Comment on "The shape and composition of interstellar silicate grains"

Abstract

The central question regarding the origin of GEMS is whether, (A) they are presolar IS amorphous silicates that survived the collapsing molecular cloud and subsequent protoplanetary disk stages of the formation of our solar system to be incorporated into IDPs (Bradley 1994), or (B) they are mostly grains formed in the solar nebula (Keller & Messenger 2004, 2007b,a, 2008). If IS amorphous silicates survived, can we recognize them? The answer is yes, but it is a conditional yes. Isotopically anomalous GEMS have been identified that, during transport through the ISM and formation of the solar system, retained some portion of the isotope signatures of their formation, in presolar CS environments. These GEMS were undeniably part of the population of presolar IS silicates. They may be analogues of the amorphous silicate grains observed in the outer disks of other young stars (van Boekel et al. 2004). The simplest explanation for those observed grains is that they are IS amorphous silicates that have escaped significant heating in the (outer) accretion disks (van Boekel 2007), and laboratory heating experiments indicate that GEMS also escaped significant heating in the solar nebula accretion disk (Brownlee et al. 2005). The isotopic compositions of most GEMS are normal (solar) within the detection limits of current analytical ion microprobes, and for those not petrographically associated with presolar material, their origin may remain an open question for the foreseeable future. However, invoking Occams razor, the similarity in the properties between isotopically anomalous and normal GEMS favors presolar origin and residence in the ISM for all GEMS, especially since both are found in petrographic association with isotopically anomalous presolar organic material in IDPs. While it is possible that some isotopically normal. GEMS formed in the solar system, it would indeed be remarkable, and probably unprecedented, that a population of grains as exotic as GEMS, found in only one class of meteoritic material (CP IDPs), having a similar size distribution, mineralogy, petrography and bulk chemical composition, arose by different mechanisms at different times in the environments of different classes of stars (evolved AGB stars versus protostellar nebulae like the solar nebula). How exciting that we do, indeed, have samples of presolar IS amorphous silicates, one of the fundamental building blocks of solar systems. © ESO 2008.