Insight into the Primordial Origins of Methane on Eris and Makemake from D/H Ratios

Deuterium, a heavy isotope of hydrogen, serves as a vital tracer in understanding the formation and evolution of the Solar System. Recent observations by the James Webb Space Telescope (JWST) have significantly expanded the dataset of D/H ratios in methane on the Kuiper Belt Objects (KBOs) Eris and Makemake, offering new insights into their chemical composition and origins. Our study explores the elevated D/H ratios in methane on these distant bodies, placing the findings within the broader context of protosolar nebula dynamics and chemistry. We propose that the methane on these KBOs has a primordial origin, challenging previous hypotheses that suggested abiotic methane production through internal heating processes.

To test this hypothesis, we employed a time-dependent protoplanetary disk model, integrated with a deuterium chemistry module to simulate the isotopic exchange between methane and hydrogen in the gas phase of the protosolar nebula. By incorporating observational constraints, such as the D/H ratio measured in methane in comet 67P/Churyumov-Gerasimenko, we refined our understanding of the primordial D/H abundance that would have been incorporated into the methane during its formation.

The results of the simulations indicate that the observed D/H ratios in methane on Eris and Makemake align well with a primordial origin. Our findings suggest that methane on these KBOs likely originated in the protosolar nebula and was sequestered in solid forms, such as pure condensates or clathrates, within the building blocks of these bodies before their accretion. These results provide a straightforward and plausible explanation for the high D/H ratios observed in methane on Eris and Makemake, without necessitating the complex internal production mechanisms previously proposed.

This work contributes to our understanding of the chemical processes in the early Solar System, suggesting that the chemical signatures preserved in the methane of KBOs are remnants of the primordial material from which the outer Solar System bodies formed.