New pyrolytic and spectroscopic data on Orgueil and Murchison insoluble organic matter: A different origin than soluble?

Abstract

Pyrolysis with and without tetramethylammonium hydroxide (TMAH), vacuum pyrolysis, and solid state 15N nuclear magnetic resonance (NMR) were used to examine the macromolecular insoluble organic matter (IOM) from the Orgueil and Murchison meteorites. Conventional pyrolysis reveals a set of poorly functionalized aromatic compounds, ranging from one to four rings and with random methyl substitutions. These compounds are in agreement with spectroscopic and pyrolytic results previously reported. For the first time, TMAH thermochemolysis was used to study extraterrestrial material. The detection of aromatics bearing methyl esters and methoxy groups reveals the occurrence of ester and ether bridges between aromatic units in the macromolecular network. No nitrogen-containing compounds were detected with TMAH thermochemolysis, although they are a common feature in terrestrial samples. Along with vacuum pyrolysis results, thermochemolysis shows that nitrogen is probably sequestered in condensed structures like heterocyclic aromatic rings, unlike oxygen, which is mainly located within linkages between aromatic units. This is confirmed by solid state 15N NMR performed on IOM from Orgueil, showing that nitrogen is present in pyrrole, indole, and carbazole moieties. These data show that amino acids are neither derived from the hydrolysis of IOM nor from a common precursor. In order to reconcile the literature isotopic data and the present molecular results, it is proposed that aldehydes and ketones (1) originated during irradiation of ice in space and (2) were then mobilized during the planetesimal hydrothermalism, yielding the formation of amino acids. If correct, prebiotic molecules are the products of the subsurface chemistry of planetesimals and are thus undetectable through astronomical probes.