The atmospheres of Jupiter and Uranus are observed to be depleted in He/H, with a possible stronger depletion in Uranus than in Jupiter, depending on the assumed deep methane mixing ratio. Remote sensing data suggest an about protosolar He/H for Neptune while for Saturn a depletion, although its magnitude is uncertain [1].  
The atmospheric He/H depletion of Jupiter together with the strong Ne/H depletion as observed by the Galileo entry probe are commonly taken evidence of helium rain at Mbar pressures [2]. This poses the question of the He/H and Ne/H abundances in the atmospheres of the ice giants if He/H phase separation takes place in their deep interiors. 
How much of a light-element component (He-H) in the deep interiors of the ice giants is required to match the observed gravity data depends on model assumptions. If He/H is present at Mbar pressures and if particle exchange between atmosphere and deep interior occurs uninhibited, their atmospheres are predicted to be highly depleted in He/H, contrary to what is observed [1].
 
Here, we assume the presence of a barrier to convection between atmosphere and interior in models of the outer planets. This boundary layer (BL) inhibits heat and particle transport. The model unifies the thermal BL assumption for the ice giants [1] with the double-diffusive BL assumption for Jupiter [3]. 
We vary the diffusivity of He and Ne in H, the strength of partitioning of Ne in He-droplets, and the He/H phase diagram to compute possible atmospheric He/H and Ne/H ratios. They are benchmarked against Jupiter and serve as predictions for Uranus to be probed by a shallow (5 bars) entry probe. A measurement would provide unique constraints on the interior structure.
 
Acknowledgement:  NN acknowledges support through DFG-grant NE 1734/3-1.
 
[1] Nettelmann N, Cano Amoros M, Tosi N, Helled R, Fortney JJ. Atmospheric Helium Abundances in the Giant Planets. SSRv 220:56 (2024) 
[2] Wilson H, Militzer B. Sequestration of Noble Gases in Giant Planet Interiors. PRL 104:121101 (2010)
[3] Nettelmann N, Fortney JJ. Jupiter’s Interior with an Inverted Helium Gradient. PSJ (2025) 

How to cite: Nettelmann, N., Bethkenhagen, M., and Bergermann, A.: Predicted He/H and Ne/H Abundances in the Atmospheres of the Ice Giants, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3223, https://doi.org/10.5194/egusphere-egu25-3223, 2025.

The rings of Uranus, composed primarily of dark, radiation-processed material, are constantly subjected to exogenic dust bombardment, which affects their composition, structure, long-term dynamics, and evolution. This research investigates the interaction of Uranus’ rings with exogenic dust, focusing on the unique seasonal dynamics driven by Uranus' extreme axial tilt of 97.8 degrees. Uranus’ tilted rotational axis likely causes seasonal variations in dust flux, resulting in asymmetrical deposition patterns and impact rates depending on its orbital position around the Sun. Micrometeoroid particles near Saturn and Jupiter provide valuable analogs for studying Uranus, as they can be categorized into interplanetary dust particles (IDPs) and interstellar dust (ISD), depending on whether their orbits are sun-bound or sun-unbound, respectively. ISD originates primarily from the local interstellar cloud (LIC) and enters the solar system in a highly directional stream, while IDPs come from sources such as comets, Oort-Cloud and Edgeworth-Kuiper Belt objects (EKBs). These dust grains, subject to forces like Poynting–Robertson drag, lose momentum and spiral inward toward the inner solar system. Colwell et al. (1998) demonstrated that interstellar and interplanetary dust particles entering the Jovian magnetosphere can be captured through energy and angular momentum exchange, eventually forming a tenuous dust ring. A similar mechanism might occur in Uranus' rings, where dust interactions play an essential role in the rings' long-term evolution. These variations are hypothesized to cause observable differences in ring over Uranus' 84-year orbital period. In this study, we explore how impacting dust transfers angular momentum and energy to Uranus' rings, leading to gradual spreading and potential long-term erosion. Also, we model impact patterns from high-velocity dust collisions, which could maintain Uranus' faint rings and influence their overall dynamics. Future missions with advanced instrumentation may provide crucial data to validate these predictions and further explore Uranus’ ring system.

