Presolar Grains

Pre-solar grains found within the Murchison CM2 meteorite. Image Source: geosci.uchicago.edu

Mineral grains that formed before our solar system. These tiny crystalline grains are typically found in the fine-grained matrix of chondritic (primitive) meteorites. Most grains probably formed in supernovae or the stellar outflows of red giant (AGB) stars before being incorporated in the molecular cloud from which the solar system formed. More exotic formation mechanisms include mass transfer within a binary star system. Presolar grains survived the collapse of the solar nebula, and also the subsequent formation of planetesimals because they consist of refractory minerals. Presolar grains in meteorites are recognized by their isotopically anomalous and highly unusual composition.

Peter Hoppe’s May, 2000 abstract in the paper “Presolar dust grains from meteorites and their stellar sources” provides an excellent baseline summary1 that has been updated to include new research2:

Primitive meteorites contain small concentrations (ppb to ppm) of presolar dust grains that have survived largely unaltered the processes that led to the formation of the solar system. Minerals identified to date include diamond, silicon carbide (SIC), graphite, silicon nitride (Si3N4), corundum (Al2O3), spinel (MgAl2O4), hibonite (CaAl12O19) and titanium oxide (TiO2). These grains exhibit large isotopic anomalies indicative of a stellar origin. Variations in the isotopic ratios of the major elements and of many trace elements contained in the grains range over more than 4 orders of magnitude. The presolar dust grains preserve memories of both nucleosynthesis in the parent stars and galactic chemical evolution. Most of the silicon carbide and corundum grains formed in the winds of red giant and asymptotic giant branch stars (AGB). Most graphite grains, some SiC and corundum grains, and all silicon nitride grains originated most likely in supernova ejecta. A few SiC and graphite grains appear to have a nova origin. The origin of the diamonds is still unknown but at least a small fraction apparently comes from supernovae. Diamonds are only 2 nm in size. The other types of presolar grains are larger and range from ~0.2 to 20 μm. These sizes are larger than those inferred for dust in the interstellar medium but are comparable to the sizes of interstellar dust in the heliosphere identified by the Galileo and Ulysses spacecraft missions.