Methane from the Beginning: A Primordial Origin for Methane on Eris and Makemake

In this study, Mousis et al. investigate the origins of methane (CH₄) on two distant Kuiper Belt Objects (KBOs), Eris and Makemake, by analyzing their deuterium-to-hydrogen (D/H) ratios. Deuterium is a heavier form of hydrogen and a valuable marker of solar system history because it behaves differently from regular hydrogen under varying temperatures. The early Solar System likely preserved these D/H differences in ices and gases. The new data from the James Webb Space Telescope (JWST), which detected monodeuterated methane (CH₃D) on Eris and Makemake, show D/H values higher than those seen in Titan’s atmosphere but lower than those in some comets, like 67P/Churyumov–Gerasimenko. While some researchers suggest this methane might have formed deep inside these bodies through heating and internal chemistry, Mousis and colleagues propose a simpler alternative: that the methane formed early in the Solar System, in the primordial gas disk called the protosolar nebula (PSN) and was captured by these KBOs during their formation.

Modeling the Early Solar System

To explore this hypothesis, the authors use a model of the PSN that calculates how temperature, pressure, and gas density change over time. They simulate how methane and hydrogen molecules exchanged deuterium as the disk cooled. This exchange process only works efficiently at certain temperatures, and below 200 K it essentially stops. The team also considers how methane would freeze out — either as a pure ice around 28 K or trapped in water ice “cages” called clathrates at 55 K. Their model includes a key parameter called the enrichment factor, which tracks how enriched methane becomes in deuterium relative to regular hydrogen. They assume the starting D/H ratio came from comet 67P, where the Rosetta spacecraft found very high deuterium levels in methane.

Simulating Deuterium Enrichment

The results show that as the disk cooled, methane could have become enriched in deuterium in a predictable way, depending on the time and location it froze out. The enrichment factor varied most rapidly between about 8 and 12 astronomical units (AU) from the Sun — a region consistent with the formation zones of the building blocks of gas giants and KBOs. For both condensation and clathrate formation scenarios, the modeled D/H ratios match those measured on Eris, Makemake, and even Titan. This suggests that the methane on these bodies might have come from the same region in the early disk and was frozen in early on. The authors argue that this supports a shared, primordial origin of methane, rather than methane being formed later through internal heating.

Discussion: A Simple Explanation Holds Up

The authors test different conditions by changing how fast the disk accreted material and how turbulent it was. Their model still fits the observed data over a reasonable range of parameters, reinforcing its reliability. They suggest that methane-rich grains likely formed quickly after about 60,000–150,000 years in the PSN and later clumped together into planetesimals — small bodies that eventually formed KBOs like Eris and Makemake. These findings also align with measurements from other icy bodies, like Enceladus and Titan, where high D/H values hint at a common reservoir of ices in the outer Solar System.

Open Questions and Implications

The paper acknowledges some remaining puzzles. For instance, if methane is primordial, why isn’t carbon monoxide (CO), another volatile, found on Eris and Makemake? The authors suggest that CO might have been destroyed in subsurface oceans or was simply not as enriched during formation. The possibility that some methane may have been produced later inside these bodies isn't ruled out entirely, especially given evidence of geological activity. However, the overall D/H patterns are consistent with methane being trapped early, offering a simpler explanation than post-formation chemistry.

Conclusion: Methane as a Time Capsule

Mousis et al.'s model provides a compelling case that the methane on Eris and Makemake is a relic from the earliest days of the Solar System, preserved in ice for billions of years. This not only explains the high D/H ratios but also supports a broader picture where many icy bodies — from moons to comets — formed from similar building blocks. Further observations of D/H ratios in other comets and interstellar methane will help refine these ideas, but for now, the results suggest that these distant worlds carry the chemical fingerprints of the Solar System’s birth.

Source: Mousis