We present first results from the James Webb Space Telescope (JWST) observations of Uranus, which provide the first spatially resolved, infrared spectra of the planet’s atmosphere spanning from 1.66 to 28.6 µm. We evaluate these unprecedented JWST NIRSpec (1.66–3.05 µm, 2.87–5.14 µm) and MIRI (4.9-28.6 μm) spectra in the context of existing observations and questions concerning Uranus’ stratospheric chemistry and thermal structure [1].

Owing to its frigid atmospheric temperatures, Uranus’ infrared spectrum is extremely weak. Much of the spectrum has never been spatially resolved before, while some had never been clearly observed at all.

From the ground, spatially resolved observations of Uranus’ mid-infrared emission are limited to imaging observations targeting the brighter regions of the infrared spectrum (i.e. ~13 µm emission from stratospheric acetylene, and 17-25 µm from the H₂ continuum). Images from the Very Large Telescope VISIR instrument at 13 µm show a stratospheric structure distinct to Uranus, with elevated radiance at high latitudes. The physical nature of this structure–-whether produced by chemical or thermal gradients–-is unclear given previously available data [1]. From space, the Spitzer Space Telescope observed Uranus' mid-infrared spectrum between ~7 and 36 µm, but it lacked the spatial resolution necessary to resolve potential thermal and chemical structure across the disk [2].

Now, with its exceptional sensitivity and outstanding spatial and spectral resolution, JWST reveals Uranus’ stratospheric temperature and chemistry with exquisite new detail, placing new constraints on hydrocarbon abundances and temperature structure across the disk.

With a projected lifetime of over a decade, JWST promises to continue providing exciting new insights into the atmospheric structure, composition, and variability of the ice giants for years to come.

[1] Roman, M.T, et al. "Uranus in northern..." AJ 159.2 (2020): 45.

[2] Rowe-Gurney, N., et al. "Longitudinal variations..." Icarus 365 (2021): 114506.

How to cite: Roman, M., Fletcher, L., Hammel, H., Melin, H., Rowe-Gurney, N., Harkett, J., King, O., Milam, S., Orton, G., Irwin, P., Moses, J., De Pater, I., and Lamy, L.: Uranus from JWST: First Results, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15491, https://doi.org/10.5194/egusphere-egu23-15491, 2023.

Knowing the composition of the giant planets is important in understanding their forma­tion and evolution history. The abundances of heavy elements, of noble gases, and isotope ratios reveals the physical and chemical conditions and processes that eventually led to their formation. The current knowledge of the composition of the giant planets is limited, with Jupiter being best studied thanks to the Galileo probe. Much less is known for Saturn, and almost nothing is known for Uranus and Neptune. Uranus and Neptune contain substantial hydrogen and helium at­mos­pheres, with bulk mass frac­tions of 5–20%. The remainder is thought to be "ices" and rocks, such as H₂O, CH₄, H₂S, and NH₃. Ura­nus and Neptune are the least-investigated planets in the solar system, but may be representative of similarly sized pla­nets com­mon in the population of exo-planets, thus provide some ground-truth.

Measurement of abundances in the atmosphere can be derived through a variety of re­mote sensing techniques, which is restricted to the upper layers of the atmosphere, but the number of useful observations from Earth is very limited. The most significant step forward in our know­ledge of giant planet internal composition was achieved with the Galileo probe into Jupiter’s atmos­phere. The prime instrument to probe the atmospheric composition on an descent probe is a mass spectrometer experiment (MSE), which comprises the actual mass spectro­meter for gas analysis, possible extensions by a gas-chromatographic pre-selection of the gaseous species, a cryogenic trap to enhance the measurement of noble gases and their isotopes, and an aerosol col­lector and pyrolysis system giving access to the composition of cloud and haze particles. To improve on the isotope measurements of selected species, a Tunable Laser Spectrometer can be added to measure the isotopic ratios with accuracy of selected molecules.

The atmos­pheric probe will enter on a specific location into Uranus’ atmosphere. Aside from technical constraints, what would be the scientific considerations for the locations? Entering at lower latitudes, perhaps near the equator where the zonal flow is retrograde or at higher latitudes with fast pro­grade zonal flows, or at a pole with very limi­ted horizontal flow, which might be easily ac­ces­sible because of Uranus’ rotation axis being close to the eclip­tic plane; at places with clouds running at constant lati­tudes or at cloud-free areas; at a dark spot (an anti­cyclonic storm) possibly providing upwelling from material; or other unique features observed on the surface.

