Did Eris Once Hide an Ocean Beneath Its Surface?
Eris, one of the largest Kuiper Belt objects, is almost the same size as Pluto and has a small moon called Dysnomia. Recent telescope observations revealed that Eris and Dysnomia are “doubly synchronous,” meaning Eris spins at the same rate that Dysnomia orbits. This unusual state suggests Eris must have been able to dissipate large amounts of energy in the past. In their new paper, Ryunosuke Akiba and Francis Nimmo test whether that energy dissipation could be explained by a warm ice shell alone, or if Eris once harbored a hidden subsurface ocean.
Why Oceans in the Outer Solar System Matter
Subsurface oceans are a big topic in planetary science. Worlds like Europa and Enceladus stay warm enough for liquid water thanks to tidal heating from giant planets. But dwarf planets like Pluto and Eris are far from such energy sources. If they can still form or maintain oceans, those oceans could survive for billions of years, making them interesting not only for geology but also for astrobiology. Eris, with its high density and possible history of giant impacts, presents a special case: could its interior structure support an ancient ocean, and could that explain its present-day orbit?
Modeling Heat and Spin
To tackle this question, Akiba modeled how Eris’s interior evolves over 4.5 billion years. Heat comes from the radioactive decay of elements in Eris’s rocky core, while an overlying ice layer conducts or convects this heat outward. If enough heat builds up at the base of the ice shell, melting begins, forming an ocean. The team also coupled these thermal models with orbital models of Dysnomia, calculating how Eris’s interior structure affects tidal dissipation and the eventual slowing of its spin.
Oceans Make Eris More Dissipative
The results show that spinning down Eris without an ocean is very difficult. A warm convecting ice shell can help, but only under extreme assumptions about how dissipative ice can be. In contrast, introducing an ocean makes Eris far more dissipative. In fact, most successful models (77–100%) include oceans at some point in Eris’s history, and when more realistic ice behaviors are assumed, this fraction rises above 98%. However, in many of these models, the oceans eventually freeze unless something insulates them (like a porous ice shell, clathrates that trap gas, or antifreeze compounds like ammonia).
Insulation and Antifreeze Effects
Different insulating mechanisms were tested. Porous ice can trap heat, allowing oceans to survive for up to 4 billion years. Clathrates, structures of ice that trap gases, also provide strong insulation, with surface clathrates especially effective. Adding antifreeze like ammonia further lowers the melting point of water ice, making it easier for oceans to persist. In these cases, nearly all simulations both matched Eris’s orbital state and maintained long-lived oceans to the present day.
Implications for Eris’s Past and Present
Akiba and Nimmo conclude that Eris likely required an ocean to explain its current orbital state, even if that ocean has since frozen. If such an ocean existed, it might have influenced surface features through cryovolcanism or chemical transport. Indeed, recent observations of methane on Eris’s surface, with isotopic signatures suggesting internal production, could be consistent with such processes. Even if Eris’s ocean is gone today, evidence points to a warmer and more dynamic past than one might expect for a frozen world at the edge of the solar system.
Source: Akiba
