A central question that remains unresolved is whether global magnetic reconnection processes continue to dominate as drivers of the outer planetary magnetospheres. Uranus offers a unique opportunity to investigate solar wind-magnetospheric interactions in the outer solar system. Here, we assess the effectiveness of magnetic reconnection in driving Uranus’ magnetospheric dynamics by modeling the associated voltage applied to the planet’s dayside magnetopause. We present theoretical predictions of these reconnection voltages under various solar wind and magnetospheric configurations using models with much heritage from the Earth and other planets [1-9] and inputs based on Voyager 2 data. Figure 1 illustrates example outputs of the model viewed from along the upstream solar wind flow direction.

Figure 1a shows the planetary magnetic field confined by the magnetopause at a selected rotational phase during Uranus’ solstice season and Figure 1b shows the magnetosheath field that results from a draped northward interplanetary magnetic field (IMF). Figure 1c identifies regions on the magnetopause where reconnection is permitted (in green) and suppressed (in black). The solid red lines indicate maxima of magnetic shear angles between the reconnecting fields – proxies for reconnection X-line locations. The reconnection voltage is then calculated using the electric field, shown in Figure 1d, to determine the potential difference along these X-lines.

In this study, we investigate how variability in the structure of the internal field with season and solar cycle modulation of the solar wind conditions impact the reconnection voltages. Voltages predicted for a full Uranian year are summarized in Figure 2. Voltages depicted in each box are calculated at eight phases over one Uranus rotation and under 100 distinct solar wind conditions that have an 11-year solar cycle dependence. Figure 2 reveals that increasing the IMF strength –mimicking conditions during solar maximum – can increase the median voltage from approximately 17 kV to 31 kV. Across one Uranian year, the median predicted dayside reconnection voltage is relatively low at 22 kV. We also note that there is no clear seasonal dependence of the reconnection voltages between solstice and equinox.

Our results suggest that any potential seasonal variations are obscured by large dependencies of the voltages on prevailing solar wind conditions and diurnal phase, as showcased by the large range of each box in Figure 2 that spans multiple orders of magnitude [10].  In the future, these results could be tested with new in situ observations of the Uranian system that could aid in revealing whether global magnetic reconnection processes dominate solar wind-magnetospheric interactions.

How to cite: Zomerdijk-Russell, S., Jasinski, J. M., and Masters, A.: Uranus’ Coupling with the Solar Wind Through Magnetic Reconnection, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-405, https://doi.org/10.5194/epsc-dps2025-405, 2025.

In 1989, Voyager 2 carried out the first, and thus far only, in-situ exploration of the Neptune system. This included a distant flyby of Neptune’s only major moon Triton – a large icy moon that may possibly be a captured Kuiper Belt Object. Based on radio occultation measurements during the flyby, Triton was found to have an intense ionosphere with peak electron densities on the order of 10⁴ cm^-3 [1]. This was surprising, as Neptune’s orbital location at 30 AU from the Sun means that the flux of ionizing solar UV photons is relatively low. It was therefore suggested that magnetospheric electron precipitation may be an important, and possibly dominant, energy input to Triton’s ionosphere [2], [3], [4]. However, subsequent modelling efforts were never able to fully explain the structure of Triton’s ionosphere from either solar photoionization or magnetospheric electron input [5], [6]. The question of what drives Triton’s intense ionosphere therefore remains unresolved.

Figure 1: Ionospheric densities at Triton as measured by Voyager 2 [1] during ingress (dashed black line) and egress (solid black line) compared to the most intense ionosphere measured by Galileo at Jupiter’s moon Callisto [7].

An important factor in determing the role of magnetospheric electron precipitation at Triton is our knowledge of Neptune’s magnetosphere and the “seed population” of magnetospheric electrons upstream of the moon. Most studies in the literature have considered either mono-energetic beams of electrons incident at the top of Triton’s atmosphere or a “best-guess” electron spectrum based on early analyses of the Voyager 2 magnetospheric measurements. An example of the most commonly used “best-guess” magnetospheric electron spectrum  [5] is shown below in Figure 2.

Figure 2:  Representative magnetospheric electron environment at Triton based on the “best guess” estimate by [5] compared to similar magnetospheric environments at Jupiter and Saturn.

Here, we revisit the question of the dominant energy input to Triton’s ionosphere using new models and analysis techniques that were not available at the time of the Voyager 2 Neptune encounter. This includes re-analyses of data from the Voyager 2 Plasma Science (PLS) and Low Energy Charged Particle (LECP) instruments near Triton’s location in Neptune’s magnetosphere, and charged particle transport modelling to simulate the resulting energy deposition and primary ionization rate due magnetospheric elctrons incident at the top of Triton’s atmosphere.

References:

[1] Tyler G. L., et al. (1989) Science, 246, 4936,1466–1473. [2] Strobel, D. F. et al. (1990) Geophys Res Lett, 17, 10, 1661–1664. [3] Majeed, T., et al. (1990) Geophys Res Lett, 17, 10, 1721–1724. [4] Yung, Y. L. and Lyons, J. R.  (1990) Geophys Res Lett, 17, 10, 1717–1720. [5] Sittler, E. C.  and Hartle, R. E. (1996) J Geophys Res, 101, A5, 10863–10876. [6] Krasnopolsky, V. A.  and Cruikshank, D. P. (1995) J Geophys Res, 100, E10, 21271. [7] Kliore, A. J.  et al. (2002) J Geophys Res Space Phys, 107, A11, 1–7.

How to cite: Nordheim, T., Luspay-Kuti, A., Grayson, R., Kollmann, P., Mandt, K., Liuzzo, L., and Vervack, R.:  Triton’s mysteriously intense ionosphere: Updates on magnetospheric charged particle fluxes and atmospheric energy deposition, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1281, https://doi.org/10.5194/epsc-dps2025-1281, 2025.

The charged particle ionosphere is the interface-region between the atmosphere below and the space environment beyond. Observations from James Webb Space Telescope (JWST) have revealed the ionosphere with unprecedented sensitivity, revealing features of this region that were hitherto undetectable using existing facilities. Here, we present the spectral analysis of JWST NIRSpec Integral Field Unit (IFU) observation obtained in January 2023 that covered most of the disk of Uranus at three separate rotational phases, providing complete longitudinal coverage. Via the spectral analysis of the discrete H3+ emission lines contained in the near-infrared spectrum, we derive maps of temperature and density of the ionosphere across the visible portion of the planet. The H3+ reveal a host of intricate structures: 1) Northern and southern auroral emission, with the southern being very confined, and the northern more extended. 2) A dark band appears along the magnetic equator, analogous to the terrestrial equatorial plasma fountain 3) the H3+ radiance distribution across the disk, which is expected to be produced primarily via the ionisation of molecular hydrogen by solar photons, show a distribution that is intricate. Overall, we see the coolest temperatures at the geometric pole, no heating associated with the southern auroral oval, and very limited heating at the northern oval. This puts stringent limits on the amount of energy at the auroral process can contribute to the overall energy budget. 

How to cite: Melin, H., Fletcher, L., Tiranti, P., Hammel, H., Milam, S., Stallard, T., Thomas, E., Knowles, K., Moore, L., and O'Donoghue, J.: The Ionosphere of Uranus as Revealed by JWST, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-647, https://doi.org/10.5194/epsc-dps2025-647, 2025.

Uranus stands as an enigma within the field of planetary science. What little we know of its ionosphere and aurorae have proven perplexing, whether this be ultraviolet (UV) or infrared (IR) emissions, considerable intensity and column density variability have been observed across the surface of Uranus’s ionosphere [Lamy et al., 2012, 2017, 2020, Lam et al., 1997, Melin et al., 2019 and Thomas et al., 2023 and 2025]. For IR emissions of the molecular ion H₃⁺, a key tracer of energy in Uranus’s ionosphere, only two observations have published spatial intensity maps with a 10 year gap between investigations, this difference prevents a direct comparison as to the variability between emission intensities, temperatures and column densities. To address this, our investigation will study the shorter term variability of Uranus by mapping multiple half planet scans (using Keck-NIRSPEC) in October 2023 and January 2025, an example shown in Fig. 1, to identify ionospheric fluctuations in key auroral and non-auroral regions. Through comparison of these intensity scans with prior JWST observations in December 2021 our discussions will focus on short-term auroral and thermal drivers in Uranus’s ionosphere.

Figure 1. Averaged global projections (~ 5hr) of the spectral radiance of the Q(1,0^-) H₃⁺ emission line from Uranus as observed by Keck-NIRSPEC on the 29^th September 2023. The intensity has been normalised in this figure with the S representing the ULS southern pole of Uranus.

How to cite: Thomas, E., Stallard, T., Melin, H., Moore, L., Knowles, K., Tiranti, P., Chowdhury, M. '., Wang, R., Roberts, K., Attwood, B., Johnson, R., O'Donoghue, J., and Mapaye, R.: Why is Uranus giving us the cold shoulder? Analysing planetary scans of H3+ infrared emissions from Uranus's ionosphere, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-967, https://doi.org/10.5194/epsc-dps2025-967, 2025.

Background: UV occultations measured by Voyager 2 (V2) during its flyby of Uranus in 1986 detected a warm stratosphere and extremely hot thermosphere [1,2], far in excess of solar irradiance [3,4] or internal heating [5]. These measurements imply that Uranus has the coldest lower stratosphere and yet the hottest thermosphere of any Solar System planet [6] (Figure 1, dotted line). Uranus also has the weakest vertical mixing of any giant planet [5]. The fundamental lack of understanding about the energy balance of Uranus’ atmosphere is an example of the “giant planet energy crisis” [7].

Furthermore, the V2 UV occultation measurements and models were in stark tension with Earth-based stellar occultations observed 1977-1996. In [8,9], we presented the results of processing 26 archival occultations using modern techniques, showing decreased stratospheric discrepancies, but thermospheric discrepancies remain. In [9], we presented a new 1-D atmospheric model based on the most reliable of these results, which shows a thermal structure similar to that of the other giant planets, and in contrast to some V2 measurements.