An Uranus orbiter will provide complementary information of the atmosphere via remote sensing, e.g. mapping the “surface” of Uranus, tracking storms, clouds, and eddies in reflected sunlight, maps of key species, abundances of hydro­carbons in the photolysis layer, and some more. This will put the entry location of the probe in a global per­spective, is its entrance at a unique surface feature, is there presen­ce of clouds and hazes, and the temporal evo­lution during the orbital observations, like con­vec­tion, upward and downward energy flow, atmospheric wave activity, which shape atmospheric features such as cloud bands and vortices. In addition, micro­wave soun­ding might probe deep inside the atmosphere.

How to cite: Wurz, P., Vorburger, A., Mousis, O., and Helled, R.: Composition Measurements of Uranus’ Atmosphere, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3990, https://doi.org/10.5194/egusphere-egu23-3990, 2023.

Despite the weak solar flux received by the ice giant planets, the storm activity of their atmospheres is intense. What is then the phenomenon responsible for this activity?

On Uranus and Neptune, a notable property draws attention: unlike the Earth, the species able to condense in the atmospheres of ice giants, methane in particular, are heavier than the ambient air, essentially hydrogen. This property makes convection difficult to start.

Convection in these atmospheres should therefore be a regime of strong intermittence where convective energy can be stored for a long time before being released in short episodes.

Our hypothesis is that this regime is at the origin of intense storms.

To study this hypothesis, we use a "cloud-resolving" model. This model is built from a dynamical core (The Weather Research and Forecasting model) solving the equations of motion, that has been initially developed for terrestrial applications and already adapted for simulations on Mars and Venus, coupled to independent physical parameterizations such as radiative transfer and micro-physics. The high resolution of the model grid can allow us to highlight moist atmospheric convection, by resolving cloud formation and dissipation.

Having successfully implemented the methane cycle in this model, we will present results from our 3D simulations, which reveal the impact of the methane cycle in tropospheric convection on Uranus & Neptune, having a special look at the methane abundance, that vary at different latitudes, and how it affects storms frequencies and intensities.

How to cite: Clement, N., Leconte, J., Spiga, A., Guerlet, S., Milcareck, G., and Selsis, F.: Impact of methane abundance on storm formation on Uranus & Neptune, revealed by a cloud-resolving model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7738, https://doi.org/10.5194/egusphere-egu23-7738, 2023.

How to cite: Neuenschwander, B. and Helled, R.: Empirical structure models of Uranus and Neptune, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5824, https://doi.org/10.5194/egusphere-egu23-5824, 2023.

We investigate the unique magnetosphere of Uranus and its interaction with the solar wind. Following previous seminal work, we developed and validated a simple yet valuable and illustrative model of Uranus’ offset, tilted, and rapidly-spinning magnetic field and magnetopause (nominal and fit to the Voyager-2 inbound crossing point) in three-dimensional space. With this model, we investigated details of the seasonal and interplanetary magnetic field (IMF) orientation dependencies of dayside and flank reconnection along the Uranian magnetopause. We found that anti-parallel (magnetic field shear angle greater than 170-degrees) reconnection occurs nearly continuously along the Uranian dayside and/or flank magnetopause under all seasons of the 84 (Earth) year Uranian orbit and the most likely IMF orientations. Such active and continuous driving of the Uranian magnetosphere should result in constant loading and unloading of the Uranian magnetotail, which may be further complicated and destabilized by sudden changes in the IMF orientation and solar wind conditions plus the reconfigurations from the rotation of Uranus itself. We demonstrate that unlike the other magnetospheric systems that are Dungey-cycle driven (i.e., Mercury and Earth) or rotationally driven (Jupiter and Saturn), global magnetospheric convection of plasma, magnetic flux, and energy flow may occur via three distinct cycles, two of which are unique to Uranus (and possibly also Neptune). Our simple model is also used to map signatures of dayside and flank reconnection down to the Uranian ionosphere, as a function of planetary latitude and longitude. Such mapping demonstrates that “spot”-like auroral features should be very common on the Uranian dayside, consistent with observations from Hubble Space Telescope. We further detail how the combination of Uranus’ rapid rotation and unique and very active global magnetospheric convection should be consistent with fueling of the surprisingly intense trapped radiation environment observed by Voyager-2 during is single flyby. Summarizing, Uranus is a very special magnetosphere that offers new insights on the nature, complexity, and diversity of planetary magnetospheric systems and the acceleration of particles in space plasmas. We still have much to learn about Uranus’ unique and intriguing magnetosphere, which might have important analogs to exoplanetary magnetospheric systems. Our hypotheses can be tested with further work involving more advanced models, new auroral observations, and unprecedented missions to explore the in situ environment from orbit around Uranus. Our results highlight why any future mission to orbit Uranus should include a complement of magnetospheric instruments in the payload.