How to cite: Shih, H.-S., Lai, I.-L., and Ip, W.-H.: Effects of Exogenic Dust on Uranus' Rings and Seasonal Variations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5547, https://doi.org/10.5194/egusphere-egu25-5547, 2025.

Uranus and its system of rings and moons represents the next frontier in the exploration of the giant planets of our solar system. In addition to being close in mass and size to the most abundant known exoplanets, Uranus has a number of interesting features that distinguish it even from Neptune, so similar in size and mass. (1) Uranus has a regular satellite system of coplanar moons that are large enough, in some cases, to have subsurface liquid water. (2) Uranus has a very large axial tilt which may be the result of an early oblique impact, or not. (3) Uranus appears to have little or no internal heat, beyond thermalized sunlight, distinguishing it from all the other giant planets. (4) Both Uranus and Neptune have super-primordial deuterium abundances measured in HD that suggest most of the heavy element fraction in their interiors to be rock rather than ice (with an admixture of ice…it is not one or the other). Properties (1), (2), (3), and (4) constitute a coupled problem; they are not separate issues. That is, one would like to know if the origin of the moons is the result of a giant impact, and whether such an impact could have produced a layered structure that would then suppress any internal heat flux, as has been suggested (Hofstadter et al). If so, why then is the D/H value in HD the same in Uranus and Neptune, where the latter has a strong internal heat flow and thus would be expected to have allowed equilibration between envelope and core, in contrast to Uranus. Further, the average system mass density of the regular Uranian satellites is consistent with the derived rock-to-ice ratio in the interior of Uranus, which in turn argues for an obliquity-generating impact sourcing deep material for the nascent satellite system.  Whether a self-consistent picture can be assembled depends on the outcome of measurements that can be made by a shallow atmospheric entry probe, such as the ⁴⁰Ar abundance (Nimmo et al), gravity measurements of the deep interior and the regular satellites, and other observations by a capable orbiter. A highly capable Uranus Orbiter and Probe mission has the potential to make profound discoveries about the properties and interior dynamics of this enigmatic world that rival what Cassini, Galileo and Juno have discovered at Jupiter and Saturn. 

Hofstadter, M., et al, 2023. https://kiss.caltech.edu/final_reports/Uranus_final_report.pdf

Nimmo et al, 2024, PSJ 5 109.

Part of this work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. © 2025. All rights reserved.

How to cite: Lunine, J. I. and Feldman, S. and the JPL Uranus Orbiter and Probe Study Team: Four coupled reasons why Uranus is so interesting, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9799, https://doi.org/10.5194/egusphere-egu25-9799, 2025.

How to cite: Duer-Milner, K., Gavriel, N., Galanti, E., Tziperman, E., and Kaspi, Y.: A mechanism for equatorial jet formation on ice giants, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12975, https://doi.org/10.5194/egusphere-egu25-12975, 2025.

Observations and modelling of albedo, cloud cover, and aerosols on Neptune (e.g. Lockwood and Thompson, Nature 280, 1979; Irwin et al, JGR Planets 127, 2022; Chavez et al, Icarus 404, 2023) have been ongoing for a long time. A feature that has yet to be fully explained is the (anti)correlation between the albedo and the 11-year solar cycle.

UV light, which varies highly with solar activity, can affect photochemistry by photolysis of methane, which can lead to haze formation (Romani and Atreya, Icarus 74(3), 1988) and under the right conditions methane can form aerosols by itself. Ionization by galactic cosmic rays can enhance aerosol nucleation rates by lowering the Gibbs free energy barrier for stable cluster formation.

In this work we calculate neutral homogeneous nucleation rates for methane as well as the corresponding ion-induced heterogeneous nucleation rates. We do this both for vertical profiles and horizontal maps of the Neptunian atmosphere.

The starting point is the ISO atmospheric profile for a hot, cold, and nominal Neptune atmosphere. This is combined with a simple methane distribution model constrained by observational values in the troposphere and stratosphere. From this the Gibbs free energy for homogeneous nucleation is calculated, giving the neutral nucleation rate.

We then find ionization rates, based on galactic cosmic ray proton flux maps generated from calculations of cut-off rigidities derived from the magnetic field model of Neptune (Connerney et al, ASR 12(8), 1992) and a particle trajectory program (the Geomagnetic Cutoff Rigidity Computer Program by Smart and Shea, 2001, Tech. Rep. No. 20010071975).

