Earth-based investigations of Martian meteoritic minerals, textures and chemical composition at the sub-micron scale have yielded discoveries of their igneous volatiles, impact-related alteration, carbonates, organic carbon, atmospheric composition and the processes that shaped them. The search for extant live requires exploration of special regions (sites where life might be able to propagate) and thereby invokes stringent planetary protection protocols. These protocols are less stringent at sites other than special regions where the search for past life would target fossil biosignatures preserved in rocks. This objective is an extension of MSL Objectives 1 through 4 (Table 1), ExoMars Objectives 2 and 4 (Table 1), and MEPAG Objective I-A, which collectively address the habitability potential of Martian environments.

Assess the evidence for pre-biotic processes, past life, and/or 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.

Discussion. The MER mission demonstrated that habitable environments existed on Mars in the past and that their geologic deposits are accessible at the surface (Squyres and Knoll, 2005; Des Marais et al., 2007). The Mars Express Orbiter OMEGA IR spectrometer mapped aqueous minerals that formed during the Noachian (Bibring et al., 2005; Poulet et al., 2005). The upcoming MSL and ExoMars missions will be able to provide information about the habitability (past or present) of their specific landing sites at even greater detail. Although ExoMars is designed to search for traces of past and present life (it should also be able to detect prebiotic organic materials), experience with Martian meteorites and, more especially, microfossil-containing rocks from the early Earth, has shown that identifying traces of life reliably is extraordinarily difficult because: (1) microfossils are often very small in size and (2) the quantities of organic carbon in the rocks that are identifiable as biogenic or abiogenic are often very low (Westall and Southam, 2006). The reliable identification of mineral and chemical biosignatures typically requires some particular combination of sophisticated high-resolution analytical microscopes, mass spectrometers and other advanced instrumentation. The particular combination of instruments that are most appropriate and effective for a given sample is often determined by the initial analyses. Accordingly, sample measurements must be conducted on Earth because they require adaptability in the selection of advanced instrumentation. . Note that the specifics of how this objective is pursued will be highly dependent on landing site selection. The search for extant life will require that the rover meet planetary protection requirements for visiting a “special region.” The localities that are judged to be most prospective for evaluating prebiotic chemistry and fossil life might not be the most favorable for extant life. However, all returned samples will assuredly be evaluated for evidence of extant life, in part to fulfill planetary protection requirements, whether or not the samples were targeted for this purpose. This objective is an extension of MSL Objective 6 (Table 1), ExoMars Objective 1 (Table 1), and MEPAG Objectives I-A, I-B and I-C, which address habitability, pre-biotic chemistry and biosignatures.

Interpret the conditions of Martian water-rock interactions through the study of their mineral products.

Discussion. Both igneous and sedimentary rocks are susceptible to a broad range of water-rock interactions ranging from low-temperature weathering through hydrothermal interactions. These processes could operate from the surface to great depths within the Martian crust. Rocks and minerals affected by such processes are significant repositories of volatile light elements in the Martian crust, and they have also recorded evidence of climate and crustal processes, both past and present. The compositions and textures of rock and mineral assemblages frequently reveal the water to rock rations, fluid compositions and environmental conditions that created those assemblages (also discussed by MacPherson et al., 2001). A significant fraction of the key diagnostic information exists as rock textures, crystals and compositional heterogeneities at sub-micrometer to nanometer spatial scales. Textural relationships between mineral phases could help to determine the order of processes that have affected the rocks. This is key to determine, for example, whether a rock is of primary aqueous origin or alternatively was affected by water at some later time in its history. Accordingly, state-of-the art Earth-based laboratories are required to read the record of water-rock interactions and infer their significance for the geologic and climate history of Mars. This objective is an extension of the discoveries of MRO, MEX, and MER that there is an extensive history of ancient interaction between water and the Martian crust. Understanding these interactions over a broad range of spatial scales is critical for interpreting the hydrologic record and records of thermal and chemical environments. This objective is an extension of MSL Objectives 1, 2 and 8 (Table 1), ExoMars Objectives 2 and 4 (Table 1), and MEPAG Objectives I-A, II-A, III-A and IV-A.

Constrain the absolute ages of major Martian crustal geologic processes, including sedimentation, diagenesis, volcanism/plutonism, regolith formation, hydrothermal alteration, weathering, and cratering.

