Formation Conditions of Enceladus and Origin of Its Methane Reservoir

Abstract

We describe a formation scenario of Enceladus constrained by the deuterium-to-hydrogen ratio (D/H) in the gas plumes as measured by the Cassini Ion and Neutral Mass Spectrometer. We propose that, similarly to Titan, Enceladus formed from icy planetesimals that were partly devolatilized during their migration within the Kronian subnebula. In our scenario, at least primordial Ar, CO, and N₂ were devolatilized from planetesimals during their drift within the subnebula, due to the increasing temperature and pressure conditions of the gas phase. The origin of methane is still uncertain since it might have been either trapped in the planetesimals of Enceladus during their formation in the solar nebula or produced via serpentinization reactions in the satellite's interior. If the methane of Enceladus originates from the solar nebula, then its D/H ratio should range between ~4.7 × 10^-5 and 1.5 × 10^-4. Moreover, Xe/H₂O and Kr/H₂O ratios are predicted to be equal to ~7 × 10^-7 and 7 × 10^-6, respectively, in the satellite's interior. On the other hand, if the methane of Enceladus results from serpentinization reactions, then its D/H ratio should range between ~2.1 × 10^-4 and 4.5 × 10^-4. In this case, Kr/H₂O should not exceed ~10^-10 and Xe/H₂O should range between ~1 × 10^-7 and 7 × 10^-7 in the satellite's interior. Future spacecraft missions, such as Titan Saturn System Mission, will have the capability to provide new insight into the origin of Enceladus by testing these observational predictions.