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

Important Questions
Some important questions concerning the composition, origin, and primordial distribution of
volatiles and organic matter in the solar system include the following:
• What are the chemical routes leading to organic molecule complexity in regions of star and
planet formation?
• What was the proportion of surviving presolar organic matter in the solar nebula, relative to
the organic compounds produced locally?
• What roles did secondary processes and mineral interactions play in the formation of organic
molecules?
• How stable are organic molecules in different space environments?
• What caused the depletions in volatile elements, relative to chondrites, observed in
differentiated asteroids and planets?
• What kinds of surface evolution, radiation chemistry, and surface-atmosphere interactions
occur on distant icy primitive bodies?
• How is the surface composition of comets modified by thermal radiation and impact
processes?
Future Directions for Investigations and Measurements
The New Horizons spacecraft will fly past Pluto in July 2015 and obtain remote sensing data on
the dwarf planet and its satellites Charon, Nix, and Hydra. It is expected that a successful encounter with
Pluto will be followed by retargeting the spacecraft to a flyby with a yet-to-be-selected Kuiper belt object.
The detailed characterization of a single small system of Kuiper belt objects—Pluto and Charon—and
maybe more if suitable candidates can be found along New Horizon’s trajectory will have to be
complemented by large ground-based telescope studies in order to continue the discovery and
characterization of a significant portion of KBOs. Organic matter in returned comet samples will provide
critical new information on organic synthesis. The study of organic matter in extraterrestrial materials
will also evolve from basic characterization of simple compounds and mixtures to understanding the
origin of different molecules. Return of samples from a range of organic-rich asteroids and comets
(including cryogenically preserved comets) will ultimately be required to fully address these questions.
How and When Planetesimals Were Assembled to Form Planets
Planet formation was hierarchical, as small planetesimals were assembled into ever-larger ones,
eventually forming the planets. The feeding zones for accretion of planets and the timing of planetary
growth remain incompletely understood. Swarms of asteroids, comets, and KBOs provide basic
information on planetesimal sizes, compositions, and orbital parameters with which to model the
assembly of planets. Studies of radiogenic isotopes in meteorites allow the timing of planet formation to
be constrained.
Theoretical studies, particularly complex accretion models developed during this decade follow
the orbital evolution of many thousands of objects and provide constraints on the timescales and widths of
feeding zones for the terrestrial planets. 30 Our understanding of the timing of accretion benefits from
improved determinations of the formation chronology of Earth, the Moon, and Mars, which have been
made using measurements of short-lived radioisotopes in samples. 31,32,33
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