How to cite: Turner, D., Cohen, I., Clark, G., Kollmann, P., and Regoli, L.: Hypotheses Concerning Global Magnetospheric Convection, Magnetosphere-Ionosphere Coupling, and Auroral Activity at Uranus, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8881, https://doi.org/10.5194/egusphere-egu23-8881, 2023.

The Uranus system hosts a diverse set of large moons, from the enigmatic Miranda with its striking coronae, to the outermost and second-largest moon Oberon with its dark and ancient surface. These moons mostly orbit within Uranus’ magnetosphere, which while relatively depleted in low energy-plasma, was found by Voyager 2 to possess surprisingly intense electron radiation belts. Prominent energetic electron absorptions associated with the Uranian moons were observed by Voyager 2, indicating that these particles interact significantly with the moons. Thus, the surfaces of the moons are continuously bombarded by high energy electrons, which are capable of breaking chemical bonds in surface material, leading to radiolytic chemistry that can alter surface composition. In addition, charged particle bombardment can alter the microstructure of surface ice as well as affect grain sizes by sputtering. Ground-based remote sensing observations have revealed planetocentric and hemispherical trends in several key spectral parameters, including the abundance of CO₂ ice, whose concentration on the trailing hemispheres of the moon hint at a possible radiolytic origin. Furthermore, possible signatures of NH₃ have been detected on the surfaces of several of the moons, including Ariel, where the sub-observer longitudinal distribution of this species appears to support spatial association with geologic features and terrains. It is known from laboratory experiments that NH₃ is readily decomposed by charged particle radiation. If NH₃ or NH₃-related compounds are present on the moons, it therefore could be an indication of recent emplacement or exposure. Here, we will present possible signatures of charged particle surface modification on the Uranian moons and discuss implications for observations by future missions to the Uranus system.

How to cite: Nordheim, T., Cartwright, R., Regoli, L., Sori, M., Menten, S., Beddingfield, C., Leonard, E., Cochrane, C., Elder, C., and Masters, A.: Charged particle bombardment– a dominant surface modification process on the Uranian moons?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16957, https://doi.org/10.5194/egusphere-egu23-16957, 2023.

In 1989 the Voyager 2 spacecraft performed a flyby of the Neptune system. In particular, a radio occultation of Triton’s ionosphere was performed on 25 August 1989. Results from this occultation experiment were published in Science by Tyler et al., and the atmospheric profiles of Triton were estimated via analysis of the real-time tracking and monitoring systems data at time resolutions of about 1 second.

In recent years, thanks to an increase in the computational power of microprocessors and an expansion in their fields of application, there has been a surge in the development of optimal estimators for stochastic signals. In this context, we present a re-analysis of the radiometric data received by the Voyager 2 spacecraft during its radio occultation experiment of Triton. Using one of the latest algorithms developed in the field of signal parameters estimation, we use the radiometric measurements received during the ingress and egress phases of the mission to accurately reconstruct the sky frequency, as received by the DSS-43 antenna at the time of occultation. In particular, by performing spectral interpolation and tuning the processing parameters, we increased the frequency resolution and detection threshold around the ingress and egress epochs to maximize the scientific return from the radiometric data collected over 30 years ago.