Motivation: Measurements of H₃⁺ emissions indicate that Uranus’ thermosphere has cooled from 725K in 1992 to 425K in 2019 [7] and continues to cool to the present [10]. New stellar occultations have the potential to identify the present-day thermal structure and allow for comparison between different atmospheric layers to better understand energy transport in Uranus’ atmosphere.

Aims: On 2025 April 08 UT, Uranus occulted a bright star, creating the best stellar occultation opportunity in decades. We observed the occultation using 18 professional telescopes in the US and Mexico, involving over 35 astronomers and observers. Details about the campaign and team can be found at https://science.larc.nasa.gov/URANUS2025/. To our knowledge, this was the largest Uranus stellar occultation campaign ever organized, and the largest simultaneous observation of Uranus since the 1986 V2 flyby campaign. We will report on the execution of the campaign, as well as preliminary results on atmospheric and ring measurements. We will discuss anticipated results and their implications for the understanding of Uranus’ thermal structure, long-term variations, and potential use of aerocapture for orbital insertion of a spacecraft around Uranus. 

Background on Stellar Occultations: An Earth-based stellar occultation occurs when a solar system body appears to pass in front of a distant star. By observing the differential refraction of starlight through the atmosphere of the occulting body, we can determine the pressure, temperature, and density vs. altitude with high accuracy and vertical resolution in the stratosphere and lower thermosphere (∼0.1–100 microbar pressures). Observations of Uranus ring occultations are used to study the rings in detail and improve Uranus’ ephemeris [11]. 

Occultation Details: This stellar occultation was initially predicted and reported in [12]. The star has designations BD+18 489, SAO 93455, Gaia DR3 57460310166762752, and is an F5 white dwarf in a physical binary system with a parallax of 8.08 mas. Its magnitude is reported as V=9.05, R=9.70, K=7.95. The occultation occurred between approximately 02 and 03 UT on 2025 April 08; the view of Earth from the occultation at the mid-point is shown in Figure 2. All observations (with the exception of the IRTF) had to begin immediately after sunset and observe low in the Western horizon, creating a challenging observation. 

Occultation Campaign: Figure 3 is a map showing the locations of each site involved in the campaign. In preparation, each telescope’s team tested different combinations of filters, cadences, sub-frame windows, and/or modes to achieve the best possible signal-to-noise without any risk of saturating. Infrared or methane absorption band filters were used where available and achieved the best signal-to-noise. Of the 18 telescopes that attempted the observation, 14 had good weather and we anticipate 4-7 will produce high quality atmospheric light curves. 

Preliminary Results: We will present preliminary results in (1) atmospheric temperature structure, (2) long-term stratospheric variations, and (3) ring occultation measurements. We will report the thermal structure of Uranus’ stratosphere and lower thermosphere from inverting these light curves and the resulting vertically averaged stratospheric temperatures in 2025. We will compare these to past occultations from 1977-1996 as well as H₃⁺ measurements of the thermosphere. We will report on the many detected ring occultations and how they are being used to improve Uranus’ ephemeris 

Implications for Aerocapture: Aerocapture is a maneuver for orbital insertion in which a spacecraft passes through the stratosphere of a planet to decelerate. If used for a mission to the ice giants, it would enable a much shorter cruise and larger science payload than a propulsive orbital insertion [13], but it is impeded primarily by large uncertainties in Uranus’ stratospheric densities [14]. We will discuss how our results may improve density measurements in support of aerocapture. 

Future Work: We are exploring how airborne and/or space-based observations of future occultations (especially a 2031 Uranus occultation of a 4th magnitude star) may improve upon the ground-based observations from 2025, based on initial work from [12]. We will discuss science and technology motivations as well as the instrument needs for the 2031 occultation. We will also report on critical lessons learned that may help with future occultation campaigns. 

References: [1] Herbert et al. (1987). [2] Stevens et al. (1993). [3] Marley & McKay (1999). [4] Li et al. (2018). [5] Pearl et al. (1990). [6] Young et al. (2001). [7] Melin (2020). [8] Saunders et al. (2023). [9] Saunders et al. (2024). [10] Luke Moore, personal communication. [11] French et al. 2024. [12] Saunders et al. (2022). [13] Soumyo et al. (2021). BAAS. [14] Report of the Aerocapture Demonstration Relevance Assessment Team 2023. [15] Mueller-Wodarg et al. (2008). [16] Orton et al. (2014).

We acknowledge NASA PSD for funding this work through grant 24-SSO24-0006.

Figure 1. Comparison of temperature-pressure profiles for atmospheres in the solar system. The V2 profile (dotted) [15] is compared to the Spitzer IR model (dot-dash) [16] and the Earth-based occultation model (dashed) [9]. Figure 14 in [9].

Figure 2. Globe of Earth showing the geometry of the occultation. Gray indicates after sunset.

Figure 3. Map of all observing sites that attempted observations. 

How to cite: Saunders, W. and Sayanagi, K. and the Uranus Stellar Occultation Campaign 2025: Uranus Stellar Occultation 2025: Report and Preliminary Results from the Largest Uranus Stellar Occultation Campaign, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-942, https://doi.org/10.5194/epsc-dps2025-942, 2025.

Although Uranus and Neptune are commonly classified as ice giants, their exact compositions remain uncertain. Recent studies on outer solar system objects challenge the traditional view that these planets are primarily icy, suggesting the idea of a rock-dominated composition. Determining the proportions of ice and rocks within Uranus and Neptune is essential for understanding their formation and the broader history of the solar system. In this work, we calculate interior structure models for Uranus and Neptune. We explore the range of structure models that meet observational constraints, assessing ice and rock fractions and analyzing their impact on the planets interior. Our results suggest that Neptune's envelope is rock-enriched, with a minimum rock fraction of around 60%, while its mantle may contain more ices. For Uranus, models with larger ice fractions (over 50%) are needed to fit the radius and gravity data. These differences between Uranus and Neptune suggest possible distinct formation and evolution paths.

How to cite: Ramirez, V., Miguel, Y., and Howard, S.: Are Uranus and Neptune really ice giants?, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-110, https://doi.org/10.5194/epsc-dps2025-110, 2025.

The morphology of the magnetic fields of Uranus and Neptune are radically different to those of all other planets in our solar system. Despite only having in-situ measurements from one Voyager 2 fly-by of each planet, it is clear that they are not dominated by an axial dipole and are instead multipolar. However, due to the rarity of large-scale multipolar planetary magnetic fields in our solar system, the focus of numerical dynamo studies has primarily been on dipole dominated models. Here, we revisit existing numerical dynamo parameter surveys, selecting cases of particular interest. We focus on multipolar models that are in the regime where rotation dominates over convective turbulence (i.e., the low local Rossby number regime, Ro_ell<1). We have also performed simulations with high Ro_ell to quantitatively disambiguate between the two dynamical regimes by comparing the characteristics of these systems, such as their field strengths, flow characteristics, and spectra. In this comparison we reconsider the best ways to characterise multipolar dynamos. While many of the existing tools sweep them into one vast category of “non-dipolar” models, we find them to display a number of vastly differing properties. Characterisation of these models allows us to assess ice giant-like properties with our existing data as well as predict what higher resolution future measurements of their magnetic fields could reveal.

How to cite: Wulff, P., Aurnou, J., Soderlund, K., and Cao, H.: Bistability of ice giant dynamo action, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-236, https://doi.org/10.5194/epsc-dps2025-236, 2025.

Uranus and Neptune present unique non-dipole-dominated magnetic fields that are also strongly non-axisymmetric (i.e., peak power for m>0). Their distinctly high power in the m = 1 component appears to be a persistent feature and, therefore, a key diagnostic for determining the mechanisms underlying magnetic field generation within these planets. Here, we highlight numerical dynamo models that successfully reproduce the large-scale features of ice giant magnetic fields, with the profile of radially varying electrical conductivity being a critical ingredient. Moreover, the magnetic fields in these dynamo models evolve rapidly with time. We thus hypothesize that Uranus' and Neptune's magnetic fields may have changed in intensity and/or orientation since the Voyager 2 flybys. This secular variation has implications for telescopic observations of auroral features (e.g., via the James Webb Space Telescope) and provides essential groundwork for the Uranus Orbiter and Probe Flagship and Triton Ocean World Surveyor New Frontier mission concepts prioritized in the Origins, Worlds, and Life Decadal Survey.

How to cite: Soderlund, K., Yan, C., Aurnou, J., and Cao, H.: Time-varying dynamo models consistent with Uranus’ and Neptune’s magnetic fields, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-439, https://doi.org/10.5194/epsc-dps2025-439, 2025.

How to cite: Esteves, L., Izidoro, A., and Winter, O.: Forming Uranus and Neptune through giant impacts: accretion scenarios with large and small impactor mass ratios in the early Solar System, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-414, https://doi.org/10.5194/epsc-dps2025-414, 2025.

Uranus and Neptune are among the least studied bodies in our Solar System, and their composition and formation processes remain subjects of ongoing debate. Observations of these icy giants have shown that both planets are significantly enriched in carbon and moderately enriched in nitrogen relative to protosolar values, challenging current planet formation models. Additionally, measurements of the deuterium to hydrogen (D/H) ratio, obtained by the Herschel Space Telescope, reveal a supersolar value, although lower than that of comets. This D/H ratio provides valuable insights into the bulk composition of Uranus, offering clues to its formation conditions.

The goal of this study is to determine whether the CO/H2O ratio of Uranus, derived from its D/H measurement, is consistent with the composition of the protosolar nebula (PSN) during its evolution. To achieve this, we use an interior model of Uranus, assuming the planet is composed entirely of ice. From this model, we derive the CO/H2O ratio of Uranus based on its D/H ratio, with the assumption that the D/H ratio in its original ices mirrors that of comets. We then compare this CO/H2O ratio to the corresponding ratio in the PSN, using a protoplanetary disk model that accounts for the evolution of species in multiple phases, including clathrate hydrates.

Our preliminary results compare the CO/H2O ratio derived from Uranus' D/H ratio with the local CO/H2O ratio in the protosolar nebula at different epochs, providing insight into the conditions that existed during Uranus' formation. Exploration of different interior models, including variations in the ice/rock ratio within Uranus, can lead to variations in the bulk CO/H2O ratio of the planet. Therefore, our study also investigates the relationship between Uranus' ice/rock ratio and its formation conditions.

