 A SAMPLING SITE SELECTION INDEX FOR MARTIAN LIFE DETECTION MISSIONS Brian D. Wade1 and Michael A. Velbel2 Michigan State University, Departments of 1Plant, Soil & Microbial Sciences and 2Earth & Environmental Sciences   Introduction: The life detection missions to be carried out by NASA's and ESA's 2020 Mars rovers will require a rigorous process for selecting sampling sites. To aid this crucial process, we have developed a Sampling Site Selection Index (SSSI) that is based upon Stoker's Habitability Index (HI) [1]. The SSSI provides semi-quantitative assessment of the relative habitability of a site and its potential to have preserved possible life. Stoker's HI: Stoker et al. [1] further developed a HI that originated [2] within NASA's advisory group for the Mars Exploration Program, the Mars Exploration Program Analysis Group (MEPAG). Stoker's HI is the product of four probabilities,  HIStoker = PlwPePchPb ,  where lw is liquid water, e is energy availability, ch is essential elements, and b is benign environment. Each probability has contributing factors (Table 1), which are environmental variables assigned values from 0 to 1 in quarter increments; see Figure 1 for an example. The probabilities are calculated as the weighted mean of their n contributing factors,  Pj = ∑ i=1 n WiFji ∑i=1 n Wi ,  where subscript j is lw, e, ch, or b, and Wi is the weight assigned to each contributing factor, Fji, based on its relative importance and reliability. SSSI for Life Detection Missions: The potential of an environment to have preserved traces of possible Martian life has been identified by MEPAG as a key criterion in selecting sites for life detection missions [3; see p. 77]. MEPAG applies this criterion to the scale of entire landing sites (e.g., Jezero crater), but it also applies to the scale of sampling sites. We have therefore expanded Stoker's HI to include a fifth probability, Ppr, biosignature preservation, resulting in an index that can be used to assess both the potential of a site relative to another to have supported possible life (i.e., relative habitability) and to have preserved traces of it,  SSSI = PlwPePchPbPpr .  Eight factors contribute to Ppr, which we based on features identified by MEPAG as being pertinent to assessing biosignature preservation potential [3; see pp. 73-78]. The Ppr factors and examples of observations that could be made by NASA's or ESA's 2020 Mars rover to evaluate those factors are given in Table 2.  Forg, organics preserved in the environment, is included in Stoker's HI as a factor for Pb, benign environment (Table 1). We have split this factor into two, Fpbc and Fbmol, resolving organics into prebiotic and biotic classes, respectively, and placed these two factors in the new probability, Ppr (Table 2). Consequently, Forg does not exist in the SSSI. Calculation of SSSI for Two Mars Analog Sites: The 2020 Mars rovers will be the first spacecraft capable of evaluating all the Ppr factors. Hence, data from previously landed spacecraft are insufficient for demonstrating the SSSI's assessment ability. We will therefore calculate SSSIs for the two most hyperarid sites in one of Earth's most Mars-like environments, Yungay [4] and María Elena [5] in the Atacama desert. Acknowledgment: This work was funded by a Michigan Space Grant Consortium graduate fellowship awarded to BDW. References: [1] Stoker C.R. et al (2010) J Geophys Res Planets, 115, E00E20. [2] MEPAG (2004) Findings of the Astrobiology Field Lab Science Steering Group. https://mepag.jpl.nasa.gov/reports/AFL_SSG_Sum_Pr es_v3.pdf [3] MEPAG (2018) Mars Science Goals, Objectives, Investigations, and Priorities. https://mepag.jpl.nasa.gov/reports/MEPAG%20Goals_ Document_ [4] Navarro-González R. et al (2003) Science, 302, 1018-1021. [5] Azua-Bustos A. et al (2015) Environ Microbiol Rep, 7, 388-394.    Phoenix: chemically etched grains; F obs2 = 0.5 Figure 1: Examples of evaluating factors. Evidence used by Stoker et al. [1] to assign values for one of the factors (see Table 1) in determining HIs for two of the six sites they examined; Phoenix HI Stoker = 0.43, Meridiani HI Stoker = 0.23. Image credits: top, Stoker et al. [1], taken from Figure 4; bottom, MER-B images 1M130671284 (left) and 1M130760791 (right). Meridiani: vugs, voids, and hematite concretions; F obs2 = 1 50 mm 50 mm   https://doi.org/10.1029/2009JE003421 https://doi.org/10.1029/2009JE003421 https://mepag.jpl.nasa.gov/reports/AFL_SSG_Sum_Pres_v3.pdf https://mepag.jpl.nasa.gov/reports/AFL_SSG_Sum_Pres_v3.pdf https://mepag.jpl.nasa.gov/reports/MEPAG%20Goals_Document_ https://mepag.jpl.nasa.gov/reports/MEPAG%20Goals_Document_ https://doi.org/10.1126/science.1089143 https://doi.org/10.1111/1758-2229.12261 . Table 1: Probabilities and factors for Stoker's HI. Probability   Factors  Plw, liquid water   Fobs1, macroscopic morphological evidence of liquid water Fobs2, microscopic observational evidence of liquid water Fobs3, mineralogical evidence of aqueous processes Fobs4, evidence for thin films of unfrozen or adsorbed water Fth1, presence of water or emplacement mechanism for water (theoretical) Fth2, temperature and pressure above triple point of water (theoretical) Fth3, stability or duration of water in liquid state (theoretical)  Pe, energy availability  Fe1, UV-shielded photosynthetically active radiation available     Fe2, presence of chemical energy source (redox couple(s))  Pch, essential elements  FCarbon, FHydrogen, FNitrogen, FOxygen, FPhosphorus, FSulfur  Pb, benign environment  FT, temperature in growth range for life     Faw, water activity high enough for growth     FpH, pH in growth range for life     Forg, organics preserved in the environment     Table 2: Factors and example observations for Ppr, biosignature preservation, included in the SSSI. Factor     Example observation  Fpbc, prebiotic chemistry polycyclic aromatic hydrocarbons; aldehydes; carboxylic acids; nucleobases; sugars  Fbmol, biomolecular signatures hopanoids; terpenoids; porphyrins; proteins; stoichiometric proportions of bioessential elements  Fmet, metabolic signatures disequilibrium of redox couples; enrichment or depletion of particular elements (e.g., Mn, Mg, Fe, Ca, Na); strong chemical gradients  Fstruc, structural signatures microfabrics and macrostructures consistent with those formed by microbial mats and microbialites  Fbmin, possible biominerals manganese concretions with organics; certain trace element bearing minerals such as siderite with strontium  Fdep, depositional conditions rapid burial by impermeable sediments such as clay; precipitated carbonates or silica; anoxia  Fpost, post-depositional conditions little or no aqueous alteration; no impact shock metamorphism; no igneous intrusions  Fiso, isotopic analysis suitability carbonate rocks in which structural, metabolic, and/or biomolecular signatures were detected  
