Saturn's Interior After the Cassini Grand Finale

We present a review of Saturn's interior structure and thermal evolution, with a particular focus on work in the past 5 years. Data from the Cassini mission, including a precise determination of the gravity field from the Grand Finale orbits, and the still ongoing identification of ring wave features in Saturn's C-ring tied to seismic modes in the planet, have led to dramatic advances in our understanding of Saturn's structure. Models that match the gravity field suggest that differential rotation, as seen in the visible atmosphere, extends down to at least a depth of 10,000 km (1/6$^{\rm th}$ the planet's radius). At greater depths, a variety of different investigations all now point to a deep Saturn rotation rate of 10 hours and 33 minutes. There is very compelling evidence for a central heavy element concentration (``core''), that in most recent models is 12-20 Earth masses. Ring seismology strongly suggests that the core is not entirely compact, but is dilute (mixed in with the overlying H/He), and has a substantial radial extent, perhaps out to around one-half of the planet's radius. A wide range of thermal evolution scenarios can match the planet's current luminosity, with progress on better quantifying the helium rain scenario hampered by Saturn's poorly known atmospheric helium abundance. We discuss the relevance of magnetic field data on understanding the planet's current interior structure. We point towards additional future work that combines seismology and gravity within a framework that includes differential rotation, and the utility of a Saturn entry probe.Comment: Invited review. Accepted for publication in "Saturn: The Grand Finale", K. H. Baines et al., eds., Cambridge University Press. All-new follow-up to previous 2016 (pre-Grand Finale) review chapter here: arXiv:1609.0632

An exploration of double diffusive convection in Jupiter as a result of hydrogen–helium phase separation

Jupiter's atmosphere has been observed to be depleted in helium (Yatm~0.24), suggesting active helium sedimentation in the interior. This is accounted for in standard Jupiter structure and evolution models through the assumption of an outer, He-depleted envelope that is separated from the He-enriched deep interior by a sharp boundary. Here we aim to develop a model for Jupiter's inhomogeneous thermal evolution that relies on a more self-consistent description of the internal profiles of He abundance, temperature, and heat flux. We make use of recent numerical simulations on H/He demixing, and on layered (LDD) and oscillatory (ODD) double diffusive convection, and assume an idealized planet model composed of a H/He envelope and a massive core. A general framework for the construction of interior models with He rain is described. Despite, or perhaps because of, our simplifications made we find that self-consistent models are rare. For instance, no model for ODD convection is found. We modify the H/He phase diagram of Lorenzen et al. to reproduce Jupiter's atmospheric helium abundance and examine evolution models as a function of the LDD layer height, from those that prolong Jupiter's cooling time to those that actually shorten it. Resulting models that meet the luminosity constraint have layer heights of about 0.1-1 km, corresponding to ~10,-20,000 layers in the rain zone between ~1 and 3-4.5 Mbars. Present limitations and directions for future work are discussed, such as the formation and sinking of He droplets.Comment: accepted to MNRAS, 21 pages, 17 figure