By combining observational data, interior models, and protoplanetary disk simulations, this work seeks to elucidate the link between Uranus' composition and the conditions of the PSN during the planet's formation.

How to cite: Benest Couzinou, T. and Mousis, O.: Uranus Revealed: What its D/H Ratio Tells Us About its Formation, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-509, https://doi.org/10.5194/epsc-dps2025-509, 2025.

The densities and moments of inertia of Jovian and Saturnian icy moons, dwarf planets, and other trans-Neptunian objects (TNOs) suggest the presence of a significant low-density carbonaceous component in their rocky cores. In a homogeneous accretion scenario, where these components are mixed in solar proportions, ices differentiate from the carbon-rich refractory core, while silicate hydration may occur. Thermal models that account for the presence of carbonaceous matter indicate that originally hydrated silicates are now largely dehydrated in the refractory cores of large moons and dwarf planets, due to interactions with volatiles released by the metamorphism of carbonaceous matter.

Progressive gas release from the slowly warming, carbonaceous matter-rich cores may sustain, up to the present day, the replenishment of ice-ocean layers with organics and volatiles, as well as outgassing to the surface. This process accounts for the observation of nitrogen, light hydrocarbons, and complex organic molecules at the surface, in the atmospheres, or in the plumes emanating from moons and dwarf planets. The formation of large carbon-rich icy bodies in the outer solar system suggests that a carbon-rich environment prevailed during ice giant planet formation—a scenario that could also lead to the formation of carbon-rich planets at the outskirts of extrasolar systems.

In the Neptunian system, Triton—presumed to be a captured TNO—shares density and surface composition characteristics with other large TNOs, including Pluto, Eris, Makemake, Gonggong, Quaoar, and others. Notably, the carbon-bearing molecules at the icy surfaces of TNOs shift from CO₂-dominated compositions in smaller objects (Pinilla-Alonso et al., 2024) to CH₄-rich compositions in the largest TNOs, including Triton (Brown, 2012; Emery et al., 2024; Grundy et al., 2024).

In the Uranian system, the latest estimates of regular satellite masses (Jacobson, 2014) reveal a power-law relationship between size and density, reflecting varying rock/ice ratios caused by fractionation processes (Reynard and Sotin, 2025). This relationship is explained by mild enrichment of rock relative to ice in the solids that aggregated to form the moons, following Rayleigh's law of distillation (Rayleigh, 1896). In the outer solar nebula, Rayleigh fractionation may account for the separation of a rock-dominated reservoir and an ice-carbon-dominated reservoir, now represented by CI carbonaceous chondrites/type-C asteroids and comets, respectively. Potential consequences for the composition of Uranus’s moons and targets for future exploration are discussed.

Acknowledgement. This work was supported by Institut National des Sciences de l’Univers through Programme National de Planétologie, by the Agence Nationale de la Recherche (ANR, project OSSO BUCO, ANR-23-CE49-0003) and by the European Union (ERC, PROMISES, project #101054470). Views and opinions expressed are, however, those of the authors only and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible for them.

How to cite: Reynard, B., Confortini, G., Delarue, C., and Sotin, C.: Icy Moons as Probes of Carbon-Rich Conditions During Giant Planet Formation, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1346, https://doi.org/10.5194/epsc-dps2025-1346, 2025.

Late gas released in the young Kuiper Belt could have significantly contributed to the carbon enrichment of the atmospheres of Neptune and Uranus.

Even before the observation of planets in extrasolar systems, there were numerous observations of analogues to our Kuiper Belt, first detected by their infrared excess (Aumann et al., 1984; Eiroa et al., 2010). ALMA has revolutionized our understanding of such debris discs, and one of the most outstanding discoveries is the observation of CO gas in approximately 30 systems thus far (e.g., Moór et al., 2017; MacGregor et al., 2017; Matrà et al. 2017). Models suggest that this gas is not primordial but is produced within the belt through collisions or sublimation of CO ices (Kral et al., 2017, 2019).

In our Solar System, it is estimated that only a small fraction of the gas could currently be produced within the Kuiper Belt, all of which would be expelled by the stellar wind (Kral et al., 2021). However, the dynamical models of the young solar system suggest the existence, for at least 10 Myr, of a massive planetesimal belt, also referred to as the primordial Kuiper Belt, with masses ranging between 5 and 50 M_⊕ (Liu et al., 2022; Griveaud et al., 2024). This massive belt would be equivalent to the extrasolar belts observed around young main-sequence stars.

Hence, our young Solar System could have hosted a secondary gas-rich disc that could have significantly altered the planets in the system. In particular, large amounts of CO gas could have been accreted onto Uranus and Neptune given that Kral et al. (2020) estimate that the accretion efficiency of gas onto planets in the debris disc is highly efficient.

Observations of Neptune and Uranus suggest that their atmospheres are enriched in carbon, with a C/H ratio approximately 80 times the protosolar value (see Guillot et al. 2023 and references therein). This could be explained by the formation process of the planets, but a significant fraction may also originate from late gas accretion. The amount of accreted carbon depends on the properties of the belt, such as its mass and lifetime, and thus on the dynamical history of the system. We therefore simulate the evolution of a gas disc for different belt origin and evolution corresponding to the Nice model with an initial compact configuration for planets (see Figure 1, Tsiganis et al., 2005; Griveaud et al., 2024) or the "rebound" instability with a more extended configuration for planets (see Figure 1, Liu et al., 2022). We also follow the evolution of the young "modern" Kuiper Belt after the dissipation of the primordial Kuiper Belt, since the Kral et al. (2021) model implies a higher mass production rate at the beginning of the belt's existence and therefore a putative gas disc for a significant timescale.

Figure 1: Schematic presenting a description of our set of simulations. The heavy belt cases are at the top (compact configuration) and middle (extended configuration), and the light cases are at the bottom. J, S, U, and N represent Jupiter, Saturn, Uranus, and Neptune, respectively. The different simulations vary in terms of the locations of planets, accretion efficiency, gas viscosity, and the belt’s mass. For the heavy belt configurations, the time when depletion starts is also a parameter. Each parameter has its own symbol and colour, as shown in the bottom part of the cartoon.

Our results indicate that the amount of carbon accreted onto Uranus and Neptune considering the modern Kuiper Belt is negligible. However, for all other dynamical models, the additional C/H ratio from the primordial Kuiper Belt is super-solar. Using a value of 50 M_⊕ belt based on the latest version of the Nice model (for a low-viscosity protoplanetary disc, Griveaud et al., 2024) leads to the highests C/H ratios, close to the observed C/H ratios for Uranus and Neptune.

To distinguish between the accretion of late gas and the planetary formation process (pebble/planetesimal accretion) from the observations, we use the S/H ratios. During planet formation, both carbon and sulphur are accreted (Mousis et al., 2024). However, since late gas is only produced by CO/CO_₂ ices, only carbon is accreted during this stage. The S/H ratio is thus a tracer of early accretion of pebbles or planetesimals. By comparing the observed C/H and S/H ratios, we obtain a rough estimate of the amount of late gas accreted into the atmospheres of Uranus and Neptune. This estimate is of the same order of magnitude as our simulation with the 50 M_⊕ belt (see Figure 2).

Figure 2: [C/H] for the fiducial simulation as a function of time. The filled areas correspond to predictions for Uranus (purple) and Neptune (blue). The uncertainties are shown via the extension of the filled areas and are due to uncertainties in the respective atmospheric masses. The solid and dashed black lines represent the maximum estimation of the gas disc contribution obtained from observations for Uranus and Neptune.

Since their atmospheres are much larger than those of Uranus and Neptune, we demonstrate that Jupiter and Saturn have experienced much less carbon enrichment due to late gas accretion. However, for our fiducial simulation, we still get significant enrichment in carbon due to late gas accretion.

We conclude our study by considering how this mechanism may be a universal process in extrasolar systems and how it could be detected in exoplanet atmospheres.

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Griveaud et al. 2024 A&A 688, A202

Guillot et al. 2023 arXiv:2205.04100

Kral et al. 2017 MNRAS 469,521

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Tsiganis et al. 2005 Nat, 435,459

How to cite: Huet, P., Kral, Q., and Guillot, T.: Late gas released in the young Kuiper Belt could have significantly contributed to the carbon enrichment of the atmospheres of Neptune and Uranus., EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-733, https://doi.org/10.5194/epsc-dps2025-733, 2025.

Uranus and Neptune are the most distant and the least explored planets within our solar system. To this day, the formation history of these ice giants remains uncertain. Better understanding of their deep atmospheric composition helps constrain where and how both planets formed. Remote sensing techniques can only probe the atmosphere down to the ~1 bar level. Similarly, an entry probe as part of the Uranus Flagship mission may only measure the atmospheric composition down to a few bars. Several disequilibrium species in the deeper troposphere are quenched to the pressure levels these measurements are made. Atmospheric models are thus needed to interpret how the measured abundances reflect the deeper atmospheric composition

Using the thermochemical & diffusion model of Cavalie et al. (2017), we aim to take advantage of such disequilibrium species to further constrain Uranus and Neptune deep atmospheric composition. We have updated the model to consider the meridional variability of several parameters in the troposphere. We investigate how the main physical and chemical parameters affect the retrieved deep elemental abundances. We present preliminary ranges of deep oxygen abundances in both planets. We discuss how such values can be used in a protoplanetary disk formation model (Mousis et al. 2024), and what it may teach us about the formation of these planets.

How to cite: Briand, T., Hue, V., Mousis, O., and Cavalié, T.: Insights on the formation of Uranus and Neptune through thermochemical modelling, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-798, https://doi.org/10.5194/epsc-dps2025-798, 2025.

