**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*

1)  Can multiple rock units be observed from orbit?

2)  Do these units have well defined stratigraphic and/or cross-cutting
    relationships?

3)  Do these units show diverse mineralogic and/or geomorphic features?

4)  How strong is the evidence that these features formed through
    interaction with water?

5)  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*

1)  How much of what will be observed by the rover can be placed into a
    geologic framework before landing?

2)  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?

3)  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

1)  Does mineralogic evidence indicate a particular habitable
    environment? If there is ambiguity, what are the options?

2)  Does geomorphic evidence indicate a particular habitable
    environment? If there is ambiguity, what are the options and how
    could the MSL payload resolve them?

3)  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*:

1)  Are all these mineral phases early and contemporaneous with
    sedimentation and/or rock alteration?

2)  How can this timing be reliably established?

3)  What is the particular mechanism/process involved in fossilization.

4)  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:*

1)  Can multiple rock units be observed from orbit?

2)  Do these units have well defined stratigraphic and/or cross-cutting
    relationships?

3)  Do these units show diverse mineralogic features indicating water?

4)  Do these units show diverse geomorphic features indicating water?

5)  How strong is the evidence that these features formed through
    interaction with water?

6)  Can multiple working hypotheses be developed for the interpretation
    of key features?

7)  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:*

1)  How much of what will be observed by the rover can be placed into a
    geologic framework before landing?

2)  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?

3)  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:*

1)  Does mineralogic/geomorphic evidence indicate a particular habitable
    environment? If there is ambiguity, what are the options?

2)  If there is ambiguity, what are the options and how could the MSL
    payload resolve them?

3)  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?

2)  How can this timing be reliably established?

3)  What is the particular mechanism/process involved in fossilization.

4)  Could mineralization destroy organic matter (rock/water ratio,
    chemical reactions)?