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

Astrobiology is the study of the origin, evolution, and distribution of life in the universe.
Although it is unrealistic to look for life on primitive bodies, we know from the study of meteorites that
many of them contain the organic ingredients for life. These organic compounds were formed as mantles
on dust grains in cold interstellar clouds and in the outer reaches of the solar nebula, and in rocks within
the warmed interiors of planetesimals as the ices in these bodies melted. The compounds are surprisingly
complex and include amino acids and other molecules that are central to life on Earth. The accretion of
such materials, late in the assembly of planets, is thought to have been a possible first step in the poorly
understood path from organic matter to organisms. Studies of the molecular forms and isotopic
compositions of the organic matter in meteorites and samples returned by spacecraft provide a prebiotic
starting point for the origin and evolution of life or an independent channel of abiologic organic
chemistry.
Specific objectives for continued advancement of this field in the coming decade include the
following:
• Determine the composition, origin and primordial distribution of volatiles and organic matter
in the Solar System;
• Understand how and when planetesimals were assembled to form planets; and
• Constrain the dynamical evolution of planets by their effects on the distribution of primitive
bodies.
Subsequent sections examines each of these objectives in turn, and identify critical questions to
be addressed and future investigations and measurements that could provide answers.
Composition, Origin, and Primordial Distribution
of Volatiles and Organic Matter in the Solar System
Meteorites and interplanetary dust particles are readily available sources of extraterrestrial
organic matter from asteroids and comets, although the volatile species and organic components of
comets and KBOs remain poorly understood. Systematic depletions in highly volatile elements in
meteorites testify to an important elemental fractionation in the early solar system, but it is not well
understood. Some organic matter in chondrites, interstellar dust particles, and Stardust comet samples has
been structurally and isotopically characterized, although the insoluble organic fractions, consisting of
huge complex molecules, are extremely difficult to analyze. Understanding the formation and evolution
of organic molecules in space and in planetesimals is essential to astrobiology. Telescopic spectral
observations of primitive bodies provide at best a tantalizing but incomplete picture of the orbital
distribution of organic matter in the early solar system.
The planetary science community has made progress in characterizing the insoluble
(macromolecular) organic matter and in analyzing the isotopic compositions of specific organic
compounds in chondrites. 21,22,23 The changes in organic matter caused by parent body alteration are now
better understood, and the fractionation of isotopes during the evaporation of volatile elements has been
modeled. Cometary organic matter and volatile elements have been analyzed in samples returned by the
Stardust spacecraft, 24,25,26 and cometary volatiles are now recognized to be heterogeneous as determined
by the Deep Impact spacecraft. 27 Spectroscopy of recently discovered KBOs has provided some
constraints on their surface compositions. 28 Comparison of meteoritic and cometary organic matter
inventories with the composition of young circumstellar disks has been facilitated by recent Spitzer
telescope observations. 29
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