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

Important Questions
Some important questions concerning how and when planetesimals were assembled to form
planets include the following:
• Are there systematic chemical or isotopic gradients in the solar system, and if so, what do
they reveal about accretion?
• Do we have meteoritic samples of the objects that formed the dominant feeding zones for the
innermost planets?
• How did Earth get its water and other volatiles? What role did icy objects play in the
accretion of various planets?
• What is the mechanical process of accretion up to and through the formation of meter-size
bodies?
Future Directions for Investigations and Measurements
Understanding the formation times of the various materials that comprise comets could constrain
the chronology of km-sized objects beyond the orbit of Neptune. Measurements of deuterium/hydrogen
in different primitive objects can be used to constraint their possible contributions to the water inventory
of Earth and other planets. Determining the deuterium/hydrogen ratio in multiple comets would quantify
the role they may have played in delivering water and organic matter to the early Earth. Spacecraft
exploration of multiple asteroids could provide information on compositional gradients in the solar
system. Improvements in numerical models for accretion could provide a more robust understanding of
feeding zones.
Dynamical Evolution of Planets by their Effects on the Distribution of Primitive Bodies
The orbital distribution of the giant planets is now thought to have been much more dynamic than
previously appreciated. Orbital perturbations of primitive bodies are the key to unraveling planet
migrations in the early solar system. Although pathways from the main belt to account for near-Earth
asteroids are now clear, the source of some asteroid populations, such as Trojans (in orbits near Jupiter)
and Centaurs (in orbits between the asteroid belt and the Kuiper belt) is not understood. The Kuiper belt
is an important reservoir of comets, although the precise delivery paths into the inner solar system remain
unclear.
Bodies exhibiting cometary activity have now been recognized within the main asteroid belt and
among the Centaur asteroids. 34 The structure of the Kuiper belt provides one of the best constraints on the
dynamical rearrangement of the giant planets, and some recent KBO studies have revised scenarios for
the early orbital history of the solar system. 35 The size distribution of main belt asteroids has been
matched to that of impactors during the late heavy bombardment about 4 billion years ago, suggesting
that the asteroid belt was the source of these impactors. 36,37 A different population of impactors is
indicated for the outer solar system judging from the cratering record preserved on Saturn’s ice
satellites. 38
Important Questions
Some important questions concerning the dynamical evolution of planets by their effects on the
distribution of primitive bodies include the following:
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