Uranus is observed to have fast surface zonal winds with speeds reaching up to 200 ms⁻¹ relative to its assumed bulk rotation. The exact decay behaviour of the winds is uncertain, but there is indirect evidence that they must decay rapidly in relatively shallow layers of Uranus. By analysing thousands of Uranus interior structure models our study investigates the Uranian zonal wind decay via past zonal gravitational harmonics measurements, as well as understanding our limitations of future zonal wind constraints with a prospective Uranus mission (as proposed by NASA’s Planetary Science and Astrobiology Decadal Survey 2023-2032). In addition, we explore the effect of zonal wind decay on planetary shapes, the relationship of zonal wind decay and planetary bulk rotation, and the effect of alternative surface zonal wind profile fits to surface wind measurements on aforementioned phenomena.

How to cite: Soyuer, D., Helled, R., Neuenschwander, B., and Morf, L.: Zonal Winds of Uranus: Analysis, Modelling and Prospects, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9646, https://doi.org/10.5194/egusphere-egu24-9646, 2024.

Understanding the atmospheric dynamics of Uranus and Neptune remains a challenging endeavour due to the limited observational data available. In this study, we employ a sophisticated General Circulation Model (GCM), known as the Generic Planetary Circulation Model, to investigate the complex meteorological phenomena of the Ice Giants, with a focus on the role of convection in the troposphere Our attention is directed towards the parametrization of convection, a crucial driver of atmospheric circulation, as it significantly impacts the transport of energy and the distribution of chemical species throughout the atmosphere. One of the unique aspects of our study lies in the consideration of methane condensation in the convection parametrization scheme based on a thermal plume model initially developed for the Earth atmospheric boundary layer (Rio & Hourdin 2008). However, unlike for the Earth, the condensable species are heavier than the surrounding atmosphere mainly composed of hydrogen. This phenomenon is suggested to be a powerful driver of intermittent storms activity detected in the atmospheres of the Ice Giants (Guillot 2022). The improved fidelity of our GCM simulations offers valuable implications for interpreting observational data and refining our understanding of the atmospheric processes governing these enigmatic outer planets.

How to cite: Le Saux, A., Guerlet, S., Spiga, A., Leconte, J., Clement, N., and Milcareck, G.: Investigating Atmospheric Dynamics of Uranus and Neptune: A General Circulation Model Approach with a Parametrization of Convection, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18317, https://doi.org/10.5194/egusphere-egu24-18317, 2024.

Uranus and Neptune have atmospheres dominated by molecular hydrogen and helium. In the upper troposphere (between 0.1 and 10 bars), methane is the third main molecule and condenses, yielding a vertical gradient in methane. Because it is heavier than the H₂/He background, methane condensation can inhibit convection and moist convective storms. Previous studies derived an analytical criterion on the methane vapor amount, above which moist convection is inhibited. In ice giants, this criterion yields a critical methane abundance of 1.2% at 80K (this corresponds approximately to the 1 bar level).

Using a 3D cloud-resolving model, we have shown (Clement et al. 2024, submitted in A&A) that this critical methane abundance governs storms inhibition and formation, concluding that the intermittency and intensity of storms depends on the methane abundance. Where methane exceeds this critical abundance in the deep atmosphere (at the equator and the middle latitudes on Uranus, and all latitudes on Neptune), frequent but weak storms form. Where methane remains below this critical abundance in the deep atmosphere (possibly at the poles on Uranus), storms are rarer but more powerful.

We use the insights of our 3D small-scale simulations to build a 1D parameterization of diffusion and convection for radiative-convective and global climate models. As 3D cloud-resolving simulations require long computation times, a radiative-convective model is needed to extrapolate heating tendencies. The combined use of these models should enable us to estimate more realistic periods between convective storms and explain the observed latitudinal sporadicity of methane clouds over a long period of time.

How to cite: Clement, N., Leconte, J., Spiga, A., Guerlet, S., Milcareck, G., Le Saux, A., and Selsis, F.: Convective storms and methane clouds on Uranus & Neptune: sporadicity and latitudinal variations revealed by a 3D cloud-resolving model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1905, https://doi.org/10.5194/egusphere-egu24-1905, 2024.

