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

DETERMINE IF LIFE EVER AROSE ON MARS
The prime focus of this goal is to determine if life is or was present on Mars. If life was there we
must understand the resources that support or supported it. If life never existed yet conditions appear to
have been suitable for formation and/or maintenance of life, a focus would then be to understand why life
did not originate. A comprehensive conclusion about the question of life on Mars will necessitate
understanding the planetary evolution of Mars and whether Mars is or could have been habitable, using
multi-disciplinary scientific exploration at scales ranging from planetary to microscopic. The strategy
adopted to pursue this goal has two sequential science steps: assess the habitability of Mars on an
environment-by-environment basis using global remote sensing observations; and then test for prebiotic
processes, past life, or present life in environments that can be shown to have high habitability potential.
A critical means to achieve both objectives is to characterize martian carbon chemistry and carbon
cycling.
Therefore, the committee’s specific objectives to pursue the life goal are to as follows:
• Assess the past and present habitability of Mars,
• Assess whether life is or was present on Mars in its geochemical context, and
• Characterize carbon cycling and prebiotic chemistry.
Subsequent sections examine each of these objectives in turn, identifies critical questions to be
addressed and future investigations and measurements that could provide answers.
BOX 6.2 Biosignatures
Life can be defined as essentially a self-sustaining system capable of evolution. But to guide the
search for signs of life on Mars requires a working concept of life that helps to identify its key
characteristics and its environmental requirements. Biosignatures are features that can be unambiguously
interpreted as evidence of life and so provide the means to address fundamental questions about the
origins and evolution of life. Types of biosignatures include morphologies (e.g., cells, and plant or animal
remnants), sedimentary fabrics (e.g., laminations formed by biofilms), organic molecules, biominerals
(e.g., certain forms of magnetite), 4 elemental abundances, and stable isotopic patterns. Because some
biosignatures are preserved over geologic time scales and in environments that are no longer habitable,
they are important targets of exploration. It is not unreasonable to anticipate that any martian life might
differ significantly from life on Earth, though Earth’s environments have been more similar to those on
Mars than to the environments of any other object in the solar system. Moreover, Mars and Earth may
have exchanged life forms through impact ejecta. Any martian life may reasonably be assumed to have
shared at least some of its basic attributes with life as we know it, which implies that any martian life also
requires liquid water, carbon-based chemistry, and electron transfer processes. 5,6
Our working concept of life should also identify environmental conditions that are most
conducive to life. A habitable environment must sustain liquid water at least intermittently and also allow
key biological molecules to survive. The elements carbon, hydrogen, nitrogen, oxygen, phosphorus, and
sulfur (CHNOPS) must be available, as they are essential for forming the covalently bonded compounds
of all known life. Organic compounds are therefore key targets, with the caveat that martian and Earthly
life might have employed different compounds. Energy drives metabolism and motility, and must be
available from, for example, light or energy-yielding chemical reactions. 7 Finally, the rates of
environmental changes must not exceed rates at which life could adapt. 8
Even if habitable environments supported the origination and evolution of life on Mars, the right
set of environmental conditions is required in order to preserve biosignatures. The study of fossilization
processes will be as important for Mars as it has been for Earth. 9 The preservation of biosignatures is
critically sensitive to the diagenetic processes that control preservation and, paradoxically, the very
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