The Astromineralogy of Interplanetary Dust Particles

Abstract

Some chondritic interplanetary dust particles (IDPs) collected in the stratosphere are from comets. Because comets accreted at heliocentric distances beyond the giant planets, presolar grains or “astrominerals” with solar and non-solar isotopic compositions are expected to be even more abundant in cometary IDPs than in primitive meteorites. Non-solar D/H and 15N/14N isotopic enrichments in chondritic IDPs are associated with a carbonaceous carrier. These H and N enrichments are attributed to extreme mass fractionation during chemical reactions in cold (10–100 K), dense interstellar molecular clouds. Enstatite (MgSiO3) and forsterite (Mg2SiO4) crystals in IDPs are physically and compositionally similar to enstatite and forsterite grains detected around young and old stars by the Infrared Space Observatory (ISO), and large non-solar oxygen isotopic compositions recently measured in some IDP silicates establish that they are presolar silicates. The compositions, mineralogy, and optical properties of a class of grains in IDPs, known as GEMS, are consistent with those of interstellar “amorphous silicates”. Non-solar isotopic compositions measured in several GEMS confirm that they are interstellar silicate grains, one of the fundamental building blocks of solar systems. Submicrometer FeNi sulfide astrominerals like those found in IDPs may be responsible for a broad ~23.5 μm feature observed around protostars and protoplanetary discs by ISO. The first sample of a known comet (81P/Wild 2) was returned to Earth in 2006 by the Stardust mission. Contrary to expectations, the “Wild 2” sample does not resemble “cometary” IDPs and it does not contain abundant astromaterials. Instead, this particular Kuiper Belt comet resembles chondritic meteorites from the asteroid belt, which reinforces the astrophysical significance of IDPs as perhaps the most cosmically primitive materials available for laboratory investigations.