Science from a Uranus Atmospheric Entry Probe

Introduction: Among the many solar system bodies that contain evidence of the origin, formation, and evolution of the solar system, the ice giants Uranus and Neptune are the last that have remained largely unexplored, limited to brief reconnaissances of Uranus in 1986 and Neptune in 1989 by the NASA Voyager 2 spacecraft. The 2023-2032 Planetary Science Decadal Survey “Origins, Worlds, and Life” (OWL) identified the ice giants as key planets for future studies. In particular, the top priority flagship mission for the upcoming decade was listed by the Decadal Survey to be the Uranus Orbiter and Probe (UOP) mission consisting of a long-term orbiter and an atmospheric entry descent probe as [1]. In-Situ vs. Remote: Particularly important to constraining formation models of the solar system and the giant planets are atmospheric noble gas abundances and isotopic ratios. The abundances of the noble gases address the conditions under which giant planets formed and evolved, including possible giant planet migration. Being chemically inert, noble gases provide no measurable radio signature and cannot be detected remotely, therefore requiring direct in situ sampling by an atmospheric probe. Additionally, understanding physical processes in the upper troposphere also requires in situ measurements from an atmospheric entry probe [2]. The deep temperatures and wind profiles are largely unknown due to hazes and clouds in the upper atmosphere, thereby limiting knowledge of dynamics, circulation, and convection in the atmosphere. Although remote sensing can measure the planetary thermal balance, measurements made in situ would provide significantly improved bounds on the depth to which solar radiation penetrates and the net transport of energy in the atmosphere. Important Atmospheric Science: The atmospheres of the giant planets are an important reservoir of noble gases. Noble gases are valuable tracers of planetary formation due to their chemical non-activity, and models for the incorporation of solids into the forming planets predict different noble gas ratios. On Jupiter and Saturn, it appears that helium rainout is occurring, and in situ measurements of He to H2 on Uranus would have important implications for the formation and thermal evolution of Uranus, and would help determine whether the ice giant planets formed with a solar He/H2 ratio. Different abundance ratios of the condensable species NH3 (ammonia), H2S (hydrogen sulfide), CH4 (methane), and H2O (water) are predicted by different Uranus formation models. Evidence for the conditions where the solids that eventually formed the giant planets formed is provided by isotopic ratios that can only be measured from an entry probe. The disequilibrium species PH3 and CO are formed in the deeper atmosphere and appear in the troposphere due to convection and vertical mixing. Measurements of PH3 and CO would help constrain vertical mixing in the atmosphere, and measurements of the vertical profiles would help determine the sources of these species in the stratosphere and the deep atmosphere. Key Probe Measurements: A Uranus atmospheric entry probe would measure the abundances of the noble gases He, Ne, Ar, Kr, Xe and respective isotopes, and would make measurements of the altitude profile of CH4, H2S, H2O, and NH3, key isotopes radios including D/H, 13C/12C, 15N/14N, and 17O/16O and 18O/16O, as well as disequilibrium species including CO and PH3. In addition to atmospheric composition measurements, other key measurements include the atmospheric thermal structure and static stability, and radiative energy structure, and the location, structure, and composition, of the cloud layers in the upper atmosphere. Atmospheric winds, waves, and turbulence could be found from Doppler tracking of the probe. In the colder outer solar system, a particularly important measurement would be to determine the ratio of the ortho to para forms of hydrogen from speed of sound measurements from an ultrasonic anemometer [3, 4]. Summary: The last class of solar system planets awaiting detailed exploration are the ice giants. With an orbiter to perform extended studies and an atmospheric entry probe to make in situ measurements of an ice giant atmosphere, models of the origin, formation, and chemical, thermal, and dynamical evolution of the solar system can be constrained, and understanding of atmospheric processes improved. Acknowledgments: Portions of this research were carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). © 2023. All rights reserved. References: [1] NASEM (2022) OWL 2023-2032 NA Press. [2] Mousis, O. et al., EGU22-13077. [3] Mousis, O. et al., Pl. Sp. Sci. 155 (2018). [4] Banfield, D. et al., 2005 IEEE Aerospace Conference. doi: 10.1109/AERO.2005.1559359.