Next: Using isotopes Up: Chapter 3: Meteorites &
Previous: An introduction to meteorites
There has long been a consensus that the solar
system was formed from part of an interstellar cloud containing dust
grains from a variety of stellar sources. However, in the 1960's and
early 1970's the most popular theory for the formation of the solar
system was that the material in the proto-solar system (the solar
nebula) was completely vaporized and mixed so that it became
isotopically and chemically homogeneous and thus lost any evidence of
its previous history. The first evidence that the solar nebula was not
homogeneous came from oxygen isotope studies which revealed an excess
of 16O in the calcium-aluminium-rich inclusions in
meteorites (Clayton et al. 1977). This was closely followed by
the detection of other isotopic anomalies for Mg, Si, Ca, Ti, etc.
(e.g. Lorin & Michel-Levy 1978; McCulloch & Wasserburg 1978;
Lee et al. 1978). These anomalies are very small and are
believed to imply severe reprocessing and dilution of the carrier
grains in the solar nebula. These were not pristine interstellar
grains. In the late 1980's, however, small refractory grains were found
in the most primitive meteorites, which have isotopic anomalies orders
of magnitude larger than those in the calcium-aluminium-rich
inclusions. These anomalies are indicative of an unaltered presolar
origin. The isotopic anomalies have become collectively known as the cosmic
chemical memory of the grains (Clayton 1978). Since their
discovery, a great deal of laboratory investigation of presolar grains
has been undertaken and has produced much data of interest regarding
interstellar and circumstellar grain nucleation, growth and composition
as well as models of stellar nucleosynthesis.
In the next few paragraphs there will be a brief outline of the
presolar grains found in meteorites, followed by a discussion of the
information available from isotopic analyses of presolar grains
concerning their history and origin.
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