Science Priorities for Mars Sample Return

VI.C.Sample Encapsulation.

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  Avoid commingling of samples. First, cross-contamination would likely occur without encapsulation and it would degrade the scientific value of samples, particularly if samples from different sites are mixed. Mixing would be a particular problem for weakly lithified and friable samples that may break apart during transport to Earth.
  
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  Retain volatile components. In addition, hydrous materials that are not maintained at Mars ambient conditions might dehydrate and form sulfur-bearing fluids that could readily react with other samples and the container.
  
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  Sample labelling for linkage to original field context. Individual samples must retain their identities after they are returned to Earth. A friable sample would lose much of its identifying characteristics if it breaks into multiple pieces during transport. It is imperative that the samples be linked to their collection sites even if the sample’s physical and chemical integrity are altered during transport.
  
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  Maintain sample mechanical integrity. Several investigations would require that the samples’ macroscopic structures, microscopic textures and mineralogical spatial relationships be preserved during collection and transport. The samples’ mechanical integrity must be preserved as well as possible. This is a particular concern for friable sedimentary rock samples that would be a major priority for MSR. Aqueous sediments could exhibit fabrics and textures at the mm- to cm-scale that are highly diagnostic of their formation and/or subsequent alteration. For example, the MER rover Opportunity documented the shapes and sizes of both grains and laminations that were consistent with the former presence of a shallow playa lake (Grotzinger et al., 2005). Much of the sulfate-rich bedrock at the Opportunity site appears to be weakly cemented and therefore seems prone to fragmentation that might destroy its valuable sedimentary textures.
  

Encapsulation is a particular issue for sedimentary rock samples. Some chemical and siliciclastic sediments are permeable and/or fragile and therefore could be highly susceptible to contamination and degradation during acquisition and transport. Many of these samples have the potential to break, disaggregate, dehydrate, and devolatilize. In addition, iron oxide and phyllosilicate materials, in particular, could adsorb volatile contaminants. Sample-to-sample contamination by water and/or organic compounds is a serious concern. Consequently, gas-tight encapsulation in inert containers is critical for samples of sedimentary materials.

An engineering trade to be evaluated would be whether a single airtight design should be used for all of the samples, or whether mass could be saved by having some vials that would be airtight and some that would be only “dust-tight”.


FINDINGS.

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  The scientific usefulness of the returned samples would depend critically on keeping them from commingling, on being able to uniquely identify them for linkage back to documented field context, and in keeping rock samples mechanically intact.
  
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  Trading sample mass for packaging material is painful, but necessary. A smaller number or mass of carefully managed samples would be far more valuable than larger number or mass of poorly-managed samples.
  
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  The encapsulation for at least some of the samples must be air-tight to retain volatile components.
  

VI.D.Diversity of the returned collection

Changes in geologic, climatic, and potential biologic processes could only be addressed by examining multiple samples that represent different intervals of time. For example, in order to understand the origin and evolution of fluids responsible for sulfate deposition, numerous sulfate-bearing samples would be required that record evolution of any fluids through time. Likewise, understanding a siliciclastic depositional environment would require determining how the rock sequence changed through time; thus a stratigraphic sequence must be sampled. Finally, understanding the evolution of the Martian interior or an individual volcanic edifice would require sampling igneous rocks produced at different times. The lithologic diversity of the sample collection must be maximized to ensure that a record of any temporal mineralogical, geochemical, and organic chemical variations has been captured in the returned collection.

The lithologic, compositional, and temporal diversity of the returned sample collection may be the single most important factor controlling the range of investigations that could be addressed using the samples. For example, many investigations involving habitability, the carbon cycle, the search for life, and the role of water on the Martian surface would require rocks containing hydrous phases. Some aspects of these investigations, as well as investigations regarding the evolution of the atmosphere, climate, surface, and interior of Mars, could only be addressed with siliciclastic sediments, igneous rocks, and regolith. Consequently, a primary exploration objective of MSR should be to maximize scientific yield by ensuring that the sample collection has the largest possible lithologic diversity. This essential objective should substantially influence both the selection of landing sites and the development of rover operation protocols. For example, mission strategies to acquire samples by visiting multiple sites are more effective at capturing a greater diversity of samples.


FINDING. A primary factor in the ultimate scientific value of MSR would be the diversity of the returned samples. The more diverse the collection, the more useful it would be in understanding the natural processes (past and present) on Mars.