Open Questions and Perspectives for Experimental Studies of Uranus’s Atmospheric Chemistry

The ice giants represent the last two and least explored planets of the solar system. The Uranus Orbiter and Probe (UOP) identified as a top priority flagship mission by the 2023 Planetary Science & Astrobiology Decadal Survey represents an important step towards the future characterization of Uranus and its moons [1]. This offers us an opportunity to draw from decades of science conducted at Jupiter, Saturn, and their moons, to advance research in preparation before the arrival of the UOP at Uranus, whose composition is still largely unknown. Atmospheric composition, formation and evolution of organic haze, and the processes influencing haze formation on Uranus and on the icy surface of its moons have all been highlighted as key topics of interest to address by the UOP [1]. As such, supportive laboratory simulations will be necessary to improve our understanding of Uranus and its icy moons, and the formation and evolution of organic molecules in the planet’s atmosphere.

For laboratory astrophysics applications, we reproduce the radiative conditions induced by cosmic rays found in planetary and astrophysical environments by utilizing heavy ions delivered by the GANIL facility (Grand Accélérateur National d’Ions Lourds, in Caen, France) [2]. Irradiation experiments of low-temperature ices and organic materials have been performed with the IGLIAS setup, part of the CIMAP-GANIL user facility CIRIL, where in situ infrared absorption spectroscopy and QMS mass spectrometry have allowed for the detection and physico-chemical characterization of organic molecules induced by heavy ion irradiation [3]. A unique and versatile multibeam irradiation platform called MIRRPLA, soon to be commissioned, will enable more realistic radiation conditions where the synergistic effect on planetary and astrophysical ices of UV photons, keV electrons, and keV-GeV ions delivered by GANIL will be studied [4]. For the ice giants, key open questions include the radiation-induced synthesis of complex organics from CH₄/NH₃/H₂O-based ices and the spectral signature of irradiation residue products relevant to Uranus’s haze. The upcoming MIRRPLA platform at GANIL will enable for the first time simultaneous UV–electron–ion irradiation, offering a realistic proxy for Uranian stratospheric and magnetospheric chemistry. We will present ongoing work from heavy-ion experiments and outline MIRRPLA’s future role in UOP preparatory science.

Acknowledgements

The authors acknowledge funding support from the French Agence National de Recherche (ANR MIRRPLA - 22-EXOR-0012 and ANR CPJ).  

References

[1] National Academies of Sciences, Engineering, and Medicine, 2023, Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023–2032; National Academies Press: Washington, DC, USA.
[2] Rangama, J., Grygiel, C., Mery, A., Rousseau, P., Rothard, H., 2022, Nucl. Phys. News. 32, No. 2 29-33.
[3] Rothard, H., Domaracka, A., Boduch, P., Palumbo, M.E., Strazzulla, G., da Silveira, E.F., and Dartois, E., 2017, Modification of ices by cosmic rays and solar wind, J. Phys. B: At. Mol. Opt. Phys. 50, 062001.
[4] Domaracka and Danger, 2023, Nuclear Physics News 33(4), 36–37.