Science Priorities for Mars Sample Return
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VI.I.Planning Considerations Involving the MSL/ExoMars CachesA decision was made in mid-2007 to add a simple caching capability to the 2009 MSL mission. As of this writing, a similar cache is under consideration for ExoMars. This capability is part of a larger strategy to utilize pre-MSR landed missions to enhance the value of sample return. It is intended that the recovery of the MSL cache will be an option for MSR, rather than a requirement. The decision would be made several years from now by future science teams and other decision-makers. The MSL cache design The MSL cache is designed to accept 0.5–1.5 cm rock samples provided by MSL's soil scoop, collected over 5–10 separate caching events (Karcz et al., 2007). The cache will have mesh sides to allow fines to filter out, leaving behind rocks. The strategy for employing the scoop to acquire either individual targeted rocks, or rock-bearing regolith would depend on further experience with prototype scoops (note that the volume of the scoop is roughly half the volume of the cache). The empty cache is specified to have a mass less than 52 g. However, the mass of the latest revision of the design (as estimated by the CAD software) is 29 g (Karcz, writ. comm., 01-07-08). A draft specification (as of this writing) is for the mass of the full container to be 200 g or less. Because the mass of the contents will be uncertain, it is likely that the science team will fill it to somewhat less than capacity--to, say, 180 g instead of 200 g. The cache and rover will not be sterilized, but any organisms on the MSL cache and rover, and in MSL's assembly, test, and launch environments will be inventoried using genetic methods. The MSL Project Science Group will direct caching operations and sample selection. The current strategy is to collect samples representative of the common rock and mineral types encountered by MSL during its traverse, rather than specifically targeting “unusual” finds.
The ND-SAG evaluated several major impacts that the MSL cache might have on the design of the first MSR mission. The following three sets of questions and answers summarize the outcome of these deliberations. These answers draw substantially from the findings of Steele et al. (2008). 1. If a cache prepared by MSL (or ExoMars, or any other future mission) is recovered, would MSR necessarily acquire additional samples in order to achieve its key scientific objectives, or would the samples in the cache be sufficient? If additional samples are required, then the MSR spacecraft must have an independent sample acquisition capability. Discussion. The cache’s samples have the advantage that they might be collected from a wider geographic area than is possible by MSR. This is particularly true for the MSL rover, which has a nominal mobility range that might be an order of magnitude higher than that of an MSR rover. For whatever reason, MSL might discover and collect unique samples that the MSR could not collect on its own. However the MSR mission might not be able to retrieve the MSL cache. For example, the MSR spacecraft might land too far away, the MSL rover might end its mission in an inaccessible location, or the cache itself might be in a state that would render its irretrievable. MSL’s cached samples will be limited to small rocks that its scoop could retrieve from the Martian surface. Isolated small rocks might be heavily weathered. Several of the MSR objectives require unaltered samples from rock interiors. The present design of the MSL sample collection and caching system will not permit the assembly and packaging of all of the samples necessary to address the scientific objectives proposed for MSR in this report. The cached samples will be neither labeled nor separated from each other; therefore the identity of each sample and therefore the information about its environmental context might be obscured during caching of the samples. Loss of context data would significantly reduce the science return, particularly for the high priority life-related objectives. Because the cache samples are not to be individually encapsulated, any mechanical disruption during transportation back to Earth might intimately mix the samples and contribute further to the loss of their individual identities. The standards for organic contamination and planetary protection for the MSL and MSR missions would probably be different (as of this writing, the MSL contamination thresholds are known, but those for MSR are not). The MSL sampling system will not be sterilized. If the MSL cache harbors biological material whose origin could not be determined (Is it from Mars? Or is it Earthly contamination?), planetary protection protocols will require that the returned cache samples be sterilized. Stringent sterilization procedures would severely degrade the scientific value of these samples for several of the MSR science objectives. Finally the MSR mission should be able to respond to any discoveries whose follow-up would require samples other than those in the MSL cache. Accordingly, the ND-SAG found that the MSR landed spacecraft should have the capability to collect at least some of the samples to be returned by itself. 2. If the MSR spacecraft must have sample acquisition capability, does the specific nature of that capability depend on whether or not it would attempt to recover a cache? Discussion. The MSL cache could substantially improve the MSR return if certain scenarios occur. For example, if the time available for MSR surface operations becomes severely limited, it might be able to collect a relatively small number of samples on its own and the MSL cache would become especially important. According to current plans, the mass and volume of the MSL or ExoMars caches are estimated to occupy about one-third of the proposed returned sample capacity. MSR therefore must acquire either two-thirds or all of the returned samples. As discussed above, the MSR spacecraft must have the capability to collect all of the sample types of interest. Therefore the design of the sample acquisition system of MSR would be independent of the decision to recover either the MSL or the ExoMars caches. Note that a key consideration in this analysis is whether a given kind of sample discovered by MSL would be sufficiently useful in an Earth-based laboratory if stored in the cache, or whether it would be preferable to have MSR recollect it.
