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

Presolar Processes Recorded in the Materials of Primitive Bodies
Traditionally, the only avenue for understanding processes that predated our own solar system has
been astronomical observations. Now, analyses of materials from primitive bodies are revolutionizing
this field. Studies of microscopic presolar grains in chondritic meteorites, interplanetary dust particles,
and comet samples returned by the Stardust mission provide critical constraints for models of the
synthesis of elements and isotopes within stars and supernovae. 1 Characterization studies of organic
matter in these materials are proceeding apace; they reveal how simple carbon-based molecules formed in
interstellar space have been processed into more complex molecules in the solar nebula and in
planetesimals. Isotopic and structural fingerprints in these molecules are allowing us to constrain how
and where these molecules formed.
Remarkable progress, enabled by significant advances in micro-analytical technology, has been
made in documenting the compositions of presolar grains incorporated into chondrites, and in linking the
various kinds of grains to the specific stellar environments in which they formed. 2,3 Further technological
advances will continue to revolutionize our understanding of stellar nucleosynthetic processes.
Somewhat surprisingly, comet samples returned to Earth by the Stardust mission were not dominated by
presolar grains, suggesting that our understanding of how presolar grains were incorporated into the solar
nebula is incomplete. 4 The challenging measurements of the stable isotopic compositions of specific
organic molecules in meteorites and Stardust grains have advanced during this decade.
FIGURE 4.2 Transmission electron microscope image of a presolar graphite grain, containing a central
core (dark) of titanium-vanadium carbide. This grain formed by condensation around another star.
SOURCE: H.Y. McSween and G.R. Huss, Cosmochemistry, Cambridge University Press, 2010, p. 131.
Copyright 2010 Cambridge University Press, reprinted with permission.
Important Questions
Important questions for understanding of presolar processes recorded in the materials of primitive
bodies include the following:
• How do the presolar solids found in chondrites relate to astronomical observations of solids
disposed around young stars?
• How abundant are presolar silicates and oxides? Most of the presolar grains recognized so far
are carbon (diamond, graphite) phases or carbides.
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