Albedo changes of Neptune related to the 11-year solar cycle have been reported since the 1970s (Lockwood and Thompson, Nature 280, 1979). For nearly two decades a clear anti-correlation between solar activity and Neptune’s brightness was observed but the relationship appeared to break down between the late 1980s (Lockwood and Thompson, Nature 349, 1991) and mid 1990s (Lockwood and Jerzykieicz, Icarus 180(2), 2006) where signs of a direct correlation instead appeared. Recent results indicate a direct correlation sustained over two solar cycles (Chavez et al, Icarus 404, 2023) prompting renewed interest in attempting an explanation.

Several parameters vary with the solar cycle, one being UV light which has a much higher variation than that of visible light. This could affect the photochemistry of Neptune’s atmosphere which is rich in methane that photolyzes at wavelengths below 200 nm and can produce haze (Romani and Atreya, Icarus 74(3), 1988). Another parameter that varies is the flux of galactic cosmic rays (GCRs) which is modulated by the solar wind. GCRs can ionize molecules leading to ion-induced nucleation (Moses et al, GRL 16(12), 1989) but only for the energies of particles which are allowed entry to the planetary atmosphere by the magnetic field. Solar activity modulates GCRs of energies up to about 20 GeV so a solar variation due to GCRs is only possible if particles of those energies can enter the atmosphere. The parameter used to describe GCR entry is the cutoff rigidity (in GV).

In this work we have used 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) to calculate a cutoff rigidity map for Neptune for vertical GCR entry. Since the magnetic field is very tilted compared to the rotational axis (by about 45 degrees) the cutoff rigidity map has interesting features as the GCRs are guided by the magnetic field lines. Thus, lower energies and therefore a higher GCR flux more susceptible to solar cycle changes are allowed to enter the atmosphere at mid latitudes, as opposed to most planets where this happens as the poles since their magnetic field is more closely aligned with the rotational axis.

Furthermore, we have used fitted GCR energy spectra in combination with the cutoff rigidity map to produce a map of solar cycle variations in GCR flux at a height of 49 km, which is at the pressure level where cosmic ray showers are initiated at Earth and also close to where Neptunian clouds are found. Where the cutoff rigidity is lowest the solar cycle variations of GCR are several tens of percents which could affect cloud formation significantly.

How to cite: Enghoff, M. B., Romani, P. N., Connerney, J. E. P., and Jørgensen, J. L.: Galactic cosmic ray cutoff rigidity and solar cycle variation at Neptune, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5294, https://doi.org/10.5194/egusphere-egu24-5294, 2024.

The international consortium GEPP has been set to conceptualize probe designs with appropriate payloads that would remain within the typical budget allocated for ESA M-class missions (currently 500 M€). The aims of the consortium are i) to conceptualize a line of generic planetary entry probes that could be targeted to the giant planets with very few modifications, ii) to make the international science community, ESA and its member states, conscious that there is an opportunity to supply a series of entry probes as part of future international collaborations, for example as part of the future NASA flagship mission towards Uranus (Uranus Orbiter Probe) or to any future NASA-led mission to the outer planets for an affordable budget, and iii) to demonstrate that an M-class budget could even fund several entry probes with well-prioritized science objectives. The model payload capabilities of each concept will be defined according to a carefully-designed science traceability matrix. Two extreme concepts shall be investigated by the GEPP Consortium, namely a highly capable parachute-descent probe including a typical payload of 30 kg of scientific instruments down to 10 bars, and a smaller parachute-descent probe designed to address top priority science objectives with selected key measurements that would address the ESA Cosmic Vision 2050 science objectives. This presentation will detail the scientific objectives for each entry probe design, as well as the content, organization and planning of the study, which is assumed to be completed by the end of 2025.

How to cite: Mousis, O. and the GEPP team: Generic Entry Probe Program (GEPP) – an international initiative promoting the development of European descent modules dedicated to the in situ exploration of giant planets, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13379, https://doi.org/10.5194/egusphere-egu24-13379, 2024.

Both Voyager 1 and 2 are equipped with Plasma Science (PLS) instruments: four Faraday cups that measure the properties (density, temperature and flow) of low energy ions and electrons. During the Voyagers journey towards the interstellar space and the flybys from the gas giants PLS gave us the first in-situ data of the solar wind in such great distances, the magnetospheric plasma properties at the gas giants, and the extent of our heliosphere along with the conditions of the interstellar medium.

