Habitability - Framework Range of sedimentary lithologies containing clay minerals and salt hydrates Interstratification of phyllosilicates and sulfates  transition in depositional environments  possible framework for understanding habitability through time Gale Mound Stratigraphy (very tentative) Lower Formation – Stacking Pattern Milliken et al. 2010 

Habitability – Implications from Mineralogy Nontronite with some Al substitution – lower mound strata Reducing and moderate pH conditions during initial phyllosilicate formation… support for: fermentive and chemoautotrophic metabolisms? redox gradients? community and biochemical diversity? If nontronite is syndepositional, potential support for past habitability

Habitability – Implications from Mineralogy Mg-sulfate mixture of monohydrate and polyhydrates – upper mound strata Very high salinity No indication for Ca- Al-, or Fe-sulfates (e.g., alunite, jarosite) no indication of extreme low-pH environment Crystalline Hematite - uppermost portion of the sulfate strata oxidation, possibly late-stage… contrasts to the more reducing conditions suggested by nontronite in the lower mound strata Interstratification of phyllosilicates and sulfates  transition between depositional environments with minimal pH drift but rise in overall Eh?

Habitability – best option, drawing from terrestrial analogs Lower Mound, nontronite-bearing strata Terrestrial nontronite – predominantly ferric iron, even though formation of nontronite is favored where the protolith contains ferrous iron Microbial redox reactions with terrestrial nontronites ferric iron reduction and (less commonly) ferrous iron oxidation CON: uncertainty in the nature of nontronite origin at Gale impacts potential habitability in-situ groundwater alteration of stratified basaltic tephra sedimentation of clays weathered from impact-related hydrothermal alteration of basaltic protolith lacustrine alteration of mafic ash other potential origins?

Habitability – second option??? Transition to Upper Mound Mg-sulfate depositional environment Very high salinity Did life adapt to extreme salinity? Merits of phyllosilicate versus sulfate strata clay minerals are likely to be more durable more dependent on preservation of primary phases than on differences in mineralogy

Biosignatures Detection - depends on generation, preservation, and measurability Generation - directly related to past habitability (discussed above) assuming past life existed on Mars. Measurability – MSL payload instruments Based on definitive nature of the biosignature and its measurability by the MSL payload (from Table 1, Summons et al., in prep). diagnostic organic molecules biogenic gases organism morphologies (cells, body fossils, casts) biofabrics (including microbial mats) isotopic signatures evidence of biomineralization and bioalteration spatial patterns in chemistry Precedence is given to organic matter (OM) detection, though, where it occurs, there is a high probability that other biosignatures may also be present.

Biosignature Preservation Potential Key factor #1: Depositional environments that favored sedimentation of OM via retention and hydrodynamic concentration Same environments favor habitability  possibility of closely linked in situ biosignature generation and burial Key factor #2 - Concentrations of sedimentary minerals that protect OM from ionizing radiation and chemical oxidation (Summons et al., in prep)(e.g., phyllosilicates) Most of the depositional scenarios hypothesized for the lower mound strata of Gale are consistent with this form of sedimentation - Exception: aeolian deposition with fluctuating near-surface water table

Biosignature Preservation Potential - Clays Mars clay mineral diagenesis low-temperature lacustrine systems or moderate temperature hydrothermal environments Degree of lithification/permeablity for the phyllosilicate strata uncertain Repeated cycles of hydration and dehydration via groundwater? induces exchange of interlayer cations, OM preservation potential would significantly diminish Possibility of significant preservation of OM with clay minerals at Gale for most depositional scenarios - Exception: those that involve very high temperatures of formation Exception: extensive organic matter degradation during transport, as in eolian recycling or extensive surface exposure.

Biosignature Preservation Potential - Sulfates Mg-sulfates subject to dissolution/ reprecipitation cycles primary precipitates will rapidly dehydrate under current Mars surface conditions at low latitudes Access to sediment several decimeters deep required to penetrate beyond the regime of diurnal and seasonal thermal cycling Loss of water and other volatiles, but less volatile organics might be retained in sulfate deposits On Earth, evaporite minerals profusely dilute most OM Scenarios for possible OM preservation in the sulfate strata: direct precipitation from surface water with little secondary modification precipitation of the sulfate salts from percolating groundwater (as in Burns Formation) total reworking with eolian transport and deposition of the sulfate minerals in a very dry environment Favorable Least favorable

Biosignature Preservation Potential - Transition Interstratification of clays and sulfates Incursion brine or leachates could lead to gypsum with clays Halogen and other salts likely Could get several different evaporite suites Diversity in salt mineralogy  diversity to biosignature preservation potential

Key habitability and biosignature questions that MSL could address at Gale Is organic matter preserved in the phyllosilicate rich strata of the lower mound? key element of habitability - organic carbon is both a nutrient and waste product for life. SAM’s EGA and GCMS experiments Follow up questions: If organic matter is present, does the composition reveal the nature of its source or alteration? (b) If organic matter is not detected, does the sedimentological, mineralogical, and chemical context indicate degradation mechanisms that may be responsible for organic matter loss? All of the the MSL payload. Even DAN, RAD, and REMS may provide essential insight into the ambient conditions they may have impacted organic matter preservation or destruction.

Key habitability and biosignature questions that MSL could address at Gale Similarly, is organic matter detectable in the sulfate-rich strata of the transition zone and upper mound? Does the composition reveal the nature of its source or processes acting upon it? Are preservation or destruction mechanisms indicated? Has groundwater dissolution and reprecipitation occurred? All of the the MSL payload

Key habitability and biosignature questions that MSL could address at Gale Do paleoenvironmental interpretations implicate processes that may have promoted deposition, preservation or alteration of organic matter or other biosignatures during early diagenesis? Larger-scale depositional context will be critical for understanding the distribution of organic matter and other biosignatures detected in sediments

Key habitability and biosignature questions that MSL could address at Gale Do organic chemical, morphological and textural, isotopic, mineralogical, and ionrganic chemical compositions of the cyclic, clay and sulfate-rich strata of the Gale mound record changes of a past ecology associated with environmental conditions at the time of deposition? Detection of a biosignature – great. Detection of several independent signatures – even better. Detection of past ecological changes – priceless! A suitable test … detection of independent relationships between two or more variables and stratigraphy that reflect time-varying processes, for example: direct correlations and/or anti-correlations in data values indirect correlations with graded beds or overall sedimentary packages Detection of OM or any biosignatures would aid the test, if it were shown to vary with the other compositional or contextual parameters.