Vision and Voyages for Planetary Science in the - Solar System ...

next step toward answering these questions would be provided through the analysis of carefully selected
samples from geologically diverse and well-characterized sites that are returned to Earth for detailed
study. Existing scientific knowledge of Mars makes it possible to select a site from which to collect an
excellent suite of rock and soil samples to address the life and habitability questions, and the technology
to implement the sample return campaign exists, or will be developed—including required entry, descent,
and landing and rover mobility systems. Existing and future analysis techniques developed in
laboratories around the world will provide the means to perform a wide array of tests on these samples,
develop hypotheses for the origin of their chemical, isotopic, and morphologic signatures, and, most
importantly, perform follow-up measurements to test and validate the findings.
BOX 6.3 Sample Return is the Next Step
The analysis of carefully selected and well documented samples from a well characterized site will
provide the highest scientific return on investment for understanding Mars in the context of solar system
evolution and addressing the question of whether Mars has ever been an abode of life.
The purpose and context of a Mars sample return has changed significantly since the early concepts
that focused solely on reconnaissance. At that time the key questions centered on bulk planetary
geochemistry, petrology, and geochronology, for which a wide range of sample types would be
acceptable. Today the emphasis is on well characterized and carefully selected rocks, with the
recognition of the critical importance of sedimentary rocks that provide clues to aqueous and
environmental conditions. While it is widely accepted that we have samples of Mars on Earth in the form
of the SNC meteorites, they are not representative of the diversity of Mars. They are all igneous in origin,
whereas recent observations have shown the occurrence of chemical sedimentary rocks of aqueous origin
as well as igneous, metamorphic, and sedimentary rocks that have been aqueously altered. It is these
aqueous and altered materials that will provide the opportunity to study aqueous environments and
potential prebiotic chemistry.
Two approaches exist to study martian materials – in situ measurements or through returned samples.
Returning samples allows for the analysis of elemental, mineralogic, petrologic, isotopic, and textural
information using state-of-the-art instrumentation in multiple laboratories. In addition, it allows for the
application of different analytical approaches using technologies that advance over a decade or more, and,
most importantly, the opportunity to conduct follow-up experiments that are essential to validate and
corroborate the results. On an in situ mission, only an extremely limited set of experiments can be
performed because of the difficulty of miniaturizing state-of-the-art analytical tools within the limited
payload capacity of a lander or rover. In addition, these discrete experiments must be selected years in
advance of the mission’s launch. Finally, calibrating and validating the results of sophisticated
experiments can be challenging in a lab, and will be significantly more difficult when done remotely. The
Viking and the ALH84001 martian meteorite experiences underscore the differences in these approaches.
It has proven difficult to reach unique conclusions regarding the existence of possible extant life or
organic materials from the Viking data because of the assumptions regarding the nature of the martian
surface materials that were necessary to design the instrument payload. For example, recent analysis of
the Phoenix data suggest that oxidizing compounds may have been present that would have destroyed any
organics during the sample heating required for the Viking instruments, raising significant questions
about the interpretation of the Viking results. In contrast, the ALH84001 experience underscores the
tremendous value of being able to perform a large number of independent analyses and follow-on
experiments. Multiple approaches in numerous laboratories have been possible over a decade because the
samples were on Earth, and new experiments could be performed to test differing hypotheses. One
essential lesson from ALH84001 is how involved sample studies can be, requiring multiple methods,
complex sample preparation, and the collective capability of Earth’s research laboratories to evaluate
complex questions. Finally, searching for evidence of extant life at Mars with a limited suite of
PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION
6-22