3.4. Interstellar Dust in the Solar System
Interstellar grains have been found in primitive meteorites and in interplanetary dust particles based on the analysis of isotopic anomalies (see Kerridge 1999 and Bradley 1999 for recent reviews). Most presolar grains identified to date are carbonaceous: diamonds, SiC, and graphite (very small TiC, ZrC, and MoC grains have also been found as inclusions in SiC and graphite grains); also identified are oxides such as corundum (Al2O3) and silicon nitride (Si3N4). One should keep in mind that much of the less refractory dust incorporated into meteorites is lost during the chemical processing used to extract the refractory grains from meteorites. Therefore, the extracted presolar grains are compositionally not representative of the bulk interstellar dust; (26) for example, the procedures used to isolate interstellar grains in meteorites are designed to deliberately destroy silicate material which constitutes the bulk of the host meteorite (see Draine 1994).
The solar system is surrounded by the local interstellar cloud with a density nH 0.3 cm-3 and moving past the Sun with a velocity 26 km s-1 (Lallement et al. 1994). Interstellar grains embedded in the local cloud with sufficiently low charge-to-mass ratios can penetrate the heliopause and enter the solar system on hyperbolic orbits (small, charged grains are deflected from the heliosphere; Linde & Gombosi 2000). The interplanetary spacecraft Ulysses and Galileo have detected over 600 grains flowing into the solar system and determined their speed, direction, and mass and therefore the mass flux (but not chemical composition since the grains were destroyed by the detection technique; Grün et al. 1993, 1994; Frisch et al. 1999; Landgraf et al. 2000). Including the mass of the large population of interstellar micrometeorites entering the Earth's atmosphere (Taylor et al. 1996; Baggaley 2000), Frisch et al. (1999) found that the total dust-to-gas mass ratio in the local interstellar cloud is about twice the canonical value determined from the interstellar extinction. They also found that there is a substantial amount of mass in large grains of ~ 1 µm in size which is difficult to reconcile with the interstellar extinction and interstellar elemental abundances.
26 We have already seen in Footnote-22 that diamond is not a major interstellar dust component. Whittet, Duley, & Martin (1990) found that the abundance of Si in SiC dust in the diffuse ISM is at most 5% of that in silicates. Back.