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

Future Directions for Investigations and Measurements
Comets and distant KBOs likely record secondary processing under a vast range of conditions;
studying such processes will require a combination of telescopic observations and challenging spacecraft
missions. Formulation of more realistic thermal models for asteroids and comets is needed.
Nature and Chronology of Planetesimal Differentiation
Melted silicate-metal asteroids and the meteorites derived from them provide information on the
formation of crusts, mantles, and cores on bodies with compositions different from Earth and under
conditions not encountered on our planet. They allow us to test the generality of hypotheses about the
differentiation of planets. From them, we learn how elements are partitioned between molten and solid
phases. The radiogenic isotope systems in differentiated meteorites provide the required information to
date differentiation processes recorded in samples of Earth, the Moon, and Mars. At the other end of the
compositional scale, the Kuiper belt is home to the largest number of silicate-ice objects, some of which it
has been suggested may have undergone internal heating and differentiation.
New age dates have revolutionized the chronology of differentiated silicate bodies, 15,16 while new
meteorite recoveries have broadened the range of differentiation styles and conditions experienced by
these bodies. 17 The rapid cooling rates determined from nickel diffusion profiles in some iron meteorite
groups suggest that glancing impacts may have stripped off their thermally insulating silicate mantles,
exposing hot, naked iron cores. 18 Thermal models to explain metal-silicate differentiation in asteroids and
silicate-ice differentiation in KBOs have become more sophisticated as more rigorous constraints have
been placed on them. 19,20
Important Questions
Some important questions concerning the nature and chronology of planetesimal differentiation
include the following:
• Did asteroid differentiation involve near-complete melting to form magma oceans, or modest
partial melting?
• How did differentiation vary on bodies with large proportions of metal or ices?
• Were there radial or planetesimal-size limits on differentiation, and were KBOs and comets
formed too late to have included significant amounts of live 26 Al as a heat source?
• What are the internal structures of Trojans and KBOs?
Future Directions for Investigations and Measurements
The Dawn spacecraft will arrive at asteroids Vesta in July 2011 and Ceres in February 2015.
Mapping and spectroscopy of Vesta and Ceres will provide new insights into differing styles of asteroid
differentiation and set the stage for geophysical exploration of asteroids by spacecraft to determine their
interior structures and compositions. Such spacecraft missions can provide ground truth for systematic
studies of KBOs with large ground-based telescopes, which might probe the state of differentiation on
bodies with a broader range of sizes and dynamical locations.
PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION
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