Voyager was particularly important for the study of the gas giants, as it was the first time we had in-situ plasma measurements from inside the magnetospheres, and PLS helped in studying not only the morphology of the magnetosphere, but also the plasma sources, dynamics, interaction with the moons, and ultimately its interaction with the solar wind plasma.

Jupiter and Saturn were visited from both Voyager 1 and 2, but only Voyager 2 visited Uranus and Neptune. The PLS data for Jupiter have been re-calibrated and archived by Bagenal+ [2017], Dougherty+ [2017], and Bodisch+ [2017]. They also developed an IDL package, VIPER (Voyager Ion PLS Experiment Response) for their analysis. For this work we re-analyzing the Voyager PLS data for Uranus and in this poster we present our methodology, the adaptation of VIPER for the Uranian conditions and the results of the re-analysis.

How to cite: Xystouris, G., Bagenal, F., and Wilson, R.: Re-analysis of Uranus Data from Voyager 2 Plasma Science Experiment, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3184, https://doi.org/10.5194/egusphere-egu24-3184, 2024.

Neutral Tori are generally a result of particles escaping from a planetary atmosphere, its rings or satellites through internal source mechanisms or interactions with the magnetosphere. These particles then form a population of co-orbiting neutral particles that provide a source of plasma to the magnetosphere as well as drive dynamics and chemistry. Thus, understanding neutral tori provides key (sometimes unique) insight into planetary magnetospheres, moon source characterization and understanding, and ultimately can provide insight to past, present and future of magnetospheres. Current increased neutral torus research had provided significant insight into Saturn and Jupiter’s magnetosphere and gas giants in general. These results indicated great potential for improving our understanding the magnetospheres of Uranus and Neptune whose orientations offer the possibility of magnetospheric configurations not previously observed. Here, we will discuss how the Gas Giant neutral torus state of understanding has signifcaintly increased and may provide an analogy for what we may expect from studying these features on Ice Giants. We also present some preliminary modeling to speculate on the possible presence of neutral tori at Uranus and Neptune.

How to cite: smith, H.: The potential for neutral tori at Uranus and Neptune, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20711, https://doi.org/10.5194/egusphere-egu24-20711, 2024.

The habitability of icy moons in the outer Solar System is linked to their ability to maintain warm subsurface oceans through time. While most heat generation in response to tidal forcing is thought to occur in icy crusts and water oceans, the actual response of silicic material to deformation under relevant planetary conditions has not previously been studied comprehensively in a laboratory setting. Similar meteoritic material is often studied at room pressure instead of at the 10s to 100s of MPas of pressure present at depth in moons, and subjected to dynamic forces to simulate impacts rather than observed under quasistatic loading and deformation. In the absence of laboratory constraints on peak strength and deformation behavior, large errors remain for estimates of heat contribution from the mantles, as well as in models of seismic and elastic properties of icy moon interiors.

We experimentally deformed samples of the Kilabo meteorite, an LL6 chondrite, under axial strain rates of 10^-5 s^-1 and confining pressures up to 100 MPa. We recorded the strength of the material, calculated energy dissipation through acoustic emission events, and measured how ultrasonic wavespeeds evolved as a function of confining pressure. Dissipative microcracking events occurred at all pressures, even at low stresses during isotropic pressurization and nominally “elastic” deformation. These events were most common at low confining pressures. The mechanical behavior of the meteoritic material also evolved as a function of confining pressure: peak strength occurred at 50 MPa laboratory confining pressure, and material continuously stiffened as pressure increased. These pressure-dependent properties indicate that larger icy planetary bodies may have stiffer, less deformable silicate layers than those found in small icy satellites. Rocky interior deformation could therefore contribute to the bulk heat budget required to maintain subsurface oceans in Ariel and Miranda, along with many other smaller icy moons in the outer Solar System.  

How to cite: Seltzer, C., O. Ghaffari, H., and Peč, M.: Experimental constraints on tidal dissipation in silicate cores of icy satellites, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-762, https://doi.org/10.5194/egusphere-egu24-762, 2024.