Giant planet formation is an outstanding question for planetary systems, including our own. Juno and Cassini have delivered unprecedented data regarding the composition, gravity field, magnetosphere, etc., which all enable better understanding the internal structures of Jupiter and Saturn an thus their formation. On the other hand, Uranus and Neptune have only been visited once, by the Voyager 2 mission. There is thus a big lack of data and the formation of these planets is still and matter of intense debate.
The deep composition of the ice giants is loosely constrained, with very few measurements performed with ground-based and earth-orbiting telescopes. One of the keys is the deep oxygen abundance. Not only does it tell us how the planetesimal ices trapped the other heavy elements, but it also has strong implications on the deep structure and mass distribution of the planet through the ice-to-rock ratio.
The detection of CO in large amounts in Neptune's troposphere in the early 1990s was taken as evidence for a water-rich interior. Subsequent models have inferred deep oxygen enrichments ranging from 250 to >500 the protosolar value. The situation at Uranus is less clear, with no deep CO detected yet. However, the possibility that its interior might be heavily convection-inhibited, as indicated by the lack of internal heat, may imply a water-rich interior for Uranus as well.
More recent observations and models seem to contradict this picture and suggest, on the other hand, that the ice giants may not be that icy after all. New observations of CO in the atmosphere of Neptune may not require such a CO-rich Neptune interior. Observations of the D/H ratio also seem difficult to reconcile with a water-rich interior.
In this debate lecture, two teams will present the ice-rich and rock-rich scenarios and the pieces of evidence that tend to back up these hypotheses. The short presentations will be followed by an open debate to discuss the best way forward in attempting to find out whether Uranus and Neptune are more "on ice" or "on the rocks".