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* Code 691, National Aeronautics and Space Administration, Goddard
Space Flight Center, Greenbelt, MD 20771; Edited by Robert P. Kirshner, Harvard-Smithsonian Center for
Astrophysics, Cambridge, MA, and approved January 12, 2000 (received for review November 6, 2000)
Understanding dynamic conditions in the Solar Nebula is the
key to prediction of the material to be found in comets. We suggest that a dynamic, large-scale circulation pattern brings processed dust
and gas from the inner nebula back out into the region of cometesimal
formation
Special Feature
Review
Constraints on nebular dynamics and chemistry based on
observations of annealed magnesium silicate grains in comets and in
disks surrounding Herbig Ae/Be stars
,
, The National
Optical Astronomy Observatories, Code 681, National Aeronautics and
Space Administration, Goddard Space Flight Center, Greenbelt, MD 20771;
§ DuPont Central Research and Development, 328/318B,
Wilmington, DE 19880-0328; and ¶ University of
Cincinnati, Physics Department, Cincinnati, OH 45221-0011
extending possibly hundreds of astronomical units (AU) from
the sunand that the composition of comets is determined by a chemical
reaction network closely coupled to the dynamic transport of dust and
gas in the system. This scenario is supported by laboratory studies of
Mg silicates and the astronomical data for comets and for
protoplanetary disks associated with young stars, which demonstrate
that annealing of nebular silicates must occur in conjunction with a
large-scale circulation. Mass recycling of dust should have a
significant effect on the chemical kinetics of the outer nebula by
introducing reduced, gas-phase species produced in the higher
temperature and pressure environment of the inner nebula, along with
freshly processed grains with "clean" catalytic surfaces to the
region of cometesimal formation. Because comets probably form
throughout the lifetime of the Solar Nebula and processed (crystalline)
grains are not immediately available for incorporation into the first
generation of comets, an increasing fraction of dust incorporated into
a growing comet should be crystalline olivine and this fraction can
serve as a crude chronometer of the relative ages of comets. The
formation and evolution of key organic and biogenic molecules in comets
are potentially of great consequence to astrobiology.
To whom reprint requests should be addressed. E-mail:
hill{at}lepvax.gsfc.nasa.gov.
www.pnas.org/cgi/doi/10.1073/pnas.051530998
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