Science Priorities for Mars Sample Return

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VI.E.In situ measurements for sample selection and documentation of field context.

The scientific value of MSR would depend critically upon the ability of the mission science team to select wisely the relatively few samples that could be returned, and on the degree to which the field context of these samples is known. In order to achieve these two functions, the MSR sample acquisition rover must be able to perform certain remote and in situ measurements.

ND-SAG has found that the instrument needs for MSR would be different in the two scenarios listed below:

Case A. The MSR rover either goes to a previously unvisited landing site, or it gets 'off the beaten track' of a previously visited site. In this case, both sample selection and documentation of context would need to be done.
Case B. The MSR rover “follows in the footsteps” of a previous rover that has already established the field context and identified the exact materials to be sampled.

For Case A, five measurements are important to support the collection of samples that could be used for a wide range of scientific objectives: 1) high quality color panoramic imaging would be essential to identify samples of interest and to determine their local geological context (e.g., Grotzinger et al., 2005). 2) A microscopic imager that examines rock and sediment textures for clues about processes and environments of formation would also be essential. In addition, microbially induced textures are one of the key indicators of life (e.g., Herkenhoff et al., 2004). 3) The mineralogy would need be determined to discriminate one rock from another and to establish geologic context of the samples (e.g., Christensen et al., 2004). Minerals reflect the processes and conditions associated with the formation of geologic materials. For example, understanding compositional variability in the igneous sample suite would require rocks that contain a range of minerals, such as olivine, pyroxene, feldspar, and oxides. Phyllosilicates, sulfates, carbonates and silica-rich minerals are excellent for retaining evidence of aqueous processes and evidence of habitable environments and life. 4) Measurements of elemental abundance have been critically important during the MER mission (e.g., Ming et al., 2006; McSween et al., 2006) and would be essential in understanding the range of variability within a field site, and in identifying the effects of unusual geologic processes. This information would be key to both sample selection and documentation of context. 5) Reduced carbon measurements would be centrally important to understanding prebiotic chemistry, habitability, and life (e.g., Schopf, 1983), therefore reduced carbon should be measured during the sample selection process. Although we could certainly detect reduced carbon in returned samples to better than 1 ppb, ppm-level sensitivity may be sufficient for screening for sample selection on Mars. The SAM instrument on MSL and the Urey and MOMA instruments on ExoMars will presumably give us important guidance on this after 2010. Finally, a rock abrasion tool would be essential to characterize the rocks adequately. Because many rocks on Mars have dusty or weathered surfaces, correctly determining the characteristics of the underlying rocks would require access to fresh surfaces.
Table 6 Rover-based Measurements to Guide Sample Selection.

Notes: Case A – MSR gets 'off the beaten track.' This assumes that a future MSR would go to either a fresh site, or outside the area studied by a previous rover, where understanding the geologic context still needs to be done. Case B – MSR follows the tracks of a previous rover, which has documented the context.
For Case B, ND-SAG has concluded that the payload could be reduced to the following two instruments: color stereo imagery and microscopic imagery. In this case, the MSR rover would not need to determine the geologic context and identify the materials to be collected - the prior mission would have achieved these tasks. However, ND-SAG also concludes that Case B has substantial risk, and it is not endorsed unless this is the only way the mission could be done. For a variety of reasons it may not be possible for the MSR rover to follow the tracks of the previous rover. In addition, MSR would not have enough functionality to make excursions off the previous rover traverse, which may be desirable to follow-up on unexpected results, including from the previous mission. The Case B rover would have minimal analytical capabilities for an extended mission after the MAV leaves. Finally, if the MSR rover follows another rover that is neither as clean nor as sterile as the MSR rover, important implications arise involving planetary protection and contamination control, and these should be evaluated further.

FINDINGS.

There is a difference in the minimum measurement capability of the MSR rover depending upon whether it would go to a previously visited site, or to a fresh site.
For a fully functional MSR rover, capable of interpreting geologic context on its own, five kinds of observations would be needed (macroscopic and microscopic imagery, mineralogy, elemental analysis, and organic carbon detection).
If MSR were restricted to previously occupied sites, and was dependent on prior information for sample selection decisions, the number of instruments could be reduced to two. However, restricting the retrieval of documented samples to previously visited sites would increase both science risk and planetary protection concerns.

VI.F.Surface Operations

In order to achieve the MSR scientific objectives and access the kinds of sites of greatest current interest, a high priority would be to deploy a spacecraft that has significant horizontal range and could navigate rough terrain. Although the current orbital imagery provides excellent context and mineralogical information in order to identify high priority landing sites, experience from the Mars Exploration Rovers has shown that the diversity of potential samples that exists at the size scale of a rover must be analyzed in-situ. The MSR lander/rover must have instrumentation that could interpret the diversity of geologic materials and help to select the highest priority samples for return (see section IV.D). Color imagery, remote spectroscopic observations and contact geochemical/mineralogical analyses constitute the minimum set of techniques would be needed to optimize sample selection. The duration of surface operations would also influence the quality of the sample collection. ND-SAG expects that a minimum of 6-12 months of surface operation would be required in order to reconnoiter a site and identify, characterize and collect a set of samples that captures its compositional diversity. To place this in context, the Opportunity rover did not complete the stratigraphic characterization in Endurance crater until Sol 315 (Squyres and Knoll, 2005) and Spirit did not identify significant indicators of aqueous processes until it reached the Columbia Hills (roughly Sol 180; Arvidson et al., 2006). Sending MSR to a previously visited site (either of the two MER sites, the MSL or ExoMars site, or potential future sites) might substantially reduce the time needed for reconnaissance, but revisiting a site might also compromise samples intended for organic analyses by increasing the risk of terrestrial contamination.

FINDING. The collection of a diverse set of rock samples from known geologic context would require significant surface mobility. Also necessary is information about the diversity of surface materials (could be collected either by a prior mission or by the MSR rover), in order to select samples that span that variation. A minimum duration for surface operations of at least 6-12 months is anticipated, depending on landing site geography/geology and relationship to prior missions.

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