The radiative heat balance of Uranus has long been a mystery amongst the solar system giant planets. Jupiter, Saturn and Neptune all emit much more power thermally (P_out) than they absorb from the Sun (P_in) with P_out/P_in having values of 1.7 to 2.6. This shows that all three planets retain a considerable amount of heat left over from formation, which they are still slowly radiating away into space. In stark contrast, Uranus appears to be unexpectedly cold. Measurements made by Voyager-2 determined a radiative heat balance ratio of only P_out/P_in = 1.06 ± 0.08 (Pearl et al. 1990), which is consistent (to within error) with Uranus being in thermal equilibrium with the Sun and thus, perhaps, having no heat of formation left over at all. Meanwhile, Voyager-2 determined a radiative heat balance ratio for Neptune of P_out/P_in = 2.61 ± 0.28 (Pearl and Conrath, 1991), which is the largest ratio determined for any of the giant planets.

How can the radiative heat balance ratios of Uranus and Neptune, the solar system’s ‘Ice Giants’ be so different? And is Uranus really in thermal equilibrium with the Sun, with no internal heat of formation left over? To answer this last question, we have performed a modelling study (Irwin et al., 2025) using our NEMESIS radiative transfer tool (Irwin et al., 2008) and a newly developed ‘holistic’ atmospheric model of the aerosol structure in Uranus’s atmosphere, based upon observations made by HST/STIS, Gemini/NIFS and IRTF/SpeX from 2000 – 2009 (Irwin et al., 2022). Taking our fitted aerosol structure and extrapolating our calculations to all wavelengths, we have made a new estimate of the bolometric geometric albedo of Uranus during the period 2002 – 2009 of p* = 0.249. The bolometric geometric albedo is the fraction of sunlight reflected by the planet back towards an observer in line with the Sun, but to determine heat balance we need to calculate the bolometric Bond Albedo, which is the fraction of sunlight incident on the planet that is scattered into all directions. With our holistic aerosol model and NEMESIS, we can calculate the appearance of Uranus to an observer at any phase angle from the Sun, and integrating these modelled curves over all phase angles we can calculate the phase integral, q, which relates the geometric albedo, p, to the Bond albedo, A, through the relation A = pq.

From this modelling we determine a bolometric (i.e., integrated over all wavelengths) phase integral of 𝑞^∗ = 1.36, and thus a bolometric Bond albedo of 𝐴^∗ = 0.338 for the period 2002 – 2009. However, to determine the overall radiative heat balance of Uranus, we first need to account for the seasonal variation in 𝐴^∗, which changes significantly during Uranus’s year due to the formation of a polar ‘hood’ of haze over the summer pole, which becomes thicker and more observable near the solstices. In addition, in terms of energy balance, we also need to account for the fact that the incident sunlight at Uranus varies significantly during its eccentric (e = 0.046) orbit about the Sun by ±10%. Also, since Uranus is significantly oblate and has high polar inclination, there is a small, but significant difference in its projected area towards the Sun between solstice and equinox, which affects the total power of sunlight received by the planet.

To estimate the orbital-average bolometric Bond albedo and radiative heat balance we used a simple seasonal model, developed by Irwin et al. (2024) to be consistent with the disc-integrated blue and green magnitude data from the Lowell Observatory from 1950 – 2016 (Lockwood, 2019). Taking all hood thickness/visibility, distance and projected area effects into account, we model how Uranus’s reflectivity and heat budget vary during its orbit and determine a new orbital-mean average value for the bolometric Bond albedo of 𝐴^∗ = 0.349 ± 0.016 and estimate the orbital-average mean absorbed solar flux to be  𝑃_in = 0.604 ± 0.027 W m⁻². Assuming the outgoing thermal flux to be 𝑃_out = 0.693 ± 0.013 W m⁻², previously determined from Voyager 2 observations, we arrive at a new estimate of Uranus’s average heat flux budget of P_out/P_in = 1.15 ± 0.06. We find, however, that there is considerable variation of the radiative heat balance with time due mainly to Uranus’s orbital eccentricity, which leads P_out/P_in to vary from 1.03 near perihelion, to 1.24 near aphelion. We conclude that although P_out/P_in is still considerably smaller than for the other giant planets, Uranus is not in thermal equilibrium with the Sun.

References. 

Irwin et al. (2008) DOI:10.1016/j.jqsrt.2007.11.006; Irwin et al. (2022) DOI: 10.1029/2022JE007189; Irwin et al. (2024) DOI: 10.1093/mnras/stad3761;Irwin et al. (2025) DOI: 10.48550/arXiv.2502.18971; Lockwood (2019) DOI: 10.1016/j.icarus.2019.01.024; Pearl et al. (1990) DOI:  10.1016/0019-1035(90)90155-3; Pearl and Conrath (1991) DOI: 10.1029/91JA01087; Wenkert (2023) DOI: 10.17189/T2R8-RK88

How to cite: Irwin, P., Wenkert, D., Simon, A., Dahl, E., and Hammel, H.: The bolometric Bond albedo and energy balance of Uranus, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-61, https://doi.org/10.5194/epsc-dps2025-61, 2025.

Uranus and Neptune are the most distant planets in the Solar System, and consequently, our observational data on them is limited. In recent years, several mission concepts have been proposed to explore these ice giants. For such missions to succeed, it is essential to develop tools that support both mission planning and the interpretation of future observations. One such tool is a general circulation model (GCM). However, no published GCM to date has successfully reproduced the observed wind patterns on Uranus or Neptune. To address this gap, we developed a new idealized GCM specifically designed to simulate atmospheric circulation on the ice giants. Using this model, we systematically investigate how the zonal wind structure and meridional circulation respond to variations in model configuration and physical parameters, with a particular focus on the depth of the simulated bottom pressure. Our results demonstrate that the circulation is sensitive to the depth of the model domain. Notably, when the model extends deep enough, the meridional structure of the radiative-equilibrium temperature becomes less critical. Additionally, we find that resolution dependence varies with model depth, indicating a significant role for eddy dynamics. This idealized GCM provides a foundational framework for developing more comprehensive models of Uranus and Neptune, ultimately supporting future observational and mission efforts.

How to cite: Kaspi, Y. and Guendelman, I.: An idealized general circulation model for the atmospheric circulation on Ice Giants , EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1033, https://doi.org/10.5194/epsc-dps2025-1033, 2025.

Atmospheric circulation on Uranus and Neptune is particularly intense and qualitatively similar between the two planets. It is characterized by a retrograde jet (westward wind) at the equator, reaching 400 m/s on Neptune, and a prograde jet (eastward wind) located at mid-latitude on each hemisphere, whose intensity reaches 250 m/s on both planets [1,2]. During the Voyager 2 flyby, the IRIS experiment revealed a complex and similar thermal structure between the two planets at tropopause level (between 70 and 800 mbar) [3,4]. A temperature minimum is observed at the equator and poles, while a temperature maximum is located at mid-latitude in each hemisphere. The origin of these jets and the associated thermal structure are still an open question within the community.

Because of its strong absorption bands, methane plays a major role in radiative exchange and therefore in the thermal structure of the atmospheres of icy giants [5]. Recently, a tropospheric latitudinal gradient in its abundance has been revealed on both planets [6,7]. Its molar fraction varies from 1 to 3% on Uranus and 2 to 6% on Neptune between the poles and equator respectively. Due to its high molecular weight compared to molecular hydrogen and helium, this results in a large difference in mean molecular weight between the poles and the equator. This variation reaches 10% on Uranus and 18% on Neptune.
Methane also varies vertically. Near 1.3 bar, methane condenses and its mole fraction decreases very rapidly in the tropopause. However, on Neptune, the stratospheric mole fraction of methane is higher than that present at the cold trap in the tropopause, suggesting that methane is being re-injected into the stratosphere by an unknown dynamic process.

These significant latitudinal and vertical variations in methane concentration raise the question of their influence on the temperature field and atmospheric circulation. Moreover the moist thermal wind equation predicts significant shear associated with the presence of this latitudinal gradient.

To reproduce the thermal and dynamic structure of ice giants, numerical simulations with a resolution of 1° were carried out between 10 bar and 0.01 mbar, using the DYNAMICO dynamic core [8] and coupling it to the seasonal radiative-convective model tailored for ice giants [5]. To take into account a variable molecular weight, the dynamic core solving the atmospheric primitive equations was modified. Parameterization of dry and moist convection required a complete rewriting of the convection criteria and enthalpy conservation to take into account of these molecular weight effects.

Simulations based on this model without latitudinal methane gradient and wet methane convection show a qualitatively similar meridional structure of the atmospheric circulation. However, the intensity of the jets is much less intense than the values obtained by cloud tracking since the Voyager 2 era. Prograde jets only reach 30 m/s, compared with 250 m/s in the observations. The simulated thermal structure also differs from that observed. The minima and maxima observed in the tropopause do not match those observed. At the tropopause, a maximum at each pole and a minimum at the equator are simulated.

By adding the latitudinal methane gradient, atmospheric circulation and thermal structure are profoundly modified. A clear acceleration of the jets has been observed, approaching quantitatively the velocity values of the jets observed on these planets. However, the simulated prograde jets are much thinner and closer to equatorial latitudes than those observed. As regards thermal structure, minima and maxima qualitatively similar to the extrema observed during Voyager 2's flyby were reproduced.

References

[1] Allison et al. (1991), Uranus atmospheric dynamics and circulation. 253–295.
[2] Limaye et al. (1991), Journal of Geophysics Research, 96:18941–18960.
[3] Orton et al. (2015), Icarus, 260:94–102.
[4] Fletcher et al (2014), Icarus, 231:146–167.
[5] Milcareck et al (2024), Astronomy & Astrophysics, 686:A303.
[6] Karkoschka and Tomasko (2009), Icarus, 202(1):287– 309.
[7] Karkoschka and Tomasko (2011), Icarus, 211(1):780– 797.
[8] Dubos et al. (2015), Geoscientific Model Development, 8(10):3131– 3150.

How to cite: Milcareck, G., Leconte, J., Guerlet, S., Le Saux, A., Clément, N., Dubos, T., Montmessin, F., Spiga, A., Bardet, D., Millour, E., Lellouch, E., Moreno, R., Cavalié, T., and Carrion-Gonzalez, O.: Radiative-convective and dynamic effect of the latitudinal and vertical methane gradient on Uranus and Neptune, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1505, https://doi.org/10.5194/epsc-dps2025-1505, 2025.

