Constraining the Effects of Pre- and Post-accretionary Processes on Ryugu Insoluble Organic Matter

Asteroidal and cometary fragments have been indicated as possible sources of the building blocks of life for the early Earth. Samples returned from the asteroid Ryugu are uncontaminated and contain organic matter. Such samples enable an investigation of the processes that formed and altered extraterrestrial organic matter. Here, the FTIR and Raman responses of insoluble organic matter (IOM) isolated from Ryugu particles and carbonaceous chondrites are reported. Several relationships were observed: (1) a strong positive correlation of CH2/CH3 plotted against aliphatics/C=C; (2) a strong negative correlation of CH2/CH3 plotted against δ15N and δ13C; and (3) strong negative correlations of FTIR band ratios plotted against Raman band parameters and D/G values. The data are not consistent with even moderate heating of the IOM, as indicated by CH2/CH3 ratios that are consistent with unheated carbonaceous chondrites. The C and N isotope data previously collected for each Ryugu particle preclude aqueous alteration based on our understanding of its effects on carbonaceous chondrite IOM. Mixing of several IOM endmembers was considered, but due to the similarities of the particle with the lowest CH2/CH3 ratio to the other particles in terms of its other spectroscopic parameters, this scenario was considered unlikely. Finally, the effects of irradiation were considered, and UV irradiation, in particular, was hypothesised to be the best explanation for the trends observed. Overall, the data highlight the importance of comprehensive data sets for revealing extraterrestrial processes, which could define the nature of organic matter reaching habitable environments, such as those of the Early Earth.