Science Priorities for Mars Sample Return

VI.G.Sample acquisition system priorities

• 
  Sampling both the weathered exterior and unweathered interior of rocks.
  
• 
  The ability to sample a continuous stratigraphic sequence of outcrops (e.g. the Burns Cliff at Meridiani Planum).
  
• 
  In the case of rocks in outcrop with differential hardness, the ability to sample both less-resistant beds and more-resistant beds.
  
• 
  Relate the orientation of structures and textures in samples to those in outcrop surfaces, bedding planes, stratigraphic sequences, and regional-scale geologic structures.
  
• 
  Maintain the structural integrity of samples.
  

The simplest way to sample granular materials, such as regolith and dust, might be using a scoop. However, it may be possible to engineer a mini-corer so that it could also be used to sample granular materials.

VI.H.Temperature.

Certain chemical species that would have great science value for MSR are also sensitive to temperatures barely above those attained in the current Martian environment. Examples include organic material as well as reactive minerals that might be common (e.g., sulfates, chlorides, and clays) yet whose stability could be compromised even at modest temperatures (<20°C). Liquid water or ice also might be present in samples, either interstitially or sorbed onto mineral surfaces. Accordingly, the temperatures experienced by samples during collection and return to Earth would be a critical issue. In order to maintain sample integrity, returned samples ideally should be kept as close as possible to the ambient temperature (and atmospheric) conditions of the location where they were collected. However the ND-SAG recognizes that if this were set as a mission requirement, it might pose a major technological challenge that may not be achievable within cost constraints. If sample integrity were seriously affected by temperature excursions, then the next best option would be to monitor the temperature history closely and also ensure that samples are fully encapsulated so that all components would be retained. Under these conditions, any chemical reactions that may take place during transport to Earth conceivably could be evaluated and reconstructed.

Elevated temperatures could compromise the integrity of organic compounds (see MacPherson et al. 2005 for a good summary table). Even at only –5°C, certain organic compounds are mobilized, and some organic compounds decompose at >-20°C. At temperatures of +50°C, significant decomposition takes place, and if samples remain at this temperature for more than about three hours, science objectives related to life goals could be seriously compromised.

Most inorganic materials should remain suitable for allowing primary scientific objectives to be achieved even if these materials experience temperatures as high as +20°C. At higher temperatures, such as +50°C, some materials (e.g., regolith, dust, clays) might deteriorate and potentially lose key scientific information. Although the kinetics of many reactions is poorly known and some metastable phases may persist well outside their nominal stability ranges, sulfate minerals are very likely to present a special challenge. For example, the hydration states of magnesium and iron sulfates are sensitive to temperature and relative humidity and changes (dehydration and/or melting) might commence at temperatures as low as –2°C. Dehydration and melting should be expected if temperatures reach 20°C. These changes have the potential to seriously influence both the chemical and physical state of the samples. For example, dehydration of MgSO₄•nH₂O from n=11 to n=1 would result in nearly a factor of four loss of mineral volume that could lead to physical disaggregation of weakly cemented samples. Release of water could result in further chemical reactions, such as dissolution of highly soluble minerals (e.g., chlorides), leaching of weakly held ions (e.g., clays) or significant lowering of pH through Fe³⁺ hydrolysis. Finally, water ice may be stable within cm of the Mars surface (e.g. Mellon et al. 2004) therefore it could occur in a regolith sample or drill core. Refrigeration and temperature monitoring would allow an accurate assessment of whether any reaction between this water and the surrounding sulfates or soluble minerals has taken place during sample return.

The ND-SAG has confidence that the MSR scientific objectives that depend upon mineral compositions could be addressed if samples were kept below about –10°C. For preservation of water, it would be preferable to hold the samples below about -20°C (MEPAG SR-SAG, 2006). There is less confidence, but it is likely, that most objectives would be met for samples that are kept below about +20°C. If samples were allowed to reach +50°C for greater than about 3 hours, the damage that ensues would seriously degrade the scientific value of the samples. It is very possible that samples containing Mg- and Fe-sulfates would be altered substantially even if temperatures approach only 20°C, but these effects could be mitigated if samples are encapsulated and their temperature history monitored. Monitoring sample temperature during transport to Earth would help determine any post-sampling melting or recrystallization. For example, MgSO₄ of unknown hydration state was identified at the Opportunity site. This has been speculated to be MgSO₄ .11H₂O, which has subsequently found on Earth (Peterson et al. 2007), is expected to dehydrate at 2C. Maintaining a low temperature would inhibit this mineralogical transformation.