In 2018, analysis of Gemini-NIFS near-infrared observations revealed the probable presence of H₂S above the main cloud deck on Neptune [1]. The spectral signature of the gas was found to be much stronger at Neptune's south pole compared to regions nearer the equator.

Conversely, analysis of Neptune's microwave emission with ALMA suggested strongly enhanced H₂S abundances at midlatitudes [2], with much less at the south pole. Determining the true variation of H₂S with latitude is crucial for understanding the tropospheric circulation of Neptune.

We present our analysis of observations of Neptune from VLT-SINFONI in 2018. Using a limb-darkening approximation, we are able to fit the reflected solar radiance from multiple zenith angles, which allows us to discriminate between gas and aerosol opacity. Despite the lower spectral resolution of this instrument compared to Gemini-NIFS, we are able to detect the H₂S spectral signature. With our radiative transfer retrieval code, archNEMESIS [3], we use nested sampling to fit a parameterised cloud model (similar to that of [4]) to these observations over a range of latitudes. We prescribe a latitudinally varying deep methane abundance derived from recent VLT-MUSE observations [5], which enables us to constrain the depth of the cloud top.

Our retrieved results are in agreement with the results derived from ALMA [2] - we find a significant enhancement of deep H₂S at Neptune's southern midlatitudes, decreasing towards the equator and the pole. Our results show a much deeper cloud top towards the pole, resulting in the increased cloud top column abundance of H₂S observed here in the previous near-infrared analysis [1].

Figure 1: A comparison of fits to a spectrum extracted from the 50°S to 60°S latitude band, with a model including H₂S (blue) and a model without H₂S (red). Note the significant discrepancy around 1.58 microns. The models are fitted to spectra at two zenith angles simultaneously.

[1] Irwin, P. G., Toledo, D., Garland, R., Teanby, N. A., Fletcher, L. N., Orton, G. S., & Bézard, B. (2019). Probable detection of hydrogen sulphide (H₂S) in Neptune’s atmosphere. Icarus, 321, 550-563.

[2] Tollefson, J., de Pater, I., Luszcz-Cook, S., & DeBoer, D. (2019). Neptune's latitudinal variations as viewed with ALMA. The Astronomical Journal, 157(6), 251.

[3] Alday, J., Penn, J., Irwin, P. G., Mason, J. P., & Yang, J. (2025). archNEMESIS: an open-source Python package for analysis of planetary atmospheric spectra. arXiv preprint arXiv:2501.16452.

[4] Irwin, P. G., Teanby, N. A., Fletcher, L. N., Toledo, D., Orton, G. S., Wong, M. H., ... & Dobinson, J. (2022). Hazy blue worlds: a holistic aerosol model for Uranus and Neptune, including dark spots. Journal of Geophysical Research: Planets, 127(6), e2022JE007189.

[5] Irwin, P. G., Dobinson, J., James, A., Wong, M. H., Fletcher, L. N., Roman, M. T., ... & de Pater, I. (2023). Latitudinal variations in methane abundance, aerosol opacity and aerosol scattering efficiency in Neptune's atmosphere determined from VLT/MUSE. Journal of Geophysical Research: Planets, 128(11), e2023JE007980.

How to cite: Penn, J., Irwin, P., and Dobinson, J.: Neptune's Latitudinal H2S Distribution: Reconciling Near-Infrared and Microwave Observations, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-502, https://doi.org/10.5194/epsc-dps2025-502, 2025.

The atmospheric composition of Uranus and Neptune provides crucial insights into their origins, evolution, and has broad implications for exoplanetary science. Clouds and hazes play a pivotal role in these atmospheres, influencing radiative transfer, dynamics, and chemistry while also serving as tracers for wind patterns. Methane and Hydrogen Sulfide clouds potentially dominate the troposphere, whereas the stratosphere contains hazes likely composed of complex hydrocarbons. These aerosol layers develop and evolve through processes such as nucleation, condensation, coagulation, and evaporation. Despite their significance, the microphysics underlying these interactions and the interplay between distinct aerosol layers remain understudied. Our research utilizes the 1D Community Aerosol and Radiation Model for Atmospheres (CARMA) to simulate the aerosol distribution and processes within Uranus’s and Neptune’s atmospheres. By varying a number of model parameters (e.g. size and vertical distribution of condensation nuclei, contact angles, atmospheric mixing, etc.), this work provides a detailed framework to study the sensitivity of the clouds to material properties and atmospheric conditions. This study not only enhances our understanding of ice giant atmospheres and informs future mission design, but also provides a foundation for interpreting observations of exoplanetary atmospheres. 

How to cite: Walker, A., Smith, S., and Gao, P.: Modeling Aerosol Microphysics in Ice Giant Atmospheres , EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1130, https://doi.org/10.5194/epsc-dps2025-1130, 2025.

Radiative transfer analyses of spectra obtained from Uranus and Neptune have revealed the presence of
a cloud layer at pressures greater than ~2 bar (1,2). The detection of hydrogen sulfide (H₂S) gas above
this cloud layer on both planets (3,4) suggests that H₂S ice is the most likely main constituent. This
interpretation is further supported by the expectation that methane (CH₄) clouds condense at higher
altitudes (5). However, due to their depth and observational limitations, our understanding of the
properties of H₂S clouds on these planets remains very limited.
To investigate the properties of H₂S clouds in the atmospheres of Uranus and Neptune, we employed a
one-dimensional cloud microphysics model originally developed for Titan and Mars (6,7). The model
includes nucleation, condensation, evaporation, coagulation, and precipitation processes, and has
previously been used to simulate haze and CH₄ cloud microphysics in the Ice Giants (5,8,9).
Figure 1 shows, as an example, simulated H₂S ice profiles for Uranus using this microphysical model.
The vertical transport of H₂S gas is simulated using an eddy diffusion coefficient (Keddʏ), which controls
the supply of vapor for cloud nucleation and particle growth. We employed the Keddʏ profiles derived
in [10] for H₂S abundances of 10× and 30× solar. Since several cloud microphysical parameters for H₂S
remain uncertain (e.g., the contact parameter), different values are tested in the simulations. In the
example shown, the model indicates cloud bases near 5.3 bar for 10× solar abundance and 6.4 bar for
30× solar. Near the cloud base, particle mean radii range from 40 to 55 μm, depending on the assumed
contact parameter and abundance. At higher altitudes, particle sizes decrease; for instance, at ~3 bar,
mean radii are around 20 μm. In general, H₂S cloud simulations produce higher opacities than CH₄
clouds.
In this work, we will present a series of cloud microphysical simulations of H₂S clouds in the Ice Giants.
Various cloud properties, such as particle size distributions and precipitation rates, will be constrained.
We will also discuss the implications of our results for the atmospheric circulation of these planets and
for the future exploration of Uranus.

Figure 1. Vertical distributions of H2S ice (g/m³) for Uranus, simulated for different values of the cloud
contact parameter and deep H2S abundances. These simulations employ the Keddʏ profiles calculated in
[10] for the corresponding H2S abundances.

References: [1] P. G. Irwin, et al., JGR: Planets, 127, e2022JE007189. [2] L. Sromovsky, et al., Icarus,
Volume 317, (2019) [3] P. G. Irwin, et al., Nature Astronomy 2, 420 (2018). [4] P. G. Irwin, et al.,
Icarus 321, 550 (2019). [5] D. Toledo, et al., A&A, 694, A81 (2025). [6] P. Rannou, et al., Science 311,
201 (2006). [7] F. Montmessin, et al., JGR: Planets 107, 4 (2002). [8] D. Toledo, et al., Icarus, 333, 1-
11, (2019). [9] D. Toledo, et al., Icarus, Volume 350, (2020). [10] H. Ge, et al., The Planetary Science
Journal,5, 101(2024).

How to cite: Toledo, D., Rannou, P., Irwin, P., de Batz de Trenquelléon, B., Roman, M., Clément, N., Milcareck, G., Apestigue, V., Arruego, I., and Yela, M.: Microphysical Modeling of Hydrogen Sulfide Clouds in the Atmospheres of the Ice Giants, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1456, https://doi.org/10.5194/epsc-dps2025-1456, 2025.

All the outer planets have much hotter thermospheres than expected from solar heating, a phenomenon known as the energy crisis, likely driven by strong energy deposition from their magnetospheres. Since the Voyager era, the upper atmospheres of Uranus and Neptune may both have cooled substantially, which can be driven by evolution of the magnetosphere-atmosphere interaction.

The upper atmospheres of the outer planets, comprised of H₂, atomic H and He, can be probed remotely through ultraviolet emissions, which constrain the composition, temperature, and extent of their thermospheres. The atomic H emission at Lyman α is the brightest UV emission line from the outer planets and has been observed at Uranus, Neptune, Saturn, and Jupiter from the earth orbit and by missions to the respective systems. In particular, the Uranian Lyman α emissions have been observed over 35 years using different platforms, including the Hubble Space Telescope, and can continue to be used to track the evolution of the thermosphere remotely, in preparation for the Uranus Orbiter and Probe mission. The interpretation of Lyman α emissions, however, is challenging due to uncertainties in spectrograph calibration and the potential presence of supplementary emission sources, such as hot hydrogen or electron-excited emissions.

We examine the emissions from Saturn, Uranus and Neptune with observations from Cassini/UVIS, HST, and Voyager 2/UVS, comparing observed brightnesses to modelled emissions from the disk of each planet using a radiative transfer model. We model the scattering of Lyman α photons, from both the Sun and the interplanetary hydrogen background, by atomic H and H₂ in the atmosphere using a model based on the doubling and adding of thin atmospheric layers. The model incorporates partial frequency redistribution which is critical for accurately modelling such optically thick atmospheres (τ>10⁴ at the Lyman α linecenter).

We find that the observed emissions from Saturn and Neptune can be well reproduced by scattering from thermal H. At Uranus, a significant portion of the Lyman α emission arises from Rayleigh scattering by H₂, due to the unusually low mixing rate that places the methane homopause at deeper pressures than at Saturn or Neptune. This leads to an optically significant column of H₂ unique to Uranus. Across all three planets, scattered solar Lyman α flux is the dominant source of emission outside of auroral regions, with an additional 10-15% contribution from scattering of the interplanetary hydrogen background.

