The ice giants Uranus and Neptune are the least understood class of planets in our solar system but the most frequently observed type of exoplanets. Presumed to have a small rocky core, a deep interior comprising ∼70% heavy elements surrounded by a more dilute outer envelope of H 2 and He, Uranus and Neptune are fundamentally different from the better-explored gas giants Jupiter and Saturn. Because of the lack of dedicated exploration missions, our knowledge of the composition and atmospheric processes of these distant worlds is primarily derived from remote sensing from Earth-based observatories and space telescopes. As a result, Uranus's and Neptune's physical and atmospheric properties remain poorly constrained and their roles in the evolution of the Solar System not well understood. Exploration of an ice giant system is therefore a highpriority science objective as these systems (including the magnetosphere, satellites, rings, atmosphere, and interior) challenge our understanding of planetary formation and evolution. Here we describe the main scientific goals to be addressed by a future in situ exploration of an ice giant. An atmospheric entry probe targeting the 10-bar level, about 5 scale heights beneath the tropopause, would yield insight into two broad themes : i) the formation history of the ice giants and, in a broader extent, that of the Solar System, and ii) the processes at play in planetary atmospheres. The probe would descend under parachute to measure composition, structure, and dynamics, with data returned to Earth using a Carrier Relay Spacecraft as a relay station. In addition, possible mission concepts and partnerships are presented, and a strawman ice-giant probe payload is described. An ice-giant atmospheric probe could represent a significant ESA contribution to a future NASA ice-giant flagship mission.

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 in situ measurements.

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