Discussion. Constraining the absolute ages of Martian rock-forming processes is an essential part of understanding Mars as a system. There are two aspects to this objective. First, dating individual flow units with known crater densities would provide a calibration of Martian cratering rates. This is critical for the interpretation of orbital data because crater chronology is the primary method for interpreting both relative and absolute ages of geologic units from orbit, and the method can be applied on a planetary scale. The scientific community has strongly advocated for the calibration of the crater chronology method since the inception of the Mars exploration program (MEPAG Investigation III-A-3). Second, we need to understand the timing of different geologic processes in the past as the planet has evolved in time and space. The suitability of the products of different geologic processes to the methods of radiometric geochronology depends on when the isotopic systems closed. Igneous rocks are by far the most useful (see summary in Borg and Drake, 2005). Constraints on low temperature processes, such as sedimentation, weathering, and diagenesis could be obtained most easily and definitively by finding sites that show discernable field relationships with datable igneous materials. For example, by determining the ages of igneous rocks that are interbedded with sedimentary rocks, the interval of time when the sediments were deposited could be constrained. In addition, the ages of secondary alteration of Martian meteorites have been measured with some success (Borg et al., 1999; Shih et al., 1998; 2002; Swindle et al., 2000). Accordingly, chemical precipitates formed during diagenesis, hydrothermal activity, and weathering may be datable using Ar-Ar, Rb-Sr and Sm-Nd chronometers. However, sophisticated Earth-based laboratories are required to perform these difficult measurements precisely, with multiple chronometers to provide an internal cross-check, and to reliably interpret the meanings of these ages. This objective is an extension of MSL Objective 1 (Table 1), ExoMars Objective 4 (Table 1), and MEPAG Objectives I-A, II-B, III-A and III-B, and has long been considered a major objective of MSR (e.g. MacPherson et al., 2001; 2002).

Understand paleoenvironments and the history of near-surface water on Mars by characterizing the clastic and chemical components, depositional processes, and post-depositional histories of sedimentary sequences.

Discussion. Experience with the Mars Exploration Rovers Spirit and Opportunity demonstrates that sedimentary rock sequences, which include a broad range of clastic and chemical constituents, are exposed and that sedimentary structures and bedding are preserved on the Martian surface. Discoveries by MRO and Mars Express further demonstrate the great extent and geological diversity of such deposits. Sedimentary rocks could retain high-resolution records of a planet’s geologic history and they could also preserve fossil biosignatures. As such, sedimentary sequences are among the targets being considered by MSL and ExoMars. Previous missions have also demonstrated that the sedimentologic and stratigraphic character of these sequences could be evaluated with great fidelity, comparable to that attained by similar studies on Earth (e.g., Squyres and Knoll, 2005; Squyres et al., 2007). The physical, chemical and isotopic characteristics of such sequences would reveal the diversity of environmental conditions of the Martian surface and subsurface before, during and after deposition. But much of the key diagnostic information in these sequences occurs as textures, minerals and patterns of chemical composition at the submicron scale. Future robotic missions might include microscopic imaging spectrometers to examine these features. However, definitive observations of such features probably will also require thin section petrography, SEM, TEM, and other sophisticated instrumentation available only in state-of-the-art Earth-based laboratories. This objective is an extension of MSL Objectives 1, 2 and 8 (Table 1), ExoMars Objectives 2 and 4 (Table 1), and MEPAG Objectives I-A, II-A, III-A and IV-A .

Constrain the mechanism and timing of planetary accretion, differentiation, and the subsequent evolution of the Martian crust, mantle, and core.