Scattering by thermal H and H₂ can replicate the observed disk emissions, without requiring additional emission sources like hot hydrogen or electron-excited emissions. However, the atmospheric model fits based on other observation sets (i.e. UV stellar and solar occultations and IR emissions) are not unique, and we therefore explore how the modeled brightnesses depend on the range of atmospheric parameters allowed by available observations.

How to cite: Stephenson, P., Koskinen, T., Bhattacharya, D., Moses, J., and Clarke, J.: Lyman α Emissions from the Outer Planets: A Comparative Study from Saturn to Neptune , EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1874, https://doi.org/10.5194/epsc-dps2025-1874, 2025.

Measurements of winds in Neptune's stratosphere obtained by ALMA Doppler spectroscopy in 2016 by [1] have shown that stratospheric zonal winds at a pressure level of 1 mbar (-180 m/s)  appear to be less intense, by a factor of two, than tropospheric winds based on cloud tracking at 1 bar from Voyager observations (-400 m/s). These data also indicate a reversal of the circulation, from retrograde to prograde, at high southern latitudes. However, the spatial resolution of these data was insufficient for a detailed characterization of the high-latitude circulation.

We present here higher spatial resolution ALMA maps of CO and HCN emission lines towards Neptune's limb to measure their Doppler shifts and derive a map of zonal winds in its stratosphere. Our observations - carried out in March-April 2025 - used the ALMA interferometer to map the CO(3-2) and HCN(5-4) rotational lines in Neptune's atmosphere at 345.796 and 354.505 GHz, respectively. These measurements were obtained using around 44 antennas of the 12m array, with an angular resolution of ~0.19'', allowing the mapping of around 12x12 independent pixels on the planetary disk (2.21").

We will present the analysis of these observations, which will include maps of the planet's zonal wind after subtraction of the planet's solid rotation, and compare these results with earlier measurements of the Doppler wind in Neptune's atmosphere [1]. Particular attention will be paid to the South Pole region, whose resolution will be improved by a factor of 2 compared with the observations of [1]. This growing ensemble of ALMA datasets opens the door to the long-term monitoring of the seasonal stability and potential variations of Neptune's stratospheric wind patterns.

Acknowledgements

ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ.

References

[1] Carrion-Gonzalez et al. 2023 A&A 674, L3

How to cite: Moreno, R., Carrión-González, O., Lellouch, E., Cavalié, T., Guerlet, S., Milcareck, G., Spiga, A., Roman, M., and Moullet, A.: Characterization of South polar-winds in Neptune’s atmosphere from ALMA observations , EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1098, https://doi.org/10.5194/epsc-dps2025-1098, 2025.

The Infrared Space Observatory and Spitzer have detected H₂O and CO₂ in the stratospheres of the giant planets and Titan (Feuchtgruber et al. 1997, Coustenis et al. 1998, Burgdorf et al. 2006). Their upper tropospheres are cold and H₂O and CO₂ condense into ice clouds (except CO₂ in Jupiter and Saturn). These detections thus imply the presence of an external source of oxygen. Such source can be: (i) local (icy rings and/or satellites), (ii) interplanetary dust particles (IDPs), (iii) large comet impacts, or (iv) a combination of them. At Uranus, H₂O probably comes from IDPs (Moses & Poppe 2017, Teanby et al. 2023). And while the dominant source of CO in Jupiter, Saturn, and Neptune seems to be comet impacts (Moreno et al. 2003, Cavalié et al. 2010, Lellouch et al. 2005), its nature remains unknown in Uranus despite the Herschel disk-averaged observations of Cavalié et al. (2014). Because the different sources have specific signatures in terms of spatio-temporal distributions, mapping observations of the stratosphere can be used to disentangle them. For instance, comets are sporadic and spatially localized (Lellouch et al. 1995), IDPs are steady, latitudinally and longitudinally uniform (Moses & Poppe 2017), and ring/satellite sources are localized (Cavalié et al. 2019). These differences have been used to constrain the origin of external sources on Jupiter (Lellouch et al. 2002, Cavalié et al. 2013), Saturn (Cavalié et al. 2010, 2019), and Neptune (Lellouch et al. 2005, Moreno et al. 2017). However, even though these sources cause different vertical and horizontal distributions in Uranus, all can reproduce the Herschel spectrum at 922 GHz (see Fig. 1). Consequently, Cavalié et al. (2014) could not discriminate the sources from this single disk-averaged line.

In October 2022, the CO(3-2) emission was mapped in the stratosphere of Uranus with ALMA (Fig. 2). We have retrieved and analyzed these data to constrain the horizontal and vertical distributions of CO. In this paper, we present our findings and their implications on the source of CO in the atmosphere of Uranus.

Fig. 1: (Left) Herschel observation of the CO(8-7) line in the stratosphere of Uranus at 922 GHz. The spectrum is expressed in terms of line-to-continuum ration (l/c). (Right) Vertical profiles obtained from 1D vertical diffusion modeling for a comet source (blue line) and from a steady flux of IDPs (red line). Figure adapted from Cavalié et al. (2014).

Fig. 2: Line area map of the CO(3-2) emission, as observed in the stratosphere of Uranus with ALMA in October 2022.

References:
Burgdorf et al. 2006. Icarus 184, 634
Cavalié et al. 2010. A&A 510, A88
Cavalié et al. 2013. A&A 553, A21
Cavalié et al. 2014. A&A 562, A33
Cavalié et al. 2019. A&A 630, A87
Coustenis et al. 1998. A&A 336, L85
Feuchtgruber et al. 1997. Nature 387, 159
Lellouch et al. 1995. Nature 373, 592
Lellouch et al. 2002. Icarus 159, 112
Lellouch et al. 2005. A&A 430, L37
Moreno et al. 2003. P&SS 51, 591
Moreno et al. 2017. A&A 608, L5
Moses and Poppe 2017. Icarus 297, 33
Teanby et al. 2023. Planet. Sci. J. 3, 96

How to cite: Cavalié, T., Moreno, R., Lefour, C., Fouchet, T., and Lellouch, E.: On the origin of CO in the stratosphere of Uranus, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-70, https://doi.org/10.5194/epsc-dps2025-70, 2025.

Over the past 20+ years, ground-based observations have determined that the surfaces of the large Uranian moons Ariel, Umbriel, Titania, and Oberon are enriched in CO₂ ice via the detection of a CO₂ “triplet band” between 1.9 and 2.1 µm [1,2]. The spectral properties of the detected CO₂ is strikingly similar to crystalline CO₂ ice measured in the laboratory. The origin of CO₂ on these moons and across the Uranus system, however, remains uncertain. CO₂ ice is unstable at the estimated peak surface temperatures of these moons (80-90 K), and it must be replenished [3]. It has been suggested that CO₂ could be formed radiolytically via irradiation of H₂O ice mixed with carbon-bearing compounds, possibly explaining the stronger CO₂ features detected on the trailing hemispheres of these satellites [1,2]. Alternatively, the large moons of Uranus are candidate ocean worlds that may host subsurface saline oceans, possibly enriched in carbonate (CO₃^2-) species and CO₂. These carbon-bearing compounds could be outgassed or exposed through geologic processes [4]. Understanding the origin of CO₂ and other carbon oxides in the Uranus system is one of the key science questions driving the rationale for future measurements made by a near-infrared (NIR) mapping spectrometer onboard a Uranus orbiter [5].

More recent observations made with the NIRSpec spectrograph (G395M, 2.9 – 5.1 µm, R ~ 1000) on the James Webb Space Telescope (program 1786, [4]) were able to identify the strong CO₂ asymmetric stretch fundamental mode (ν₃), roughly spanning 4.2 to 4.4 µm, a wavelength range inaccessible from the ground due to telluric CO₂ absorption. The CO₂ ν₃  mode is a factor of ~1000 stronger than the CO₂ triplet band measured in prior ground-based observations, and therefore, we predicted that the Uranian moons would exhibit strong and possibly saturated CO₂ ice absorption features across the 4.2 to 4.4 µm wavelength range, similar to NIRSpec data of Neptune’s moon Triton [6]. However, the ¹²CO₂ ice bands detected by NIRSpec on the trailing hemispheres of these moons are surprisingly weak, and instead, are convolved with scattering peaks that obscure these absorption features (Figure 1). NIRSpec also revealed ¹²CO₂ on the leading hemispheres of these moons and hyper-volatile CO ice on their trailing sides. Furthermore, ¹³CO₂ is present, primarily on the inner moons Ariel and Umbriel, and CO₃-bearing species, carbon chain oxides (C_XO₂), and CN-bearing organics (nitriles) may also be present on Ariel and Umbriel (Figure 1). Thus, the surfaces of the large Uranian moons are enriched in carbon oxides, especially Ariel [4].

Figure 1: JWST/NIRSpec data (G395M) of the large Uranian moons, normalized to 1 at 4.14 µm and offset vertically for clarity.

At first glance, the detected species and their hemispherical distributions are broadly consistent with prior ground-based observations, supporting radiolytic production of CO₂ and other carbon oxides via charged particles trapped in Uranus’ magnetosphere. However, none of the data display notable hydrogen peroxide (H₂O₂) combination modes near 3.51 µm, features that laboratory experiments have shown to emerge in irradiated H₂O ice substrates (< 100 K), in particular when mixed with small amounts of CO₂ [7], representing conditions relevant to the surfaces of Uranus’ moons. Furthermore, icy satellites at Jupiter and Saturn tend to display darker and redder trailing hemispheres over UV/VIS wavelengths due to irradiation by corotating plasma, but observations made with Hubble’s Space Telescope Imaging Spectrograph (~200 – 550 nm) indicate no such hemispherical asymmetry in albedo for the large moons of Uranus [8]. Because Uranus’ magnetosphere is notably offset from the orbital plane of its satellites (~59°) and seemingly devoid of heavy ions (i.e., Cⁿ⁺, Oⁿ⁺), it is possible that moon-magnetosphere interactions may be somewhat limited at Uranus. Instead, perhaps CO₂ is primarily native to the large moons and exposed by geologic processes, such as cratering, tectonism, cryovolcanism, and outgassing.

