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

• What are the abundances and distributions of different classes of asteroids, comets, and
KBOs?
• How do the compositions of Oort cloud comets differ from those derived from the Kuiper
belt?
Future Directions for Investigations and Measurements
Although progress has been made in assessing whether various kinds of interplanetary dust are
derived from comets or asteroids, there remains some uncertainty about the parent objects of some kinds
of primitive meteorites. Determining the ages of chondrite components that record specific nebular
processes is required to produce a timeline for major events in the solar nebula. Further refinements in
analyzing solar wind samples are needed to define isotopic ratios in the Sun. Sampling additional comets
is necessary to understand the diversity within this large population of poorly studied primitive bodies.
Obtaining comet samples from the surface, as opposed to dust ejected from a comet nucleus, is a high a
priority. Increasing the number of known KBOs may reveal the environments in which different classes
of objects formed.
Effects and Timing of Secondary Processes on the Evolution of Primitive Bodies
The asteroidal parent bodies of most meteorites have been altered by internal heating, reactions
with aqueous fluids (produced by melting accreted ices), and impacts. Telescopic spectral measurements
of asteroids indicate that the nature and intensity of alteration differs with orbital position. Secondary
processes on primitive bodies control the mineralogy, nature of organic compounds, and volatile element
abundances. Abundant meteorite samples have allowed us to quantify the conditions under which these
secondary processes occurred and to understand their timing in asteroids. However, the extent of such
secondary processes in comets and KBOs is not understood at all.
The conditions and timescales for metamorphism and aqueous alteration in asteroids have been
quantified, 12 and considerable progress has been made in modeling asteroid thermal histories. 13 A
consensus has apparently been reached that decay of the short-lived radionuclide 26 Al was the primary
heat source for asteroids, based on new isotopic analyses of meteorites and its success in thermal
evolution models. Recent spacecraft missions to asteroids and comets have documented secondary
processes, including extensive impact cratering on asteroid surfaces and smooth flows of erupted
materials on comet nuclei 14
Important Questions
Some important questions concerning the effects and timing of secondary processes on the
evolution of primitive bodies include the following:
• To what degree have comets been affected by thermal and aqueous alteration processes?
• How well can we read the nebular record in extraterrestrial samples through the haze of
secondary processes?
• What is the relationship between large and small KBOs? Is the small population derived by
impact disruption of the large one?
• How do the impact histories of asteroids compare to those of comets and KBOs?
• How do physical secondary processes such as spin-up result from non-gravitational forces,
the creation and destruction of binary objects, and space weathering?
PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION
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