Chemical attack on fragments of asteroids

Abstract

Meteoritic organic matter has been studied widely, especially the solvent soluble or free organic matter (FOM) fraction. However, the different components that make up the insoluble or macromolecular organic matter (MOM) fraction have drawn little attention, with most studies focussed on the overall nature of this organic polymer. The current study has employed a series of analytical techniques, gas chromatography-mass spectrometry (GC-MS), Fourier transform infrared (FTIR) and Raman spectroscopy, in conjunction with chemical degradation and high pressure procedures, in order to probe the nature of the MOM and FOM fractions of the CM2 chondrites: Murchison and Mighei. GC-MS has revealed that the FOM fraction is easily contaminated by microbial activity, but that there are a considerable variety of pyranone related compounds and aromatic acids that are likely indigenous. FTIR spectroscopic mapping of the FOM and MOM fractions supports the strong relationship between meteoritic organic matter and phyllosilicates, consistent with the generation of portions of meteorite organic matter, including many FOM and some LOM compounds, via aqueous alteration. Ratios of CH2 to CH3 calculated for asymmetric stretching indicate that Murchison has shorter chain length and/or more highly branched aliphatic compounds than Mighei. Petrographic studies indicate a higher degree of aqueous alteration for Mighei than Murchison and this suggests that the CH2/CH3 ratios might be explained by some degradation of the aromatic rings during aqueous alteration, which could generate the longer aliphatic chains and increase the CH2/CH3 ratio of Mighei compared to Murchison. The inverse relationship between the ratio of phyllosilicates to anhydrous silicates and silicate Si-O stretching to carboxyl hydroxyl stretching indicate the Murchison parent body accreted with or synthesised a higher abundance of carboxyl rich organic matter than that of Mighei. The refractory organic matter (ROM) component of MOM, which is isolated after chemical degradation, demonstrates a statistical similarity between both meteorites in FTIR and Raman spectroscopy; an observation that suggests a common organic progenitor may have been accreted by all CM chondrites and possibly all carbonaceous chondrites. Pressure is an important, but often neglected parameter relating to the origin of meteoritic organic matter. Model compounds, representing the oxygen and aromatic containing functionalities of MOM, have revealed the importance of intermolecular hydrogen bonding under pressure. Hydrogen bonding has been observed to facilitate esterification and is a plausible process by which portions of MOM could be generated.