Deep Atmosphere Composition, Structure, Origin, and Exploration, with Particular Focus on Critical in situ Science at the Icy Giants

Core accretion is the conventional model for the formation of the gas giant planets. The model may also apply to the icy giant planets, Uranus and Neptune, except that it may take upward of 50 Myr for them to form at their present orbital distances, which is beyond the maximum 5 Myr lifetime of the solar nebula. A plausible alternative is formation in the region of the gas giants, followed by migration to their present locations at 20 and 30 AU. Another alternative is the gravitational instability model, which is much faster and does not require the formation of a core first. In either scenario, heavy elements (mass > helium) provide the critical observational constraints. Additionally, helium and neon abundances in the observable troposphere are indicators of the interior processes in the megabar region. We investigate the atmospheric regions most suitable for accessing the above elements. Volatiles containing some of the elements (C, N, S, O) undergo condensation on the icy giants. On the other hand, noble gases (He, Ne, Ar, Kr, Xe), which are chemically inert, non-condensible, and uniform all over the planet, can provide the best constraints to the formation and migration models of Uranus and Neptune. Only entry probes are capable of measuring the key elements and isotopic ratios. They are accessible at 5-10 bars, except for the condensibles. Data from an orbiter on gravity, magnetic field, upper atmospheric composition and the maps of ammonia and water with depth would be a valuable complement to the situ measurements.