Science Priorities for Mars Sample Return
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V.H.DustDust is the pigment of Mars, supplying the reddish hue to the Red Planet. Thick accumulations of dust are a significant component of the Martian surface. The globally extensive high albedo, low thermal inertia regions of Mars may contain a meter or more of dust (Christensen, 1986). Intermediate albedo regions like those visited by four of the five landed missions show a patchy dust cover that is several cm thick in places. Even the low albedo surface of Meridiani Planum includes isolated occurrences of dust in the lee of obstacles as well as mixed into the regolith (Yen et al., 2005). This dust is carried aloft during seasons of atmospheric turbulence, encircling the globe and then falling out over time onto all exposed surfaces both natural and human-made. Despite the ubiquity of dust and the multitude of orbital and surface analyses applied to it, some of the details of its mineralogy and chemistry remain elusive. Without these details, an important window into the weathering and alteration history of Mars remains closed (see also MacPherson et al., 2001; 2002), and questions about its potential hazard to human explorers are left unanswered. B FINDING. In order to acquire enough dust mass, and to do so relatively quickly, ND-MSR-SAG recommends that a single dust sample of at least 5g should be collected from a surficial geological deposit. eginning with telescopic observations, the bright regions of Mars were recognized as rich in oxidized iron. Visible/near infrared (VNIR) spectra are reasonably well matched by certain palagonitic tephras from Hawaii [Singer, 1982], which are described as hydrated amorphous silicate materials containing nanophase ferric oxide particles. The role of water in altering the dust and/or its parent material has been recognized in subsequent years with orbiter observations of spectral features attributable to a water-bearing phase(s) (e.g., Murchie et al., 1993) including the possibility of zeolite (Ruff, 2004). Thermal infrared spectra provide evidence that a few weight percent of carbonate minerals may be present in the dust (Bandfield et al., 2003). Measurements by the MER rovers clearly show that sulfur is enriched in the dust (Yen et al., 2005) and that virtually all dust particles, which very likely are agglomerates, contain a magnetic phase (Bertelsen et al., 2004) that probably is magnetite (Goetz et al., 2005). Although Martian dust shows evidence for aqueous alteration, the presence of olivine demonstrates that water did not play a dominant role in its formation (Goetz et al., 2005). V.I.Depth-resolved suiteSeveral of the life-related MSR objectives assign high priority to returning samples that contain reduced carbon. Because the surface of Mars is oxidized, organic matter might exist only at depth. Even if MSR is unable to acquire organic-bearing samples, it is important to acquire data in order to model the preservation potential of reduced species and thereby determine where organic matter might be accessible. The organic carbon measurements of the Viking landers indicated clearly that the surface (regolith) of Mars is oxidized to such an extent that any volatile organic components are being continuously destroyed. Although organic carbon compounds are raining down continuously from carbonaceous chondrites, cometary material, interplanetary dust particles, and micrometeorites (Flynn and McKay, 1990), the Viking experiments found no trace of them (Klein, 1978, 1979). It is hypothesized that prebiotic compounds that are relatively nonvolatile have been destroyed. Although there is indication that reduced organic compounds survive in the parent lithologies of Martian meteorites (Steele et al., 2007 and references therein), chemical modelling suggests that the depth of the oxidized surface layer is of the order of cm to several meters (Dartnell et al., 2007). Various oxidizing agents have been proposed, including OH, HO2 and H2O2 species produced by photolysis of atmospheric water vapor (Zent and McKay, 1994; Zent et al., 2003). These species could form complexes with metals in the Martian regolith to create peroxy radicals. Another source of oxidation could be UV-silicate interactions that trap oxygen, resulting in highly oxidized dust and soil particles, or perhaps even unknown “super-oxidants". Models indicate that impact “gardening” of the regolith could mix the oxidant(s) to depths of a few meters (Zent, 1998). Kminek and Bada (2006) concluded that over geologic time scales, ionizing radiation destroys organic matter (specifically, amino acids) to depths of at least 1.5 to possibly 2 m, although Dartnell et al. (2007) have shown that this effect is intrinsically linked to the amount of shielding of organic materials. Permeability-based modelling estimates that oxidants penetrate to depths between 10 cm to 5 m in the regolith, depending on the model, time of exposure and the nature of the regolith material (Bullock et al., 1994). Thus it might be desirable to obtain samples from as deep as 3 m into regolith. Although it would be preferable to collect a set of samples from several depths, an alternative would be to collect a single larger sample from the maximum depth reached. Regarding bedrock and detached rocks, the depth of oxidation presumably depends principally on time and the permeability and reactivity of the rock. Analyses of RAT holes during the MER mission indicate that Hesperian-age basalts have remained largely unoxidized within <1 cm of their surfaces (McSween et al., 2006). Data from Martian meteorites has shown that reduced carbon could be detected within carbonates from 3.6Ga on Mars (Steele et al., 2007, Jull et al., 1997, Flynn et al., 1998). Sedimentary bedrock at the MER Meridiani site has been oxidized to greater undetermined depths. A rock core at least several cm in length from an outcrop would allow the change with depth in composition (organic, inorganic, oxidation state) due to surface oxidation to be determined. An important strategic consideration is that MSL (2009) and ExoMars (2013) will both collect data that will either increase or decrease the priority of the depth-resolved sample suite (see Fig. 1). MSL will carry a highly sensitive organic detection system (the SAM instrument) and obtain samples by drilling 5 cm into rocks and wheel-trenching up to tens of cm into regolith. ExoMars will also carry a very sensitive organic detection instrument (MOMA) and an oxidant detector (MOD). They will characterize gradients with depth in oxidation state, as well as the organic carbon, using so-called Vertical Surveys (VS), obtaining samples at 50-cm depth intervals from the surface down to 2 m. Two such VS acquisitions are planned for the nominal mission. If MSL discovers organic carbon at shallow depths in either rocks or regolith, the importance of a depth-resolved set of samples for MSR would decline. If MSL fails to detect organic carbon in shallow samples, but ExoMars detects it in deeper samples, the importance would increase substantially.  
Figure 1. Importance of sampling to a depth of 2-3 m by MSR, given various potential scientific results from MSL and ExoMars. |