Book Review: MAPS 40-4, 2005 April, pp. 655-656
The origin of chondrules and chondrites, by Derek Sears.
Cambridge University Press, 2004, 209 pp.
$110.00, hardcover (ISBN 0-521-83603-4).
Chondrites are the end products of nebular processes that operated in the
protoplanetary
disk and geological processes that operated on asteroids. Disentangling the
effects of these
two kinds of processes has been a continuing challenge for chondrite
researchers for the last
50 years. Sears infers that chondrules did not form in the solar nebula and
argues that impact
processing on asteroids was much more important. He includes a brief review of
Ca-Al-rich
inclusions and the possible role of nebular condensation in their formation,
but concludes that
Ca-Al-rich inclusions are evaporative residues and byproducts of chondrule
formation.
In the final chapter, he outlines his preferred origin for chondrules and
chondrites: within a few
million years after the formation of the oldest solar system solids, massive
impacts on the larger,
volatile-rich, carbonaceous asteroids produced plumes of melt droplets, gas,
dust, and fragments.
These plumes enveloped the asteroids, gradually depositing chondrules and Fe,Ni
metal grains
that had been aerodynamically sorted by size and density. Interestingly, he
suggests that most
North American researchers favor nebular mechanisms for chondrule formation,
whereas most
European and Japanese researchers favor all impact origin. However, recent
models proposed
specifically for CB chondrites by workers in North America resemble Sears'
concept.
Sears traces the birth and evolution of diverse rnodels for chondrule origins
and includes references
to 800 papers on chondrules and chondrites published between 1772 and 2003. His
historical approach
ensures that this book will be a valuable reference in many libraries. Where
else can you discover who
first compared the composition of the Sun's surface with that of the chondrites
(the famous American
astronomer, H.N. Russell in the Astrophysical Journal in 1929) or who first
published quantitative models
for heating asteroids of diverse sizes with 26Al (J.M. and M.A. Herndon in a
Meteoritics paper in 1977)?
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