A Post Accretionary Origin for Meteoritic and Cometary Organic Solids?

Introduction: The insoluble organic matter (IOM) contained with in all chondritic meteorites constitutes a significant reservoir of extraterrestrial organic matter that in some cases may be as high as 3.5 wt. % [1]. Whereas considerable effort has been made to develop a molecular understanding of meteoritic IOM, there remains considerable uncertainty regarding its origins, i.e., was IOM formed prior to the origin of the solar system, as consequence of nebular processes, or as consequence of post accretionary processing. Notwithstanding this uncertainty, recent studies have shown that IOM carries in its molecular structure a rich history of parent body chemical evolution, e.g., chemical oxidation [2]. More recently it has been shown that chondritic IOM precisely records the integrated thermal history associated with parent body metamorphism [3]. A significant conclusion of this later study is that primitive IOM (as occurs in type 1 & 2 chondrites) is clearly not formed through hot processes and IOM that occurs in type 3 chondrites is derived directly from heating of primitive IOM. With the successful capture of cometary organic matter from the Stardust mission the opportunity to analyze and compare organic solids from a comet with organic matter of primitive meteorites was finally possible. In these first analyses, it became evident that the cometary organic particles exhibit a wide range in their respective molecular structures and elemental chemistry [4,5]. One subset of particles, the oxygen rich particles, exhibit what appears to be a clear chemical connection with primitive IOM (e.g., that present in Murchison CM2). A comparision of CXANES spectra of two oxygen rich particles with that of Murchison is shown in Figure 1. Wild 2 particle A exhibits a simple (in terms of diversity of organic functional groups) absorption spectrum revealing a predominance of alcohol moieties; Wild 2 particle B, with slightly lower O/C exhibits a more complex spectrum with significant peaks at 285 and 286.5 eV. The peak at 286.5 eV has been shown to be directly related to the degradation of sugars, e.g., polysaccharides [6]. The peak at 285 eV can be attributed to either olefinic or aromatic carbon. In comparision, Murchison’s IOM exhibits considerably greater intensity at 285 eV (consistent with the high aromatic carbon content [2]), but also exhibits intensity at 286.5 eV. Figure 1: C-XANES spectra of IOM from Murchision (CM2) and two organic particles extracted from Stardust aerogel collectors. Particle Wild 2 Org A is characterized by having a large atomic O/C and a CXANES spectrum dominated C-OH absorption at 289.5 eV (peak C). Particle Wild 2 Org B has a lower O/C and in addition to C-OH, peaks at 285 and 286.5 eV (A & B) are clearly evident corresponding to aromatic/olefinic and vinyl keto moieties.