MARS RETURNED SAMPLE SCIENCE: PLANNING CONSIDERATIONS RELATED TO THE INORGANIC CONTAMINATION OF GEOLOGICAL SAMPLES

Mars sample return is among the top-priority objectives of planetary science recommended by the NRC Decadal Survey (2013-2022). In-situ science on Mars enables studies of rocks or soils in their natural environment, but is often insufficient, owing to limited analytical or spatial resolutions, for definitively addressing high priority science goals, such as evidence for extant or extinct lift or its chemical precursors, and establishing quantitative constraints of planetary evolution of Mars (e.g., age, context and processes of accretion, and early differentiation).

NASA recently announced plans to collect and cache a set of martian rock and soil samples via its proposed Mars 2020 Rover. These samples may be retrieved from Mars and transported to Earth at some future time, should the U.S. Government choose to do so. A key planning question is the maximum amount of inorganic contamination, expressed as elemental concentrations, which can be allowed on these samples, and would be consistent with their potential future scientific use.

The inorganic contamination level can be expressed as a fraction of the naturally occurring concentration of each element in martian samples. However, it is unrealistic to define contamination levels for all 90 stable elements in the periodic table. A practical approach is to define elements that are especially important to high-priority science goals (e.g., McLennan et al., 2012). In addition to determining which elements should be constrained, different elements have different levels of sensitivity. For example, data for elements used in geochronology (e.g., Rb-Sr, Sm-Nd, U-Th-Pb) are normally interpreted at the 3^rd significant figure, implying a need for strict contamination control. For elements used in other geochemical studies, the data are typically interpreted at the 2^nd significant figure, so the contamination control constraints can be less strict. We have considered several approaches using analytic data from the shergottites, nakhlites and chassignites to construct a table with specific values to be used in MSR contamination control planning. We propose that the best solution is to use the composition of a single meteorite, Tissint, which is a depleted shergottite. We welcome input from all geochemists on this approach.