Planetary Entry Probe Explorations of an Ice Giant
Résumé
The largely unexplored ice giants Uranus and Neptune fill the gap in size between the terrestrial and gas giant planets. To date, our knowledge of ice giant composition and atmospheric processes is limited to space-based or Earth-based observations. Remote sensing is not capable of providing the essential measurements of noble gas and key isotope abundances, however. Only in situ exploration by an entry probe can reveal the well-mixed atmosphere beneath the cloud tops wherein resides pristine materials from the location and epoch of ice giant planet formation. Reflecting the processes of ice giant origin, formation, and evolution, possibly including evidence of giant planet migration, chemically inert noble gases are of particularly high importance. With no detectable radio signature, the essential measurement of noble gas abundances requires an in situ probe. An atmospheric entry probe would sample well into the cloud-forming regions of the troposphere far below regions directly accessible to cloud top remote sensing. A concept for an ice giant entry probe mission includes a spacecraft to carry and deliver the probe to the ice giant atmospheric entry interface point and subsequently act as a receiving station for the probe science telemetry. The primary goal of an ice giant atmospheric probe would be to measure the well-mixed abundances of the noble gases He, Ne, Ar, Kr, Xe and their isotopes, as well as the altitude profile of heavier elements C, N, S, and P, key isotope ratios 15N/14N, 13C/12C, 17O/16O and 18O/16O, and D/H, and disequilibrium species CO and PH3. The ice giants represent the last unexplored class of planets in the solar system and the most frequently observed type of exoplanets. Extended studies of one or both ice giants, including in situ with an entry probe, are necessary to further constrain models of solar system formation and chemical, thermal, and dynamical evolution, the formation and evolution of atmospheres, atmospheric processes, and to provide additional ground-truth for improved understanding of exoplanetary systems.