From the ionization rates the ion-induced nucleation rate can be found. We then compare the neutral and ion-induced nucleation rates for a vertical profile to identify which altitudes are dominated by which process.

Horizontal maps at altitudes where the nucleation rates are not dominated by neutral nucleation can then be generated. By using latitudinal variations in the methane concentration in combination with the ionization maps we can identify a geographic distribution of where ion-induced nucleation may play a significant role in the generation of aerosols.

How to cite: Enghoff, M. B., Romani, P. N., Connerney, J. E. P., and Jørgensen, J. L.: Methane nucleation in the atmosphere of Neptune, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15795, https://doi.org/10.5194/egusphere-egu25-15795, 2025.

The importance of studying the Ice Giants is highlighted by NASA’s recent designation of a mission to Uranus as its top priority in upcoming space exploration initiatives [1]. Modelling Galactic Cosmic Ray (GCR) ionization, along with the resulting chemical and electrical profiles, is crucial for interpreting data from a descent probe, as it will provide a detailed characterization of the descent region [2]. A comprehensive global model could influence mission planning by identifying optimal descent locations for maximum scientific return and by guiding recommendations for the necessary instrumentation. In this work, we used CORSIKA8 [3] to model how GCR air showers deposit energy to ionization at incremental pressures. The input parameters such as the pressure profile, as well as energies and fluxes of incident primary particles were scrutinized for robust results. The energy deposited by GCRs was used to calculate the ionization rate in the lower stratosphere and upper troposphere of Uranus. Our results show that the peak of ionization, known as the Regener-Pfotzer (RP) maximum – a universal parameter across planetary atmospheres, occurs at approximately 10⁴ Pa, which is consistent with other planets and existing literature [4], [5].

In addition to geomagnetic cut-off rigidity, which determines the minimum GCR energies based on the magnetic field, we examined the impact of Uranus' asymmetric and complex magnetic field on air shower evolution. A key focus was the parameter RP maximum, representing the pressure at which the ionization rate peaks. Although characterizing secondary particle deflections under varying magnetic fields is challenging due to numerous sources of randomness, sensitivity analysis revealed that RP maxima are significantly influenced by magnetic field variations. This prompted a global investigation into RP maxima variations, resulting in a pioneering ionization rate profile. Our analysis showed positively correlating trends between RP maxima and horizontal magnetic field strength. RP maxima were observed to occur at deeper pressures near the poles, with notable hemispheric differences driven by the stronger magnetic field at the southern pole compared to the northern. Given Uranus' large scale height, these pressure differences translate to altitude variations exceeding 25%. These findings have important implications for Uranus' atmospheric chemistry, cloud formation, and electrical conductivity, particularly with respect to geomagnetic latitude variations.

[1] Choi, C. Q. (February 2023). Uranus up close: What proposed NASA 'ice giant' mission could teach us. Space.com. Retrieved from https://www.space.com/nasa-uranus-orbiter-and-probe-mission-objectives

[2] Hueso, R., & Sánchez-Lavega, A. (2019). Atmospheric Dynamics and Vertical Structure of Uranus and Neptune’s Weather Layers. Space Science Reviews, 215:52. https://doi.org/10.1007/s11214-019-0618-6

[3] Engel, R., Heck, D., Huege, T., et al. (2019). Towards a Next Generation of CORSIKA: A Framework for the Simulation of Particle Cascades in Astroparticle Physics. Computing and Software for Big Science, 3, 2. https://doi.org/10.1007/s41781-018-0013-0

[4] Molina-Cuberos, G., et al. (2023). The Low-Altitude Ionosphere of the Ice Giant Planets. Journal of Geophysical Research: Planets. https://doi.org/10.1029/2022JE007568

[5] Nordheim, T., et al. (2020). Cosmic ray ionization of Ice Giant atmospheres. 22nd EGU General Assembly, held online 4–8 May, 2020, id.6977 [poster].

How to cite: Al-Khuraybi, O., Aplin, K., and Gambaruto, A.: Modelling Uranus' Global GCR Ionization Profile: Unveiling Geomagnetic Latitude Variations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16177, https://doi.org/10.5194/egusphere-egu25-16177, 2025.