Other observations by JWST (program 4645) have revealed that CO₂ is present across the Uranian system, including in its rings, ring moons, and irregular satellites [9,10]. The origin(s) of CO₂ on these smaller bodies and rings is presumably varied, but almost certainly includes native sources of CO₂, especially on the far-flung irregular satellites that orbit well beyond the influence of Uranus’ magnetosphere and exhibit strong 4.27 µm and 2.7 µm features, consistent with native CO₂. Similar to the large moons, CO₂ ice is unstable on these small bodies and rings and must be replenished and trapped in more refractory compounds.

To gain a more complete understanding of the origin and nature of these species will require a Uranus orbiter equipped with a NIR mapping spectrometer and a charged particle suite making measurements during close passes [11]. We will present NIRSpec results and analyses for carbon oxides on the large moons Ariel, Umbriel, Titania, and Oberon and provide an update on the developing picture of the origin of carbon oxides in the Uranus system in the JWST era.

[1] Grundy, W. et al., 2006. Icarus 184.

[2] Cartwright, R.J. et al., 2022. The Planetary Science Journal, 3, 8.

[3] Sori, M.M. et al., 2017. Icarus, 290, pp.1-13.

[4] Cartwright, R.J. et al., 2024. The Astrophysical Journal Letters, 970, L29.

[5] National Academies of Sciences, Engineering, and Medicine 2023. Origins, Worlds, and Life:

Planetary Science and Astrobiology in the Next Decade.

[6] Wong, I. et al., 2023. AGU Fall Meeting Abstracts. P44B-08.

[7] Mamo et al. PSJ, submitted.

[8] Cartwright, R.J. et al., 2022. AAS/DPS Meeting 54, abstract 106.01.

[9] Belyakov, M. et al., 2024. AAS/DPS Meeting 56, abstract 405.02.

[10] Belyakov, M. et al., April 2025. Ice Giant Systems Seminar Series.

[11] Cartwright, R.J. et al., 2021. The Planetary Science Journal, 2 (3), p.120.

How to cite: Cartwright, R. J., Grundy, W. M., Holler, B. J., Raut, U., Nordheim, T. A., Neveu, M., Glein, C. R., Hedman, M. M., Belyakov, M., DeColibus, R. A., Villaneuva, G. L., Protopapa, S., Tegler, S. C., Beddingfield, C. B., Quirico, E., Castillo-Rogez, J. C., and Roth, L.: JWST Observes the CO2-rich Surfaces of Uranus’ Large Moons , EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-182, https://doi.org/10.5194/epsc-dps2025-182, 2025.

How to cite: Davis, R., Belyakov, M., Brown, M., and Wong, I.: JWST Reveals Phyllosilicates on the Small Inner Moons of Neptune, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-181, https://doi.org/10.5194/epsc-dps2025-181, 2025.

Introduction: Ceres is the only inner Solar System dwarf planet (2.77 AU, 940-km-diameter). There are intriguing parallels between Ceres and two of the large Uranian satellites, located at 19.19 AU: Umbriel (1,169-km-diameter) and Oberon (1,523-km-diameter). All three bodies are mid-sized objects, differentiated into icy shells and rocky interiors, and lack atmospheres or substantial present day tidal heating. The limited coverage and low resolution data of Umbriel and Oberon (~4-5 km/pixel, <40% coverage from Voyager 2) hampers our ability to define and test hypotheses about the origin and evolution of these moons. Because of the similarities between Umbriel, Oberon and Ceres, and because Ceres currently has the highest resolution datasets of any icy body (e.g., 35 m/pixel, ~100% coverage from Dawn), we leverage Dawn data to gain insights into specific surface features on Umbriel and Oberon.

Similar surface features on Ceres, Umbriel and Oberon: The overall morphometry and albedo contrast of the bright deposit in Umbriel’s Wunda crater (131-km-diameter) (Figure a) is reminiscent of the bright salt deposits in Ceres’ Occator crater (92-km-diameter), called faculae (Figure b). Wunda’s bright deposit is interpreted as cold-trapped CO₂ ice (Sori+2017) or a cryovolcanic deposit (Plescia1987). Sodium-rich carbonates, organics and phyllosilicates may be present somewhere on Umbriel (Cartwright+2023). Occator’s bright faculae are salt deposits (De Sanctis+2016, Raponi+2019) interpreted to originate from brines that were emplaced from a subsurface impact-induced melt reservoir (Bowling+2019, Quick+2019, Scully+2020), which likely thermally merged with the remnants of an ancient subsurface ocean (Hesse&Castillo-Rogez2019).

We also find that the overall morphometry of an unnamed mountain on Oberon and Ceres’ Ahuna Mons are comparable in limb views (Figures c & d). Oberon’s 11 km high mountain may be the central peak of an impact crater (Smith+1986, Blanco-Rojas+in review), a type of tectonic feature, remnant equatorial ridge, or volcanic construct (Schenk&Moore2020). On Ceres, Ahuna Mons is ~4 km high and interpreted as an extrusive cryovolcanic dome (Ruesch+2016), formed by a slurry of brine and rock particles from the remnant subsurface ocean, sourced near the crust/mantle boundary (Ruesch+2019).

The aforementioned observations lead us to propose two new hypotheses:

• Hypothesis #1: Brines sourced from an impact-induced melt reservoir (which may have been refreshed from a subsurface ocean) formed the bright deposit in Wunda crater on Umbriel.
• Hypothesis #2: A slurry of brine and solids from the subsurface ocean formed the mountain on Oberon as a cryovolcanic dome.

We further explore our hypotheses using presently-available data via image analysis.

Wunda image analysis: We hypothesize that the dark central area in Wunda is a central peak, contributing to the annulus shape of Wunda’s bright deposit. Occator’s faculae are six times brighter than Ceres’ average surface (Schröder+2017). Thus, it is necessary to stretch the images of Occator to enhance detail in both the surroundings and in the faculae. We investigated whether similar stretch variations would enhance detail within, and around, Wunda’s bright deposit. We found that stretching the Umbriel data does not enhance particular areas within Wunda’s bright deposit as it did with the Ceres data. This is likely due to the Dawn Framing Camera (Schröder+2013) having a higher dynamic range and higher bit depth than the Voyager ISS NAC (Smith+1977). Moreover, Dawn’s orbital mission enabled fine-tuning of exposure times to image Occator’s faculae, which was not possible during Voyager 2’s singular flyby of Umbriel. Thus, the Voyager images cannot capture the full brightness variation of the bright Wunda deposit. Given the change in size of Occator’s faculae in images before and after stretching, we assume that the diameter of the Wunda bright material (~80 km) as measured by Smith+1986 is an overestimate.

Oberon image analysis: Ruesch+2016 found that the aspect ratio of  Ahuna Mons is 0.24, which is consistent with extrusive volcanic domes on the Earth and Moon (aspect ratios of ~0.2). We measure the aspect ratio of Oberon’s mountain to be 0.25^+0.25_-0.13, consistent with a cryovolcanic dome. Oberon’s mountain may alternatively be the central peak of an impact crater (Smith+1986). The aspect ratio of central peaks on Ganymede is 0.05 (Bray+2012) and of Aeneas crater on Dione is 0.06 (Moore+2004, Schenk1991), significantly lower than the (cryo)volcanic domes. However, the aspect ratio of the central peak of Herschel crater on Mimas is 0.24 (Moore+2004, Schenk1989). Therefore, further data is needed to discern between the cryovolcanic dome and central peak hypotheses.

Future observations: We define the types of observations a potential future Uranus Orbiter and Probe mission could make to further test our hypotheses. The mission could balance observations of the previously observed <40% portions of Umbriel and Oberon to resolve open questions about the formation of features, with observations of the >60% unobserved portions to better understand the global diversity and distributions of features. We derive that panchromatic images should require a spatial resolution of 1 km/pixel to resolve structure within Wunda’s bright deposit and the summit of Oberon’s mountain, both of which are required to test our hypotheses. The camera should have an appropriate dynamic range and exposure times to successfully image Wunda’s bright deposit. The composition of the bright deposit in Wunda is a key test for identifying its source (cold-trapped CO₂ ice or subsurface brines), which we find could be tested with an IR spectrometer with a spectral range of 1-5 μm and spatial resolution of 8 km/pixel. Geophysical data from at least two flybys of Umbriel could test for the presence of a subsurface ocean. A new proposed passive sounding technique using Uranian Kilometric Radio (UKR) emissions could be used for ocean detection at Oberon (Romero-Wolf+2024).

Conclusions: We eagerly anticipate an orbiter in the Uranian system with the capability to test our hypotheses and predictions about the origin of Wunda’s bright deposit on Umbriel and the mountain on Oberon, which would contribute to the overall understanding of the evolution of these moons and their habitability potential (Weber&Leonard2024).

Figure. (a) Bright deposit in Wunda crater, Umbriel, ~4 km/pixel. (b) Faculae in Occator crater, Ceres, ~4 km/pixel. (c) The unnamed mountain on Oberon, ~5 km/pixel. (d) Ahuna Mons on Ceres, ~5 km/pixel.

How to cite: Scully, J., Denton, C. A., Castillo-Rogez, J., Sori, M., Leonard, E., Beddingfield, C., Cartwright, R., Elder, C., Mitchell, K., and Nordheim, T.: Leveraging Ceres to Gain Insights into the Candidate Ocean Worlds of Umbriel and Oberon that Orbit Uranus, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-381, https://doi.org/10.5194/epsc-dps2025-381, 2025.

Naiad is the innermost moon of Neptune, orbiting within the synchronous orbit at less than 2 Neptune radii.  It was recently shown that Naiad is in a peculiar 73:69 mean motion resonance with Thalassa, Neptune’s next innermost moon [1]. Combining Naiad’s mass, estimated from its resonant interaction with Thalassa, combined with its size and shape estimated from Voyager 2 images [2], yield a bulk density of ~0.8 g/cm^3, similar in density to other small icy moons. 

