Colloquium: Internal fractionation of atmospheric volatiles on magma ocean exoplanets
Low-mass exoplanets in a fully to partially molten state open a novel window into key processes that shape the earliest, high-temperature evolutionary regimes of rocky planets; a critical barrier to connect planetary accretion with long-lived climate states. I will outline how internal magma ocean dynamics and chemical differentiation between core-forming metals and atmospheric species influence the feedback mechanisms between largely molten interiors and volatile envelopes. The physical and chemical coupling between magma layers and their equilibrating atmospheres can fractionate the dominant compounds observable in the upper atmosphere to a degree that is testable with current instrumentation. As key examples, (1) nitrogen species can be suppressed in H/He-dominated atmospheres if the volatile envelope is in direct contact with a molten interior – the NH3 depletion observed in K2-18b can be fully explained by an internal magma ocean; (2) the total volume of atmospheric water vapour is closely related to the phase state of the rocky planetary mantle – many slightly under-dense super-Earths are consistent with a runaway greenhouse / magma ocean state. With an emphasis on a few individual, and then statistical ensembles, of super-Earth and sub-Neptune exoplanets, I will outline observational tests to distinguish internal phase state and evolutionary scenarios.