However there are differences in perspective within the ND-SAG regarding how closely the MSR rover would have to follow the footsteps of a previous rover in order to take advantage of that previously acquired information. Opinions range from the view that sampling the same stratigraphic bed would be adequate to the view that the same drill hole must be re-sampled. The further the MSR rover departs from the tracks left by the previous rover, the more important it would be for MSR to have its own instruments to guide its sampling operations. In addition, the presence of instruments on MSR would allow the mission to follow up on unexpected discoveries and/or to pursue scientific questions that were not addressed by the previous mission. There is also a concern about contamination should MSR re-visit sites. The MSL contact instruments and tools would be decontaminated to a level that allows for the confident detection of Martian organics if present, but this is not true of MER. Does this mean that we would need to avoid sampling directly any location previously sampled by a MER or MSL? If so, returning to a MER site may not provide the benefit of a reduced payload because if we want to sample "sister" materials rather than the exact same materials, we might need a fairly sophisticated instrument payload rather than a minimal one. This topic needs further discussion by the community.
Steele et al. (2007) recently assessed the expected scientific value of the MSL cache. Their results are summarized with respect to the 11 candidate scientific objectives for MSR in Table 10. For Objectives 7 (Regolith), 10 (Gas Chemistry), and 11 (Polar), the MSL cache will not make any contribution because it will not have the capability to acquire and store regolith, atmosphere, and ice samples. For the rest of the MSR scientific objectives, the contribution of the cached samples would likely be ‘minimal’ to ‘some’, depending on what MSL encounters and the nature of the samples selected for caching. The priorities for additional sampling by MSR reflect the comparison of the objectives for the MSL cache and the MSR mission and are summarized in Table 10. In its discussions both internally and externally, ND-SAG has observed that the two main sectors of the Mars science community that would be able to make the most use of returned samples, namely astrobiologists and geologists, tend to view the strengths and limitations of the MSL cache differently. For our astrobiology objectives, the limitations are a major concern, whereas for at least some geologists, the strengths are more important. In order for MSR to maximize its scientific return, the samples need to serve the needs of BOTH astrobiology and geology, and we encourage all sectors of the community to be sensitive to other points of view. In summary, for each of the 11 candidate scientific objectives for MSR, ND-SAG concludes that the expected contribution from the cached samples alone would not be sufficient to achieve the main scientific objectives proposed, and that additional sample acquisition/packaging by MSR would greatly enhance the science return. The MSL cache has the potential to increase the diversity of the samples; however it would imply a larger complexity for MSR in operations and possibly a larger mobility.
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Notes on Table 10. For MSL Objective 8 (Risks to Human Explorers), there is a need to acquire both regolith and dust samples. Because samples in the MSL cache will be exposed to the Martian environment during the period of time between MSL and MSR, the samples will be covered with dust. Although this dust could be extracted after return to Earth, the amount would be very little. For MSL Objective 9 (Oxidation), the degree of relevancy of the cached samples depends largely on the design (including depth range)