Discussion. Studies of Martian meteorites have provided a fascinating glimpse into the fundamental processes and timescales of accretion (e.g., Wadhwa, 2001; Borg et al., 2003; Symes et al., 2008; Shearer et al., 2008) and subsequent evolution of the crust, mantle, and core (e.g. Treiman, 1990; Shearer et al., 2008). Martian meteorites also record a history of fluid alteration as shown by the presence of microscopic clay and carbonate phases (e.g. Gooding et al. 1991, McKay et al. 1996, Bridges et al. 2001). Although the trace element and isotopic variability of the Martian meteorite suite far exceeds that observed in equivalent suites of basalts from Earth and Moon (Borg et al., 2003) the apparent diversity of igneous rocks identified by both orbital and surface missions far exceeds that of the meteorite collection. This implies that an extensive record of the differentiation and evolution of Mars has been preserved in igneous lithologies that have not been sampled. Samples returned from well-documented Martian terrains would provide a broader planetary context for the previous studies of Martian meteorites and also lead to significant insights into fundamental crustal processes beyond those revealed by the Martian meteorites. Key questions include the following: (1) When did the core, mantle, and crust first form? (2) What are the compositions of the Martian core, mantle, and crust? (3) What additional processes have modified the crust, mantle, and core and how have these reservoirs interacted through time? (4) What processes produced the most recent crust? (5) What is the evolutionary history of the Martian core and magnetic field? (6) How compositionally diverse are mantle reservoirs? (6) What are the thermal histories of the Martian crust and mantle and how have they constrained convective processes? (7) What is the nature of fluid-based alteration processes in the Martian crust? Coordinated studies of Martian meteorites and selected Martian samples involving detailed isotopic measurements in multiple isotopic systems, the study of microscopic textural features (melt inclusions, shock effects), and comparative petrology and geochemistry are needed to answer these questions definitively. These data will provide the basis for model ages of differentiation that are placed in the context of solar system evolution. They will also permit some of the compositional characteristics of crust, mantle, and core to be determined, which in turn will allow geologic interactions between these reservoirs to be evaluated, as well as their thermal histories to be elucidated. The tremendous value of this approach has been validated by geochemical studies on the returned lunar samples that have been more informative than any other means in deciphering the geologic history of the Moon. This objective is an extension of MSL Objective #1 (Table 1), ExoMars Objective #4 (Table 1), and MEPAG Objectives I-A, II-A, III-A and III-B and has long been considered a major objective of MSR (e.g. MacPherson et al., 2001; 2002).

Determine how the Martian regolith was formed and modified, and how and why it differs from place to place.

Discussion. The Martian regolith preserves a record of crustal, atmospheric and fluid processes. Regolith investigations would determine and characterize the important ongoing processes that have shaped the Martian crust and surface environment during its history. It is a combination of broken/disaggregated crustal rocks, impact-generated components (Schultz and Mustard, 2004), volcanic ash (Wilson and Head, 2007), oxidized compounds,, ice , aeolian deposits and meteorites. The Viking, Pathfinder and MER landers have also revealed diverse mineral assemblages within regolith that include hematite nodules, salt-rich duricrusts, and silica-rich deposits (e.g. Ruff et al. 2007; Wanke et al. 2001) that show local fluid-based alteration. The regolith contains fragments of local bedrock as well as debris that were transported regionally or even globally. These materials would accordingly provide local, regional and global contexts for geological and geochemical studies of the returned samples. Martian surface materials have also recorded their exposure to cosmic ray particles. Cosmic ray exposure ages obtained at Apollo landing sites have helped to date lunar impact craters (e.g. Eugster, 2003). Regolith returned from Mars should provide similar information that could in turn be used to constrain the absolute ages of local Martian terrains. An MSR objective would be to examine returned samples of regolith mineral assemblages in order to determine the abundances and movement of volatile-forming elements and any organic compounds in near-surface environments and to determine their crustal inventories. The abundance of ice in the regolith varies dramatically across the Martian surface. At high latitudes water ice attains abundances of tens of weight-percent below the top few tens of cm. Inventories of water ice at near equatorial latitudes are less understood but ice might occur below the top few cm (Feldman et al. 2004). The regolith is assumed to harbor large fraction of the Martian CO₂ and H₂O inventories but their abundance has not yet been accurately determined. This objective is an extension of MSL Objectives 1, 2, 3, 4, 6, 7, 8 (Table 1), ExoMars Objectives 1, 2 and 3 (Table 1), and MEPAG Objectives I-A, I-B, I-C, II-B, III-A and IV-A.

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.

