Study of Uranian Satellite Volatiles

Non-water ices and non-water volatiles have been spectroscopically observed in the outer solar system and in comet comae. The presence of ammonia (NH₃) and ammonia hydrates on satellites in the Uranian system, such as Oberon, Titania, Ariel, and Umbriel, has been predicted (and observed, in the case of the latter two moons),^1,2 and may increase the likelihood of the existence of subsurface oceans on these worlds.³ Castillo-Rogez et al. show that high accretion of CO₂- and NH₃-ices on dwarf planets and icy moons create stronger magnetic fields than would be expected from subsurface oceans of pure water.⁴ Additionally, the high volatility of oxygen makes the observed abundance of O₂-ice in comet comae and on the surface of Ganymede puzzling. However, for many species of volatiles, which are observed or hypothesized on icy satellites and Kuiper Belt Objects, the assumed vapor pressures have been extrapolated from laboratory studies at much higher temperatures and pressures than are appropriate for the surface conditions of these bodies.⁵ In the Astrophysical Materials Laboratory at Northern Arizona University, we have investigated the vapor pressures of several species, including NH₃, CO₂, and O₂, at temperatures applicable for surface conditions for the Uranian system, and relevant for the Neptunian system, as well as other planetary bodies in the outer Solar System. Our results show that these species are stable as ices at higher temperatures than expected from previous thermodynamic models, which may have implications for the stability of non-water ices on icy bodies. Acknowledgements: This work is supported in part by the NAU REU program under NSF grant 1950901. References: [1] R. J. Cartwright et al. (2020) Astrophysical Journal Letters, 898(1), L22. [2] R. J. Cartwright et al. (2023), The Planetary Science Journal, 4(3), 42. [3] C. J. Bierson & F. Nimmo (2022) Icarus, 373, 114776. [4] J. C. Castillo-Rogez et al. (2022) Geophysical Research Letters, 49(16), e2021GL097256. [5] N. Fray and B. Schmitt (2009) Planetary and Space Science, 57(14-15), 2053-2080.