Stable stratification of the helium rain layer yields vastly different interiors and magnetic fields for Jupiter and Saturn

At sufficiently high pressures (>~ Mbar) and low temperatures (10³-10⁴ K), hydrogen and helium become partly immiscible. Interpretations of Jupiter and Saturn's magnetic fields appear to favor the existence of a statically stable layer near the Mbar pressure level. We seek to demonstrate that the phase separation of hydrogen and helium is capable of producing a layer of static stability and an internal structure consistent with magnetic field measurements. From experimental and computational data for the hydrogen-helium phase diagram we ƒind that moist convection and diffusive convection are inhibited, implying a stable helium rain layer in both Jupiter and Saturn, but with a significant difference in terms of structure and evolution: In Jupiter, helium settling leads to a stable yet super-adiabatic temperature gradient that is limited by conductive heat transport. The phase separation region should extend on only a few tens of kilometers instead of thousands in current-day models, and be characterized  by a sharp increase of the temperature of about ~500 K for standard phase separation diagrams. In Saturn, the fact that helium rains occurs much deeper implies a helium flux that is much larger, relatively to the total planetary mass. We find that the significant (positive) latent heat associated with helium condensation implies that a large fraction, perhaps close to 100%, of the planet's intrinsic heat flux, may be locally transported by the sinking helium droplets. This implies that contrary to Jupiter, the temperature gradient in that region may be much lower, perhaps even subadiabatic, leading to Saturn possessing a much more extended helium-rain region, consistent with interior models constrained by seismological constraints. This also accounts, at least qualitatively, for the differences in strength and characteristics of the magnetic fields of the two planets: Jupiter's strong, complex, magnetic field and high Lowes radius (80 to 83% of the planetary radius) is consistent with the existence of a narrow stable helium-rain region and a convective metallic hydrogen envelope below. On the other hand, Saturn's ten times weaker, axisymmetric field may be consistent with a dynamo powered by compositional motions in a mostly stable helium-rain metallic region. Dedicated models are required to test these hypotheses.