Jupiter's Interior with an Inverted Helium Gradient

Abstract

Jupiter's gravity field observed by NASA's Juno spacecraft indicates that the density in the 10--100 GPa region is lower than one would expect from a H/He adiabat with 0.5-5x solar water abundance as has been observationally inferred in Jupiter's atmosphere, supported by the 2--4$\times$ solar enrichment in the heavy noble gases and other volatiles observed by the Galileo entry probe. Here, we assume that Jupiter's envelope harbors a radiative window at ~0.975-0.99 RJ. This outer stable layer (OSL) delays particle exchange and accelerates the cooling of the deep interior. Consequently, the He-depletion at the Mbar-level where H/He phase separation occurs would be stronger than seen in the atmosphere. We find that the inverted He-gradient across the OSL leads to atmospheric heavy element abundances that are up to dZatm=0.03 (+2x solar) higher than for adiabatic models. With an additional inverted Z-gradient, Zatm up to 3x solar is possible. Models with 1x solar Zatm have a dilute core confined to the inner 0.2-0.3 MJ (0.4-0.5 RJ), smaller than in adiabatic models. Models with 3x solar Zatm have a largely homogeneous-Z interior at 1x solar. The low observed atmospheric Ne/He ratio suggests that Ne is transported through the OSL as efficiently as He is and at an enhanced diffusivity as is characteristic of double diffusive convection. Better knowledge of the H/He-EOS in the 10--100 GPa region and of the H/He phase diagram is needed to understand Jupiter's interior structure.