Discussion Points and Science Criteria

The principal objective of the MSL mission is to find and interpret evidence for the environmental history of Mars, information that is critical to any attempt to understand its possible habitability.  This will be best achieved by selecting a landing site with the following characteristics.   All of the criteria and questions listed below pertain to characteristics of the landing site that can be ascertained before landing.  All sites will be ranked according to the degree to which the data allow the questions to be answered.

Diversity: To mitigate the risk of disappointment and ensure the greatest chance for science success, we want the greatest number of possible science objectives at a chosen landing site.  We need multiple science targets, and these targets should be as strongly differentiated as possible.  Thus, a landing site with both morphologic and mineralogic evidence for past water is better than a site with just one of these criteria.  Furthermore, a site with spectra indicating multiple hydrated minerals is better than a site indicating just one.  A site with multiple styles of stratigraphic expression and inferred depositional mechanisms is better than a site with a single mode of stratigraphic expression. For example, a site with stratigraphically differentiated spectroscopic evidence for both clay minerals and sulfate salts would constitute a rich site.
Critical questions
Can multiple rock units be observed from orbit?
Do these units have well defined stratigraphic and/or cross-cutting relationships?
Do these units show diverse mineralogic and/or geomorphic features?
How strong is the evidence that these features formed through interaction with water?
Can multiple working hypotheses be developed for the interpretation of key features, and if so, can the MSL payload enable us to differentiate among alternatives?

Context: Rocks and soils investigated by MSL must be put into a larger, more regional context. This regional context is important for constraining past processes which may have led to habitable environments.  Our ability to do this will depend on how well rock units can be traced or otherwise correlated based on physical and textural attributes, as well as mineralogic signatures. 
    Critical questions
How much of what will be observed by the rover can be placed into a geologic framework before landing?
Can local observations be placed into a more general regional context and, if this is possible, how confidently can this be done?  Can the MSL payload be used to resolve alternative interpretations?
Do we reliably know the age of the identified rock units?

Habitability:  Not all outstanding questions concerning the geologic history of Mars lead directly to understanding its environmental history, and therefore prospects for habitability.  Almost all planetary processes, those on Earth included, can be viewed in a general way as contributing toward understanding its environmental history.  Consequently, to be meaningful for MSL it is essential to adopt a more focused view that makes specific predictions that can be incorporated into an exploration strategy for MSL.  The essential issue here is to identify a particular geologic environment (or set of environments) that would support microbial life.   The spacecraft can then be directed to interrogate promising rock and soil masses for clues that might lead to the detection of chemical, mineralogic, and textural features than would confirm the presence of a habitable environment at the landing site.
    Critical questions
Does mineralogic evidence indicate a particular habitable environment?  If there is ambiguity, what are the options?
Does geomorphic evidence indicate a particular habitable environment?  If there is ambiguity, what are the options and how could the MSL payload resolve them?
Can minerals or morphologies detected from orbit be used as reliable indicators of past water pH, rock-water ratios, or water activity?
Fossil/Biosignature Preservation:   On Earth, the preservation of fossils and other biosignatures depends on particular physico-chemical conditions that result in early mineralization of organic matter to preserve, or entomb morphologic and chemical fossils in the rock record.  The early diagenetic history of clay minerals is critical to their ability to sequester organic substances.  These conditions embrace a very specific subset of the much broader set of conditions that enable life to be present in any given geologic environment.  Simply put, life may be present everywhere on Earth’s surface, but only rarely does it get fossilized.  On Mars, how might have early preservation of organic matter and/or delicate textures proceeded?   Hematite, other iron oxides, sulfate minerals, phyllosilicate minerals, silica, and possibly chloride minerals have all been suggested as possible substrates for fossil preservation. Indeed, all are known to facilitate the preservation of fossil morphologies and molecules on Earth.  Some – iron oxides, sulfate salts – additionally can preserve isotopic signatures of biogeochemical processes.
Critical questions:
Are all these mineral phases early and contemporaneous with sedimentation and/or rock alteration? 
How can this timing be reliably established? 
What is the particular mechanism/process involved in fossilization.
Could mineralization destroy organic matter (rock/water ratio, chemical reactions)?




Evaluation Questions for Voting

1)  How diverse is the site?  A high score should be awarded for sites that show strong evidence of rich mineralogy as well as rich geomorphology and stratigraphy, and that both of these attributes be clearly related to the presence of water.  Ideal Example: a site with strong mineral signatures, expressed in discrete stratigraphic sequences, in which the mineralogy varies from unit to unit, and in which different units contain geomorphic evidence for subaqueous processes.

Vote on the following:
Can multiple rock units be observed from orbit?
Do these units have well defined stratigraphic and/or cross-cutting relationships?
Do these units show diverse mineralogic features indicating water?
Do these units show diverse geomorphic features indicating water?
How strong is the evidence that these features formed through interaction with water?
Can multiple working hypotheses be developed for the interpretation of key features?
Can the MSL payload enable us to differentiate among alternatives?


2) How refined is the geologic context?    A high score should be awarded for sites which have a clear geologic context and which offer the rover a well-defined set of objectives.  The mission can always hope to count on “surprises” as a back up for discovery, but its best to have a hypothesis-driven framework to begin with at the outset.  Ideal Example:  Two groups of rocks can be inferred to have formed by aqueous processes, and one is clearly Hesperian age, the other Noachian age, and these units can be shown to have a superposed, unconformable relationship.  Internally, one or both groups of rocks can be further subdivided and its internal stratigraphy and mineralogy, if studied by the rover, are representative of similar types of rocks at other locations on Mars; the discoveries of the rover can be extended more broadly away from the landing site.

Vote on the following:
How much of what will be observed by the rover can be placed into a geologic framework before landing?
Can local observations be placed into a more general regional context and, if this is possible, how confidently can this be done?  Can the MSL payload be used to resolve alternative interpretations?
Do we reliably know the age of the identified rock units?


3)  How habitable was the environment(s) represented by the landing site?  Do the minerals and morphologic/stratigraphic relationships provide strong and direct evidence for habitability?  What do theoretical considerations of these observations suggest?  Ideal Example:  High silica content rocks are observed in mound-shaped structures consistent with spring deposits.  Phyllosilicate/sulfate/silica enrichment in layers contained within enclosed basin; composition varies from layer to layer.

Vote on the following:
Does mineralogic/geomorphic evidence indicate a particular habitable environment?  If there is ambiguity, what are the options?
If there is ambiguity, what are the options and how could the MSL payload resolve them?
Can minerals or geomorphic features detected from orbit be used as reliable indicators of past:
a.    duration of water
b.    water pH
c.    rock-water ratio
d.    water activity (salinity)


4) How high is the fossil preservation potential?  The goal here is to evaluate what mineral(s) may have precipitated early, or other processes such as clay mineral adsorption, would have caused entrapment of organic compounds and/or preservation of biogenic textures.  Also, what processes would have destroyed fossil preservation?  Ideal Example:  Precipitation of mineral at low temperature that impregnated cellular structure to entomb organics; mineral aggregate has such low permeability that oxidation during contemporaneous or subsequent pore fluid circulation is excluded.
Vote on the following:
1)  Are all these mineral phases early and contemporaneous with sedimentation and/or rock alteration? 
How can this timing be reliably established? 
What is the particular mechanism/process involved in fossilization.
Could mineralization destroy organic matter (rock/water ratio, chemical reactions)?

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