Moderate D/H ratios in methane ice on Eris and Makemake as evidence of hydrothermal or metamorphic processes in their interiors: Geochemical analysis

Dwarf planets Eris and Makemake have surfaces bearing methane ice of unknown origin. This ice can provide important insights into the origin and evolution of volatiles in the outer solar system. Deuterium/hydrogen (D/H) ratios were recently determined from James Webb Space Telescope (JWST) observations of Eris and Makemake (Grundy et al., 2024b), giving us new clues to decipher the origin of methane. Here, we develop geochemical models to test if the origin of methane could be primordial, derived from CO2 or CO (“abiotic”), or sourced by organics (“thermogenic”). We find that primordial methane (as currently understood) is inconsistent with the observational data, whereas both abiotic and thermogenic methane can have D/H ratios that overlap the observed ranges. This suggests that Eris and Makemake either never acquired much methane during their formation, or their original inventories were removed and then replaced by internally produced methane. Because producing abiotic or thermogenic methane likely requires temperatures above ∼150 °C, we infer that Eris and Makemake have rocky cores that underwent substantial radiogenic heating. Their cores may still be warm/hot enough to make methane. This heating could have driven hydrothermal circulation at the bottom of an ice-covered ocean to generate abiotic methane, and/or metamorphic reactions involving accreted organic matter could have occurred in response to heating in the deeper interior, generating thermogenic methane. Additional analyses of relevant thermal evolution model results and theoretical predictions of the D/H ratio of methane in the solar nebula support our findings of elevated subsurface temperatures and an apparent lack of primordial methane on Eris and Makemake. It remains an open question whether their D/H ratios may have evolved subsequent to methane outgassing. We also suggest that lower-than-expected D/H and 84Kr/CH4 ratios in Titan's atmosphere disfavor a primordial origin of methane there as well. Recommendations are given for future activities to further test proposed scenarios of abiotic and thermogenic methane production on Eris and Makemake, and to explore these worlds up close so that we can see if they bear additional evidence of endogenic processes.