Speaker: Raymond T. Pierrehumbert, University of Oxford
Abstract: Astronomical observations directly probe the properties of only the outer portions of a planet's atmosphere. When both mass and radius observations are available, the resulting mean density provides further, though highly degenerate, constraints on the composition of the interior. In this talk, I will discuss the kinds of inferences that can be drawn when the two kinds of information are put together. The emphasis will be on planets whose fluid layer is composed of H2 and H2O with various proportions, potentially interacting with a silicate core. An important physical consideration constraining plausible interior structures is that for liquid water interiors, the solubility of H2 is constrained by Henry's Law solubility, whereas for supercritical water interiors H2 (and other gases) are completely miscible with the interior. We will discuss the range of possible H2:H2O ratios in the outer atmosphere that can be compatible with a supercritical water atmosphere. Although an H2 layer is miscible with a supercritical water interior, there is a stable density jump at the interface, which inhibits mixing between the two layers; an essential missing piece of the puzzle is the quantification of the rate of such mixing. Once mixing begins, the moistening of the H2 layer leads to additional phenomena, including both water vapour feedback and generation of steep radiative layers near the interface through compositional stabilization of the lower atmosphere. I will also discuss thermal evolution models and implications of interaction of the H2:H2O fluid layer with a basal magma ocean. K2-18b and GJ1214b will be used as the archetypes of two very different types of subNeptunes, but I will also discuss results from a recent JWST survey of subNeptunes selected to have densities compatible with a potentially H2O-rich composition.