Presolar Grains

Presolar grains are the solid matter that was contained in the interstellar gas before the sun was born, but those terms have come to mean presolar grains that are found in meteorites to meteorite researchers. Such grains comprise only about 0.1 percent of the total mass of presolar particulate matter, but they are of the type found in meteorites. Such found examples are isotopically-distinct clusters of material found in the fine-grained matrix of primitive meteorites, such as chondrites. Their isotopic differences from the surrounding meteorite suggest that those clusters predate the solar system. Crystallinity in these clusters ranges from that of micrometre-sized silicon carbide crystals, down to that of diamond and unlayered graphene crystals with fewer than 100 atoms. The refractory grains have condensed thermally within slowly cooling gases to achieve their mineral structures, probably in supernovae or the stellar outflows of red giant stars, and later admixed into the molecular cloud from which the solar nebula separated to form our solar system. Presolar grains identified in the laboratory by their abnormal isotopic abundances consist of refractory minerals which survived the collapse of the solar nebula, and the subsequent formation of planetesimals.

In the 1960s, neon and xenon components of trapped noble gases with unusual isotopic ratios were discovered in primitive meteorites, but their origin and the types of matter that contain them was a mystery. In the mid-1970s Donald D. Clayton predicted unusual isotopic compositions within thermally condensed grains during mass loss from stars of differing types and argued that such grains exist throughout the interstellar medium and provide isotopic evidence of their prior interstellar residence in early-solar-system samples. He defined several differing types of presolar grains (STARDUST from red giant stars, SUNOCONs from supernovae, NEBCONs from nebular accretion, and NOVACONs from novae), but his suggestions lay dormant for a decade until these grains were discovered within meteorites. The first unambiguous demonstration of the existence of stardust within meteorites came from the laboratory of Edward Anders in Chicago, who found that the xenon isotopic abundances contained within an insoluble carbonaceous residue that remained after the meteorite bulk was dissolved by strong acids matched almost exactly the predictions for red-giant stardust. There followed a decade of intense experimental searching to isolate single grains of those xenon carriers.

In 1987 diamond and silicon carbide grains were found to be carriers of noble gases. Significant isotopic anomalies were in turn measured within the structural chemical elements of these grains.

Because identified presolar stardust grains formed within outflowing and cooling gases from specific stellar nucleosynthesis sources, the isotopic composition of their elements is that of the stars and differs from the isotopic composition of solar-system matter as well as from the galactic average. These isotopic signatures often fingerprint very specific astrophysical nuclear processes, and thus they go beyond proving interstellar origin to providing independent insight into nuclear processes within the parent stars.