A closer inspection of Naiad’s dynamical and collisional environment leads to some contradictory conclusions. Based on its elongated shape and bulk density,  we show that Naiad is likely already within the Roche limit, suggesting it must have significant cohesive strength to hold itself together. This is difficult to reconcile with the idea that Neptune’s inner satellites should be weak, reaccumulated icy fragments from Neptune’s original satellite system that was destroyed during the capture of Triton [3,4]. We also demonstrate that Naiad may need ~Gyrs to reach its present inclination through the current 4th-order resonance with Thalassa. Due to the bombardment of Neptune by scattered disk objects, however, Naiad and Thalassa should both have much shorter collisional lifetimes depending on their collisional strengths and the size-frequency distribution of impactors [5, 6], meaning that this pristine resonance should have been destroyed. We discuss these contradictory ideas, some possible resolutions, and their implications for the Neptune system.

Acknowledgements: H.A. was supported by the French government, through the UCA J.E.D.I. Investments in the Future project managed by the National Research Agency (ANR) with the reference number ANR-15-IDEX-01.

[1] M. Brozović, M. R. Showalter, R. A. Jacobson, R. S. French, J. J. Lissauer, and I. De Pater, “Orbits and resonances of the regular moons of Neptune,” Icarus, vol. 338, p. 113462, Mar. 2020, doi: 10.1016/j.icarus.2019.113462.

[2] E. Karkoschka, “Sizes, shapes, and albedos of the inner satellites of Neptune,” Icarus, vol. 162, no. 2, pp. 400–407, Apr. 2003, doi: 10.1016/S0019-1035(03)00002-2.

[3] D. Banfield and N. Murray, “A dynamical history of the inner Neptunian satellites,” Icarus, vol. 99, no. 2, pp. 390–401, Oct. 1992, doi: 10.1016/0019-1035(92)90155-Z.

[4] C. B. Agnor and D. P. Hamilton, “Neptune’s capture of its moon Triton in a binary–planet gravitational encounter,” Nature, vol. 441, no. 7090, pp. 192–194, May 2006, doi: 10.1038/nature04792.

[5] J. E. Colwell, L. W. Esposito, and D. Bundy, “Fragmentation rates of small satellites in the outer solar system,” J. Geophys. Res. Planets, vol. 105, no. E7, pp. 17589–17599, Jul. 2000, doi: 10.1029/1999JE001209.

[6] K. Zahnle, P. Schenk, H. Levison, and L. Dones, “Cratering rates in the outer Solar System,” Icarus, vol. 163, no. 2, pp. 263–289, Jun. 2003, doi: 10.1016/S0019-1035(03)00048-4.

How to cite: Agrusa, H., Ćuk, M., Nesvorny, D., and Marschall, R.: The Curious Case of Neptune's Naiad, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-816, https://doi.org/10.5194/epsc-dps2025-816, 2025.

The Uranian satellite system has recently attracted increased interest due to its complex dynamical and geological history, particularly in the context of the proposed high-priority NASA Flagship mission to Uranus. To date, Voyager 2 remains the only spacecraft to have explored the system, providing observations of the five major regular moons: Miranda, Ariel, Umbriel, Titania, and Oberon. These moons display evidence of both ancient, heavily cratered terrains and signs of more recent resurfacing events. The nature of these moons arises from differences in their formation and composition, as well as from their coupled orbital and thermal evolution – each aspect influencing the other over time. Among the various forces at play, tidal interactions are particularly significant. Tides introduce dissipation in the system through friction processes, in which orbital and rotational energy is dissipated as heat in either (or both) the surface or interior of a planet or satellite. Additionally, moons within the system gain angular momentum from their hosting planet, resulting in an outward orbital migration. Variations in orbital distance can lead to encounters with mean-motion resonances. Although the system currently lacks any mean-motion resonance, the non-negligible orbital eccentricities of the moons and Miranda’s high inclination suggest that past resonant interactions may have played a significant role in shaping their current orbits (Tittemore & Wisdom 1988, 1989, 1990; Ćuk et al. 2020; Gomes & Correia 2024).

Past studies of the orbital evolution of the Uranian moons adopted a medium-low tidal dissipation rate for Uranus, as predicted by equilibrium tide theory (Goldreich & Soter 1966). Previously proposed evolutions constrained the value of Uranus's dissipation within ranges that would prevent capture into the 2:1 mean-motion resonance between Ariel and Umbriel. The reason, as shown by Tittemore & Wisdom (1990), is that it seems not to be possible to escape such a resonance, which would be incompatible with the present configuration.

However, recent theoretical developments and measurements indicate that Uranus may exhibit a higher tidal dissipation rate than previously assumed (e.g., Nimmo 2023; Jacobson & Park 2025). This enhanced dissipation leads to a faster orbital migration of its satellites, consistent with the so-called resonance locking, as proposed by Fuller et al. (2016). Consequently, resonant interactions that were previously considered unlikely now need to be reassessed. In particular, Ariel’s fast migration implies that crossing the 2:1 mean-motion resonance with Umbriel is almost certain and may have occurred in quite recent times (within the last 1 Gyr). This fact indicates that the entire orbital history of the Uranian moon system may need to be revised.

In this presentation, we focus on the potential crossing of the 2:1 mean-motion resonance between Ariel and Umbriel, where Ariel undergoes a rapid migration. Capture into this strong resonance may have induced significant tidal heating within Ariel, potentially explaining its resurfacing. To investigate this resonant encounter, we present numerical simulations based on an ad hoc dynamical model. Assuming a resonance locking regime, we show the possible outcomes of the resonant encounter. In particular, we find that for small initial eccentricities, the moons are always captured into resonance, as already described in Tittemore & Wisdom (1990). In addition, we explore mechanisms for exiting the 2:1 resonance, possibly involving resonant perturbations from other moons of the system. We present evolution histories that depend primarily on Ariel’s tidal dissipation rate, the timing of the Ariel-Umbriel resonance capture, and the initial orbital elements of the other satellites. We propose a parametric study, dependent on tidal parameters, capable of reproducing orbital element distributions consistent with the current orbital configuration of the system.

As a product of the above analysis, we derive constraints on the tidal parameters of Uranus and its moons. This study may therefore serve as a valuable input for future space missions – such as the Uranus Orbiter and Probe space mission concept – by guiding both astronomical and geophysical measurements. A higher tidal dissipation is expected to have a more significant impact on orbital dynamics and related observational uncertainties, potentially producing detectable signatures that future missions could confirm or rule out.

References:

Ćuk, M., El Moutamid, M., & Tiscareno, M. S. (2020). Dynamical history of the Uranian system. The Planetary Science Journal, 1(1), 22.

Fuller, J., Luan, J., & Quataert, E. (2016). Resonance locking as the source of rapid tidal migration in the Jupiter and Saturn moon systems. Monthly Notices of the Royal Astronomical Society, 458(4), 3867-3879.

Goldreich, P., & Soter, S. (1966). Q in the Solar System. Icarus, 5(1-6), 375-389.

Gomes, S. R., & Correia, A. C. (2024). Dynamical evolution of the Uranian satellite system II. Crossing of the 5/3 Ariel–Umbriel mean motion resonance. Icarus, 424, 116254.

Jacobson, R. A., & Park, R. S. (2025). The Orbits of Uranus, Its Satellites and Rings, the Gravity Field of the Uranian System, and the Orientation of the Poles of Uranus and Its Satellites. The Astronomical Journal, 169(2), 65.

Nimmo, F. (2023). Strong tidal dissipation at Uranus?. The Planetary Science Journal, 4(12), 241.

Tittemore, W. C., & Wisdom, J. (1988). Tidal evolution of the Uranian satellites: I. Passage of Ariel and Umbriel through the 5:3 mean-motion commensurability. Icarus, 74(2), 172-230.

Tittemore, W. C., & Wisdom, J. (1989). Tidal evolution of the Uranian satellites: II. An explanation of the anomalously high orbital inclination of Miranda. Icarus, 78(1), 63-89.

Tittemore, W. C., & Wisdom, J. (1990). Tidal evolution of the Uranian satellites: III. Evolution through the Miranda-Umbriel 3:1, Miranda-Ariel 5:3, and Ariel-Umbriel 2:1 mean-motion commensurabilities. Icarus, 85(2), 394-443.

How to cite: Rossi, M. and Lari, G.: Orbital evolution of the Uranian moons in a fast-migration regime, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-530, https://doi.org/10.5194/epsc-dps2025-530, 2025.

Neptune is the only giant planet that lacks a regular satellite system. Triton constitutes over 99% of the Neptunian satellite system's mass and follows a retrograde and synchronous orbit, suggesting an early capture from the protoplanetary disk followed by tidal circularization. Discovered in 1949 by G. P. Kuiper, Nereid is the third largest Neptunian satellite. While typically classified as an irregular satellite due to its eccentric and inclined orbit, Nereid is unique among this population. Of the irregular satellites, Nereid has the lowest pericenter (0.012 Hill Radii), the highest mean eccentricity (e = 0.75), the lowest prograde inclination relative to the ecliptic of 6 degrees, and the largest radius at 175km -- twice as large as the next largest irregular satellite, Saturn's Phoebe. Its compositional properties also stand out: it is bluer and higher albedo (0.25) than similarly-sized Centaurs and Kuiper belt objects. We present JWST near-infrared spectroscopy of Nereid, finding it has a distinct composition among the giant planet satellites and small bodies observed with JWST NIRSpec thus far, with abundant water ice and a notable blue slope in the near-infrared that is unlike that of any Kuiper belt object. The uniqueness of Nereid's spectrum suggests the moon was not captured during the giant planet instability from the same population that formed the Kuiper belt. Instead, we propose that Nereid was once a regular satellite. By simulating the capture of Triton alongside an initially regular Nereid, we show the existence of a plausible dynamical pathway in which Nereid was initially a regular satellite that Triton perturbed onto its current orbit during a phase of rapid tidal circularization.

How to cite: Belyakov, M., Davis, R., Brown, M., and Wong, I.: Nereid was a Regular Satellite of Neptune, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-180, https://doi.org/10.5194/epsc-dps2025-180, 2025.