In Situ Exploration of the atmospheres of the Ice Giants

The ice giants Uranus and Neptune are the least understood class of planets in our solar system, while planets of their size, the most frequent among exoplanets, represent a common outcome of planet formation.  Presumed to have a small rocky core, a deep interior comprising ∼70% heavy elements surrounded by a more dilute outer envelope of H₂ and He, Uranus and Neptune are fundamentally different from the better-explored gas giants Jupiter and Saturn. Because of the dearth of missions dedicated to their exploration, our knowledge of their composition and atmospheric processes is primarily derived from a single Voyager 2 flyby of each, complemented by subsequent remote sensing from Earth-based observatories, including 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 are not well understood. Exploration of ice giant systems is therefore a high-priority science objective as these systems (which link together the magnetospheres, satellites, rings, atmosphere, and interior of these planets) challenge our understanding of planetary formation and evolution. Here we describe the main scientific goals to be addressed by future in situ exploration of an ice giant's atmosphere. An atmospheric entry probe targeting the 10-bar level, approximately 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 governing the structure and composition of planetary atmospheres. The battery-powered probe would descend under parachute to measure composition, structure, and dynamics. In our favorite scenario, an Ice Giants orbiter performing a comprehensive exploration of the system would be used to deliver the probe to the atmosphere and to relay its data back to Earth. Following the successful architecture of the Cassini-Huygens mission, we envision that the probe would be delivered by ESA and the orbiter by NASA, with possible technical contributions of one Agency to the other's platform, on the basis of technical and programmatic considerations. The science payloads of the two platforms would be shared between NASA and ESA members states on the basis of scientific merit and technical/funding resources.