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Biologically relevant abiotic extraterrestrial soluble organic matter (SOM)has been widely investigated to study the origin of life and the chemicalevolution 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 besidesthe biologically relevant ones, and the reaction mechanisms thatcomprehensively explain the complex nature of SOM are unknown. Besides, theinitial reactants, which formed before asteroid accretion, wereuncharacterized. We examined the mass distribution of SOM extracted from threedistinct Tagish Lake meteorite fragments, which exhibit different degrees ofaqueous alteration though they originated from a single asteroid. We reportthat mass distributions of SOM in the primordial fragments are well fit by theSchulzZimm (SZ) model for the molecular weight distribution patterns found inchain growth polymerization experiments. Also, the distribution patternsdiverge further from SZ with increasing degrees of aqueous alteration. Theseobservations imply that the complex nature of the primordial SOM (1) wasestablished before severe alteration on the asteroid, (2) possibly existedbefore parent-body accretion, and (3) later became simplified on the asteroid.Therefore, aqueous reactions on asteroids are not required conditions forcultivating complex SOM. Furthermore, we found that overall H over C ratios ofSOM decrease with increasing aqueous alteration, and the estimate of H lossfrom the SOM is 10% to 30%. Organics seem to be a significant H2 source thatmay have caused subsequent chemical reactions in the Tagish Lake meteoriteparent body.

Aqueous Alteration on Asteroids Simplifies Soluble Organic Matter Mixtures