Discussion. Returned samples could help to accomplish four tasks that are required to prepare for human exploration of Mars (see Appendix II). These tasks include: 1). Understanding the risks that granular materials at the Martian surface present to the landed hardware (Investigation IVA-1A), 2) Determining the risk associated with replicating biohazards (i.e., biological agents, Investigation IVA-1C), 3) Evaluating possible toxic effects of Martian dust on humans (Investigation IVA-2), and 4) Expanding knowledge of potential in-situ resources (Investigation IVA-1D). The human exploration community has consistently advocated that these tasks are essential for understanding the hazards and to plan the eventual human exploration of Mars at an acceptable level of risk (Davis, 1998; NRC, 2002; Jones et al., 2004). Regarding possible Martian biohazards, analyses of robotically returned Martian samples might be required before human missions could commence, in order to quantify their medical basis and to address concerns related to planetary protection from both a forward and back contamination perspective (Warmflash et al, 2007). This objective is an extension of MSL Objective 7 (Table 1), ExoMars Objective #3 (Table 1), and MEPAG Objective IV-A.

For the present-day Martian surface and accessible shallow subsurface environments, determine the preservation potential for the chemical signatures of extant life and pre-biotic chemistry by evaluating the state of oxidation as a function of depth, permeability, and other factors.

Discussion. The surface of Mars is oxidizing, but the composition and properties of the responsible oxidant(s) are unknown. Characterizing the reactivity of the near surface of Mars, including atmospheric (e.g. electrical discharges) and radiation processes as well as chemical processes with depth in the regolith and within weathered rocks is critical investigating in greater detail the nature and abundance of any organic carbon on the surface of Mars. Understanding the oxidation chemistry and the processes controlling its variations would aid in predicting subsurface habitability if no organics are found on the surface, and also in understanding how such oxidants might participate in redox reactions that could provide energy for life. Potential measurements include identifying species and concentrations of oxidants, characterizing the processes forming and destroying them, and characterizing concentrations and fluxes of redox-sensitive gases in the lower atmosphere. Measuring the redox states of natural materials is difficult and may require returned samples. This objective is an extension of MSL Objective 1, and 8 (Table 1), ExoMars Objective #2 (Table 1), and MEPAG Objectives I-A, III-A and IV-A.

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.

Discussion. The modern chemistry of the Martian atmosphere reflects the integration of three major processes, each of which is of major importance to understanding Mars: 1). The initial formation of the atmosphere, 2). The various processes that have resulted in additions or losses to the atmosphere over geologic time, and 3). The processes by which the atmosphere exchanges with various condensed phases in the upper crust (e.g., ice, hydrates and carbonates). Many different factors have affected the chemistry of the Martian atmosphere, however if the abundance and isotopic composition of its many chemical components could be measured with sufficient precision, definitive interpretations are possible. We have already gathered some information about Martian volatiles from isotopic measurements by Viking and on Martian meteorites (Owen et al., 1977; Bogard et al., 2001). In addition, MSL will have the capability to measure some, but not all, of the gas species of interest with good precision. This leaves two planning scenarios: If for some reason MSL does not deliver its expected data on gas chemistry, this scientific objective would become quite important for MSR. However, even if MSL is perfectly successful, it will not be able to measure all of the gas species of interest at the precision needed, so returning an atmosphere sample could still be an important scientific objective for MSR. This objective is an extension of MSL Objective 5 (Table 1) and MEPAG Objectives I-A, II-A, II-B, and III-A.

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, isotopic ratios, and detailed stratigraphy of the upper layers of the surface.

Discussion. The polar layered deposits represent a detailed record of recent Martian climate history. The composition of the topmost few meters of ice reflect the influence of meteorology, depositional episodes, and planetary orbital/axial modulation over the timescales of order 10⁵ to 10⁶ years (Milkovich and Head, 2005). This objective addresses the priorities of MEPAG Investigation IIB-5. Terrestrial ice cores have contributed fundamentally to interpreting Earth’s climate history. Similar measurements of Martian ices could be expected to reveal critical information about that planet’s climate history and its surface/atmosphere interactions (Petit et al., 1999; Hecht et al., 2006). The ability of ice to preserve organic compounds (and, potentially, organic biosignatures) may help address objectives associated with habitability and pre-biotic chemistry and life (MEPAG Goal 1; Christner et al., 2001). By exploring lateral and vertical stratigraphy of active ice layers and facilitating state-of-the-art analyses of returned materials, a rover-equipped sample return mission would significantly improve our understanding beyond what the Phoenix stationary lander is expected to achieve at its single high-latitude site. This objective is an extension of MEPAG Objectives I-A, II-A, II-B, and III-A.

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