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

• How do the compositions of presolar grains and organic molecules vary among different
comets?
Future Directions for Investigations and Measurements
While previous laboratory work has focused on presolar grains extracted from meteorites by
harsh chemical treatments, future efforts will exploit new technologies to locate and analyze presolar
grains in situ in the host meteorites, so as to identify less refractory materials. Obtaining presolar grains
and organic matter from additional comet sampling missions and from interplanetary dust particles will
allow us to understand how these materials were distributed in the solar nebula and preserved in solar
system solids.
Condensation, Accretion, and Other Formative Processes in the Solar Nebula
Primitive asteroids and the meteorites derived from them were witnesses to events and processes
in the inner solar nebula, while comets formed in the outer reaches of the solar system and thus record a
broader array of nebular environments. The innermost portions of the nebula were hot, causing
interstellar solids to melt, evaporate, and recondense as refractory minerals. The outer portions of the
nebula were cold enough to condense ices, profoundly changing the bulk compositions of accreted
planetesimals and planets. The thermal and chemical conditions of various nebular regions, the processes
that occurred in these regions, the timing of such processes, and the subsequent transport of materials
between nebular regions, can all be constrained from studies of primitive body materials. The nature of
the accretion process is also revealed by the size distributions of planetesimals whose assembly was
arrested before they reached planetary masses.
Significant progress has been made in constraining the nature and mixing during formation, and
the formation chronology, of primitive bodies using short-lived radioisotopes. 5,6 Dynamic nebular mixing
is indicated by the diverse components in comet samples returned by the Stardust spacecraft, and the
composition of the Sun is now constrained by analyses of solar wind samples from the Genesis
spacecraft. 7 The 16 O-rich isotopic composition of oxygen in the Sun is one of the biggest surprises in
cosmochemistry. 8 Another startling revelation is that melted and differentiated asteroids formed earlier
than chondritic asteroids, 9 implying that the early building blocks for the terrestrial planets were already
differentiated. 10 Kuiper belt object size distributions, now moderately well known for bodies greater than
100 km, are serving as our best tracer of the accretion phase in the outer solar nebula. 11
Important Questions
Some important questions concerning condensation, accretion, and other formative processes in
the solar nebula are as follows:
• How much time elapsed between the formation of the various chondrite components, and
what do those differences mean?
• Did evaporation and condensation of solids from hot gas occur only in localized areas of the
nebula, or was that widespread?
• What are the isotopic compositions of the important elements in the Sun?
• Which classes of meteorites come from which classes of asteroids, and how diverse were the
components from which asteroids were assembled?
• How variable are comet compositions, and how heterogeneous are individual comets?
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