Data suggest that most super-Earths formed as gas-rich sub-Neptunes, but whether super-Earths still have atmospheres is unknown. We identify a pathway by which sub-Neptunes can develop long-lived 2-200 bar H2O-dominated atmospheres during their conversion to Super-Earths, and show that this is a common outcome for efficient interaction between a nebula-derived atmosphere and oxidized magma, followed by atmospheric loss. H2O that is made by reduction of iron oxides in the magma is highly soluble in the magma, forming a dissolved reservoir that is protected from loss so long as the H2-dominated atmosphere persists, and whose large size buffers the H2O atmosphere against loss after the H2 has dispersed. This idea is imminently testable with JWST and has numerous implications for the interpretation of mass-radius data for transiting super-Earths.
In addition to this new result, we will also summarize results from our recently published study of the transition over time from sub-Neptune atmospheres to super-Earth atmospheres (Kite & Barnett PNAS 2020). We describe the (narrow) range of conditions under which super-Earths will have secondary atmospheres Gyr after primary-atmosphere loss. The figure (from Kite & Barnett PNAS 2020) shows selected processes (italics) and reservoirs (upright font) in the model. Atmosphere-interior exchange is central to the transition from primary to secondary atmospheres. Timescales are approximate.