Presolar grains

Presolar grains are interstellar solid matter in the form of tiny solid grains that originated at a time before the Sun was formed (presolar: before the Sun). Meteoriticists often use the term to represent stardust, grains that originated within a single star and which they extract from meteorites for study. Because most interstellar grains are not stardust from a single star, however, being instead interstellar cloud matter accreted by smaller presolar grains, most presolar grains are also not stardust. Logically, all stardust are presolar grains; but not all presolar grains are stardust. This confusing terminology is heavily entrenched among 21st century meteoriticists who prefer to use the terms interchangeably, however, so it is best to live with both usages or to write presolar stardust grains for stardust.

Presolar stardust grains formed within outflowing and cooling gases from earlier presolar stars. The stellar nucleosynthesis that took place within each presolar star gives to each granule an isotopic composition unique to that parent star, which differs from the isotopic composition of our solar system's matter as well as from the galactic average. These isotopic signatures often fingerprint very specific astrophysical nuclear processes that took place within the parent star and prove their presolar origin.

In the 1960s, the noble gases neon and xenon were discovered to have unusual isotopic ratios in primitive meteorites. Their origin and the type of matter that contained them was a mystery. These discoveries were made by vaporizing a bulk sample of a meteorite within a mass spectrometer, an instrument that is able to count the relative abundances of the isotopes of noble gases that are trapped in the meteorite sample. During the 1970s similar mass spectrometers discovered more components of trapped xenon isotopes. Competing speculations about the origins of the xenon isotopic components were advanced, all within the existing paradigm that the variations were created by processes within an initially homogeneous solar gas cloud.

A new theoretical framework for interpretation was advanced during the 1970s when Donald D. Clayton introduced stardust into planetary science. At the same time he rejected the popular belief among meteoriticists that the solar system began as a uniform hot gas. Instead he predicted that unusual but predictable isotopic compositions would be found within thermally condensed interstellar grains that had condensed during mass loss from stars of differing types. He argued that such grains exist throughout the interstellar medium. Clayton's first papers using that idea in 1975 pictured an interstellar medium populated with supernova grains that are rich in the radiogenic isotopes of Ne and Xe that had defined the extinct radioactivities. Clayton defined several different types of stardust presolar grains likely to be discovered: stardust from red giant stars, sunocons (acronym from SUperNOva CONdensates) from supernovae, nebcons from nebular condensation by accretion of cold cloud gaseous atoms and molecules, and novacons from nova condensation. Despite vigorous and continuous active development of this picture, Clayton's suggestions lay unsupported by others for a decade until such grains were discovered within meteorites.

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