Carbonaceous Chondrites and Their Role in the Origins of Life on Earth: Discovering Prebiotic Molecules in Asteroid Ryugu

An artist’s impression of meteors impacting ancient Earth. This image depicts the arrival of the carbonaceous chondrites, which are thought to have delivered prebiotic molecules responsible for forming early life. Credit: NASA’s Goddard Space Flight Center Conceptual Image Lab.

Asteroid (162173) Ryugu is a pristine carbonaceous asteroid recently sampled by JAXA during the Hayabusa2 mission. Chemical, isotopic, and contextual measurements suggest that Ryugu is genetically linked to Ivuna-type (CI) carbonaceous chondrites. Carbonaceous chondrites are important preservers of organic matter. Previous studies detected prebiotic molecules, including the building blocks of ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), e.g., purine and pyrimidine nucleobases. Results led many researchers to conclude that primitive carbonaceous chondrites may have been the source of prebiotic molecules responsible for the emergence of life on Earth. However, the extent of terrestrial contamination of chondrite meteorites is challenging to quantify.

The Hayabusa2 mission sampled a pristine carbonaceous asteroid without exposing it to the atmosphere and biosphere on Earth, which provides a rare opportunity to study pristine extraterrestrial organic matter that has not been compromised by terrestrial contamination. Preliminary studies of returned Ryugu samples identified various organic molecules, including amino acids, amines, and carboxylic acids. Recently, during the investigation of soluble organics in Ryugu, Yasuhiro Oba from the Institute of Low Temperature Science at Hokkaido University and colleagues identified vitamin B3 (niacin) and uracil, one of four nucleobases in RNA. Organics were extracted using solvent techniques and measured using a recently developed analytical technique (HPLC/ESI-HRMS) for a small-scale study of nucleobases at parts per billion to parts per million precisions. The discovery of prebiotic molecules in a pristine carbonaceous chondrite strongly suggests that carbonaceous chondrites preserved prebiotic molecules of interest and delivered these molecules to early Earth. Preserved molecules initially formed either on the parent body during aqueous alteration or externally from photochemical reactions in ice mantles within the molecular cloud or outer protoplanetary disk. Further study of the organic matter preserved in samples returned from missions to asteroid Ryugu (C-type) and Bennu (B-type) could provide important insights into the origins of life on Earth. READ MORE