Silicon Carbide in Comet Wild 2 & the Abundance of Pre-solar Grains in the Kuiper Belt

Introduction: One of the most remarkable findings from of the study of comet samples returned by the Stardust mission is the apparent scarcity of pre-solar grains. Unlike common expectations, the abundance of pre-solar grains in comet Wild 2 appears to be less than in primitive meteorites, micrometeorites and IDPs. The best measurements of the abundances of isotopically anomalous pre-solar grains in Wild 2, ~17ppm [1] and ~11ppm [2], have been made by nanoSIMS analyses of debris on the walls of craters in Al foil. These results are significantly lower than the ~375 ppm abundance characteristic of primitive IDPs [3] and lower than primitive meteorites. If pre-solar grains are indeed less abundant in Wild 2, a Kuiper belt comet, than in asteroidal materials, this finding will have profound implications on solar nebula processes and mixing. As pointed out by Stadermann and Floss [1], a lower abundance of pre-solar grains in a body formed at the edge of the solar nebula could be caused by dilution with solar nebula products that were radially transported above the nebular mid-plane. In this paper we use the detection of pre-solar SiC in Wild 2 materials captured in aerogel as a means to determine if the Al crater results reflect the true abundance of isotopically anomalous pre-solar grains in comet Wild 2 or if they are lower limits. Al craters vs aerogel tracks: Stardust collected comet dust in two ways: A) impact into aluminum foil and B) impact into low-density silica aerogel. Craters in aluminum are advantageous for searching for pre-solar grains because most of the components of an impacting particle are concentrated in a very limited area, the crater wall. This contrasts with impacts into silica aerogel where fragmentation can distribute contents over a volume as long as a centimeter. Disadvantages of the Al foils include a smaller number and size of impactors, as well as the heating effects that result from impact onto a solid metal substrate. As pointed out previously [1,2], crater formation in Al may destroy a fraction of pre-solar grains. Most Wild 2 particles were collected in low-density aerogel but capture tracks in aerogel have special difficulties for detection of pre-solar grains. In addition to being dispersed over a wide area, isolated submicron components are also systematically degraded or destroyed during capture. For a fragmenting comet particle , composed of both submicron and larger than mi-cron components, the larger components are …