Planning considerations related to the organic contamination of Martian samples and implications for the Mars 2020 Rover.

s and papers in preparation describe the ‘‘tentative’’ detection of dichloropropane and chlorobenzene at levels of a few tens of nanograms per gram in a core drilled into a mudstone at Yellowknife Bay (e.g. Summons et al., 2014a,b). A significant caveat here is that all three missions used thermolysis and pyrolysis to volatilize organic compounds so that they are amenable to gas chromatographic and mass spectrometric analysis. A series of studies (Navarro-Gonzalez et al., 2006, 2010, 2011; NavarroGonzalez and McKay, 2011) suggested that the presence of perchlorate and other oxidants in the martian regolith renders the Viking results difficult to interpret because of the high probability of either oxidizing or chlorinating indigenous organic molecules during heating. These observations have been contested (Biemann, 2007). As a result of these complications, the Panel did not rely on the earlier non-detections of organics in Mars regolith. The more recent detections of chlorobenzene (or possibly its aromatic precursors) and other chlorohydrocarbons by MSL (e.g., Freissinet et al., 2014) are considered to provide likely lower limits for these compounds. The OCP briefly considered organic concentrations in analog terrestrial rocks as another constraint on what to expect on Mars. Fine-grained sedimentary rocks that have not been oxidized or weathered constitute the type of sample that we might hope to return from Mars. On Earth, similar rocks commonly contain > 0.1% (100 ppm) TOC, and contain many individual biomarkers at levels > 1 ppm. Even those sediments considered to be relatively poor in organics contain > 0.01% (10 ppm) TOC (Mayer, 1994), and yield individual biomarkers at levels > 10 ng/g (e.g., Lipp et al., 2008). Soils from the Atacama Desert are reported to have 32 ppm TOC (Navarro-Gonzalez et al., 2010), an order of magnitude below typical ‘‘organic poor’’ marine sediments. Subcritical water extraction of subsurface Atacama soils ( Jungay region) followed by derivatization and capillary electrophoresis of the fluorescently labeled amines has demonstrated individual amines and amino acids at the 50– 100 ng/g level (Skelley et al., 2007). The earlier MSR SSG II panel considered a more extreme example of organic-poor sediments (i.e., a highly oxidized sedimentary rock that had been buried and undergone thermal maturation). Although such rocks have similar levels of TOC (approximately 0.01%), diagenesis has rendered most biomarkers into macromolecular kerogen, which is not extractable. They thus estimated expected concentrations for hydrocarbon biomarkers of 0.1–1.0 ng/g in such a rock (see Section 2.4 for details of this calculation). There are several limitations to using such terrestrial analogs to predict concentrations of martian organics. First and foremost, there is no a priori reason to expect that concentrations of organics in terrestrial rocks would be indicative of those on Mars. Indeed, valid arguments can be made for their being either higher or lower than indigenous martian concentrations. For example, with lower organic input and more oxidizing subsurface conditions, martian rocks might have lower organic concentrations. If microbial activity were more limited (or absent) on Mars, residual organic concentrations might be higher. Second, it is unclear which terrestrial rocks, sediments, or soils we should choose as appropriate analogs. Even considering that oxidized rocks represent a reasonable lower bound for those found on Mars, the goal of the mission is clearly not to sample and return the most oxidized martian rocks. Indeed, it is unclear whether scientific goals could be met with such a rock even given zero organic contamination. The third concern expressed by the OCP is that hydrocarbons (the dominant biomarkers in thermally mature terrestrial rocks, and those considered primarily by the MSR SSG II report) may not be the class of organics that are most abundant or interesting on Mars. With no active tectonics to deeply bury sediments under reducing conditions, biomolecules (or even meteoritic organic compounds) might be transformed to more oxidized species rather than more reduced ones. In summary, consideration of terrestrial analog rocks indicates that organic concentrations in martian rocks might span a huge range around those directly measured in meteorites. We therefore conclude that this line of argument provides little firm footing on which to construct quantitative limits. A final constraint on expected concentrations in the absence of martian biota, previously considered by the OCSSG report and by Benner et al. (2000), can be derived from estimated rates of delivery of organic carbon to the surface of Mars by meteorites. Meteorites deposit an estimated 2.4 · 10 g/year of organic carbon to the martian surface (Flynn, 1996). If allowed to accumulate over 3 billion years, and given a Martian surface area of 3.6 · 10 m, this would result in 20 kg/m of organic carbon. Assuming a mixing depth of 1 m and rock density of 4 g/mL results in a predicted TOC concentration of 5 mg/g. A much more conservative mixing depth of 100 m would lower this to 50 lg/g. If we presume that this organic carbon has a molecular makeup similar to that of Murchison (see Table 7), where functional classes of molecules represent approximately 0.05% of TOC, we predict approximately 2.5 lg/g of each class of organics (1 m mixing depth). Further assuming that each class comprises 10–100 compounds, this yields a final prediction of approximately 20–200 ng/g per compound (or 0.2–2 ng/g for the 100-m mixing depth). These estimates span Table 7. Distribution of Carbon in the Murchison CM2 Meteorite