Presolar grains
These isotopic signatures often fingerprint very specific astrophysical nuclear processes[1] that took place within the parent star or formation event and prove their presolar origin.
In the 1960s, the noble gases neon[5] and xenon[6] 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, in order to count the relative abundance of the isotopes of the very small amount of noble gases trapped as inclusions.
[7] 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 rejected the popular belief among meteoriticists that the Solar System began as a uniform hot gas.
[8][9] 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.
[8] 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.
The first unambiguous consequence of the existence of presolar grains within meteorites came from the laboratory of Edward Anders in Chicago,[11] who found using traditional mass spectrometry that the xenon isotopic abundances contained within an acid-insoluble carbonaceous residue that remained after the meteorite bulk had been dissolved in acids matched almost exactly the predictions for isotopic xenon in red giant dust condensate.
Significant isotopic anomalies were in turn measured by improvements in secondary ion mass spectrometry (SIMS) within the structural chemical elements of these grains.
The presolar component can be identified in the laboratory by their abnormal isotopic abundances and consists of refractory minerals which survived the collapse of the solar nebula and the subsequent formation of planetesimals.
The refractory grains achieved their mineral structures by condensing thermally within the slowly cooling expanding gases of supernovae and of red giant stars.
Hence, particles bearing the signature of stellar nucleosynthesis provide information on (i) condensation processes in red giant atmospheres, (ii) radiation and heating processes in the interstellar medium, and (iii) the types of particles that carried the elements of which we are made, across the galaxy to the Solar System.
