CHEMICALLY ANOMALOUS, PREACCRETIONALLY IRRADIATED GRAINS IN INTERPLANETARY DUST FROM COMETS

Nonstoichiometric grains with depletions of magnesium and silicon (rel ative to oxygen) and inclusions of iron-nickel metal and iron-rich sul fides have been identified in interplanetary dust particles from comet s. These chemical anomalies accumulate in grains exposed to ionizing r adiation. The grains, known as GEMS (glass with embedded metal and sul fides), were irradiated before the accretion of comets, and their infe rred exposure ages, submicrometer sizes, and ''amorphous'' silicate st ructures are consistent with those of interstellar silicate grains. Th e measured compositional trends suggest that chemical (as well as isot opic) anomalies can be used to identify presolar interstellar componen ts in primitive meteoritic materials.