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

BUILDING NEW WORLDS: UNDERSTANDING SOLAR SYSTEM BEGINNINGS
What Were the Initial Stages and Conditions and Processes of Solar System Formation
and the Nature of the Interstellar Matter that Was Incorporated?
A nearby supernova explosion may have initially triggered the collapse of the local molecular
cloud and thereby the onset of solar system formation. 3 Many primitive bodies—asteroids, comets,
meteorites, Kuiper belt objects, Trojan asteroids, and bodies in the distant Oort cloud—still contain intact
records of this very early period. Examination of their minerals and their isotopic and molecular
chemistry can reveal the physical conditions under which they formed and provide our best view into this
earliest chapter of solar system formation. In fact, we may see isotopic evidence of such a supernova
explosion in ancient meteorite samples. 4 The least-processed of the primitive meteorite samples preserve
tiny presolar grains, whose isotopic compositions reflect the nucleosynthetic processes in stars and
supernovae that preceded solar system formation. 5 These presolar stellar remnants provided key
ingredients (e.g. metals and silicates) for the accretion of planets.
In the last decade major progress has been made in linking the compositions of presolar grains in
chondritic meteorites to the specific stellar environments where they are formed. 6 Unexpectedly, presolar
grains were in low abundance in comet samples returned by the Stardust mission, signaling our limited
understanding as to how and where presolar grains were incorporated into the solar nebula. 7 Recent
studies of organic matter in these materials are starting to reveal how carbon-based molecules formed in
interstellar space are further processed and incorporated. Most of the presolar grains recognized so far are
carbon (diamond and graphite) or carbides; 8 important questions remain as to the abundance of presolar
silicates and oxides and how the compositions of presolar grains and organic molecules differ among
comets.
After the Sun formed, the solar nebula gradually began to coalesce and form clumps that, in turn,
accreted into planetesimals. In the inner solar system where conditions were hotter, primitive asteroids
and meteorites record early events and processes in the solar nebula whereby interstellar solids melted,
evaporated, and condensed to form new compounds. Farther out, beyond the “snow line”, it was cooler
and volatiles condensed as ices; here the giant planets and their satellite systems began to form. In this
region and extending farther out where temperatures were extremely low minimizing chemical
processing, the parent objects of comets formed. They retain the most pristine records of the initial
chemistry of the outer parts of the solar nebula. The size distributions of objects in the Kuiper belt reveal
the nature of accretion in the outer region that was arrested early, ceasing their growth. 9,10 Mixing of
materials between nebular regions is clearly shown by the diverse components in the Stardust comet
samples. 11 It also now appears that some differentiated asteroids formed earlier than the primitive
chondrites, showing that the accretional sequences were far more complex than once thought. 12
Over the next decade important breakthroughs in our understanding of presolar materials and
early nebular processes will certainly come from applying ever advancing analytical techniques in the
analysis of meteorites, interplanetary dust, and Stardust samples. However, greater potential to achieve
major steps in our understanding of presolar and nebular cosmochemistry would come from the analysis
of samples returned directly from the surfaces of comets. Stardust samples have dramatically expanded
our knowledge of presolar sources and nebular processes. Eventually, the greatest scientific
breakthroughs in addressing these questions will come from returning surface samples whose volatiles
have been cryogenically preserved.
PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION
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