1. 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 

2. 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

3. 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?

4. 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?

5. 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

6. 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.

7. 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

8. 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.

9. 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

10. 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

11. 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.

12. 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

13. 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

14. 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.