What Juno and Cassini have told us about Giant Planet interiors*

Juno is in orbit about Jupiter and will likely continue collecting data for several more years. Cassini collected data during close encounters within the Saturn ring plane for several months in 2017. The main source of information is the gravity data and magnetic field data, augmented in the case of Juno by microwave radiometry data that inform us about atmospheric composition, a key boundary condition for the interior. Juno gravity indicates that the innermost part of Jupiter is enriched in heavy elements (everything heavier than H and He), perhaps ot the extent of around 10 or 15 Earth masses (~5% of the planet mass) but this enrichment is not in the form of a discrete core but is instead diluted by the overlying H and He. This is consistent with current ideas for planet formation, where very high temperatures lead to evaporation and mixing of incoming solids. The distribution of heavier elements in the region where hydrogen begins to metallize is less clear and may be affected by and entangled with the possible rain-out of helium. Saturn, unlike Jupiter, benefits from ring seismology: the existence of ring structure that must be attributed to density anomalies within the planet. In both Jupiter and Saturn, the region of magnetic field generation extends out beyond the metallic hydrogen region in to primarily molecular hydrogen that is sufficiently hot and compressed that it has significant conduction electrons. Coupling of the magnetic field to the zonal flow extends out further still, to a region where the electrical conductivity is of order 1 S/m. Although there has been much improvement in our understanding of H-He, the uncertainties, both in theory and experiment, remain large enough that they limit our ability to make full use of the spacecraft results. We need experiments at relevant temperatures and pressures, since most current experiments are either too hot or too cold at the pressures of importance.