Since Voyager 2 transmitted two coherently related signals (at S and X bands), the two series of sky frequencies can be combined to isolate the Doppler frequency shift due to dispersive effects. The latter is used to compute the electron number density profiles inside the Triton ionosphere using a classical Abel transform-based method. Also, a Monte Carlo procedure is used to evaluate uncertainties in the derived profiles. The results of this analysis are consistent with those presented in the past literature, with the only difference of a slight (but important for the planning of future missions) shift in the ionosphere's peak altitude due to the use of updated Voyager and Triton ephemerides. The methods and data analysis approaches presented in this work are very relevant to the exploration of the Ice Giants, in particular for radio science observations of satellite tenuous exospheres and ionospheres.

How to cite: Togni, A., Caruso, A., Buccino, D., Zannoni, M., Oudrhiri, K., and Tortora, P.: Voyager 2 Radio Occultation of Triton revisited using modernized analysis tools, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9714, https://doi.org/10.5194/egusphere-egu23-9714, 2023.

Introduction

During the only flyby of Triton by Voyager 2 in 1989, a dense ionosphere was observed (Tyler et al. 1989). Results were surprising as the solar irradiation of this satellite is ten times lower than on Titan, and yet its ionosphere is denser. Thus, electronic precipitation from Neptune’s magnetosphere was hypothesized to bring the needed extra input energy (Krasnopolsky et al. 1993), as high energy electrons have been observed by the spacecraft in this area (Krimigis et al. 1989).  To understand how this precipitation could impact the composition of Triton’s atmosphere, we coupled an electron transport code to a photochemical model of this atmosphere.

Methodology 

We used the electron transport code TRANS that was utilized to compute the transport of electrons in various planetary atmospheres (see Gronoff et al. 2009 and references therein). We adapted it to Triton’s conditions and used the results from Strobel et al. (1990) and Sittler and Hartle (1996) to compute the input precipitation. This led us to calculate the mean magnetic field and the mean precipitation before adjusting it depending on energy, as detailed in Sittler and Hartle (1996). We then coupled TRANS with our most recent photochemical model of Triton’s atmosphere (Benne et al. 2022) by using TRANS outputs to compute the reaction rates of the electro-dissociation and electro-ionization reactions. Iterations were performed between the two codes until steady state was reached. After determining the nominal composition of the atmosphere, we ran a Monte Carlo simulation to characterize the effect of chemical uncertainties on the model results.

Results

With our previous model presented in Benne et al. (2022), we found a peak electronic number density larger by a factor of 2.5 to 5 compared to the one derived from Voyager 2 observations. By coupling the photochemical model with TRANS, we find that our electronic profile is now in agreement with these measurements, resulting from a significant decrease of the electro-ionization rate. In contrast with the results of Benne et al. (2022), Krasnopolsky and Cruikshank (1995) and Strobel and Summers (1995), the main ionization source is solar EUV radiation instead of magnetospheric electrons. This work also allows us to better understand how the varying magnetic environment impacts the atmospheric chemistry.

References

[1] Tyler, G. L. et al. Science 246, no. 4936 (December 15, 1989): 1466–73.

[2] Krasnopolsky, V. A. et al. Journal of Geophysical Research 98 (February 1, 1993): 3065–78.

[3] Krimigis, S. M. et al. Science 246, no. 4936 (December 15, 1989): 1483–89.

[4] Gronoff, G. et al. Astronomy & Astrophysics 506, no. 2 (November 2009): 955–64.

[5] Strobel, Darrell F. et al. Geophysical Research Letters 17, no. 10 (1990): 1661–64.

[6] Sittler, E. C., and R. E. Hartle. Journal of Geophysical Research: Space Physics 101, no. A5 (May 1, 1996): 10863–76.

[7] Benne, B. et al. Astronomy & Astrophysics 667 (November 2022): A169.

[8] Krasnopolsky, Vladimir A., and Dale P. Cruikshank. Journal of Geophysical Research 100, no. E10 (1995): 21271.

[9] Strobel, D. F., and M. E. Summers. 1995, 1107–48. Cruikshank, Dale P., Mildred Shapley Matthews, et A. M. Schumann. « Neptune and Triton », 1995.

How to cite: Benne, B., Benmahi, B., Dobrijevic, M., Cavalié, T., Loison, J.-C., Hickson, K., Barthélémy, M., and Lilensten, J.: Coupling a photochemical model of Triton's atmosphere with an electron transport code, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6543, https://doi.org/10.5194/egusphere-egu23-6543, 2023.