Aqueous Alteration on Asteroids Simplifies Soluble Organic Matter Mixtures

TL;DR

This study investigates how aqueous alteration on asteroids affects soluble organic matter, revealing that complex organic mixtures likely formed before asteroid alteration and are simplified by aqueous processes, with implications for organic evolution in space.

Contribution

It demonstrates that primordial organic matter predates asteroid alteration and that aqueous processes simplify complex organic mixtures, challenging previous assumptions about organic synthesis in space.

Findings

Mass distribution fits SchulzZimm model for primordial SOM

Aqueous alteration reduces hydrogen content in SOM

Complex organic mixtures likely formed before asteroid alteration

Abstract

Biologically relevant abiotic extraterrestrial soluble organic matter (SOM) has been widely investigated to study the origin of life and the chemical evolution of protoplanetary disks. Synthesis of biologically relevant organics, in particular, seems to require aqueous environments in the early solar system. However, SOM in primitive meteorites includes numerous chemical species besides the biologically relevant ones, and the reaction mechanisms that comprehensively explain the complex nature of SOM are unknown. Besides, the initial reactants, which formed before asteroid accretion, were uncharacterized. We examined the mass distribution of SOM extracted from three distinct Tagish Lake meteorite fragments, which exhibit different degrees of aqueous alteration though they originated from a single asteroid. We report that mass distributions of SOM in the primordial fragments are well fit by the SchulzZimm (SZ) model for the molecular weight distribution patterns found in chain growth polymerization experiments. Also, the distribution patterns diverge further from SZ with increasing degrees of aqueous alteration. These observations imply that the complex nature of the primordial SOM (1) was established before severe alteration on the asteroid, (2) possibly existed before parent-body accretion, and (3) later became simplified on the asteroid. Therefore, aqueous reactions on asteroids are not required conditions for cultivating complex SOM. Furthermore, we found that overall H over C ratios of SOM decrease with increasing aqueous alteration, and the estimate of H loss from the SOM is 10% to 30%. Organics seem to be a significant H2 source that may have caused subsequent chemical reactions in the Tagish Lake meteorite parent body.