Atmospheric Entry Probes for in situ Exploration of the Ice Giant Planets

Understanding the formation and evolution of the solar system and the formation of the giant planets is constrained by inherent limitations in the capabilities of remote sensing. In situ exploration of planetary atmospheres provides key measurements not possible from remote observations, remarkably demonstrated at Jupiter by the Galileo probe, where key measurements included the determination of noble gas abundances and the precise measurement of the Jupiter helium mixing ratio. In this paper, we describe the primary scientific goals to be addressed by future in situ exploration of the ice giants Uranus and Neptune, placing in situ explorations of the gas giants, including the Galileo probe and a future Saturn probe, into a broader solar system context. An ice giant atmospheric entry probe reaching 10 bars would provide insight into both the formation history of the solar system and the giant planets, and the structure and composition of, and physical processes at play within ice giant atmospheres. An entry probe as an element of a future ice giant flagship mission would descend under parachute to measure the abundances and isotopic ratios of the noble gases, D/H in H₂ and ¹³C/¹²C, and the thermal structure and dynamics from the upper atmosphere down to the deepest region from which the probe is able to return data, perhaps 10-20 bars or more. Probe data would be returned to Earth using a Carrier Relay Spacecraft as a relay station. The relay spacecraft, particularly if it is an orbiter with a suite of remote sensing instruments, can significantly enhance the science return from the probe; remote sensing provides the global context from which to understand the probe's local measurements of weather and cloud properties. One or more small atmospheric probes could represent a significant ESA contribution to a future NASA New Frontiers or Flagship Ice Giant mission.