Observations of thermal emission from Uranus and Neptune have been made over a broad wavelength range from ground-based platforms, airborne observatories, Earth-proximal spacecraft and from the Voyager-2 flybys in the 1980s.  Observations since the Voyager flybys have included long-wavelength observations of disk-averaged radiances from the Infrared Space Observatory and the Herschel Space Observatory covering the far-infrared to millimeter range. We present recent airborne spectra from SOFIA covering 17-35 µm, together with Akari and Spitzer spectroscopy at wavelengths extending down to 7 µm, below which contributions from reflected sunlight and potential auroral emissions may confuse the signature of thermal emission.  We also show how these disk-averaged spectra are complemented by ground-based filtered imaging and spectroscopy at 8-10m telescopes, which have enabled spatially resolved measurements, complementing those of Voyager IRIS from several decades ago. The critical insights into the structure, chemistry and dynamics of the atmospheres of these Ice Giants attest to the need for significant parts of this spectral region to be included in the instrument complement to be assigned to spacecraft sent to these planets.  A vigorous program of Earth-based observations in the accessible spectral range should accompany the spacecraft capability in order to track potential seasonal and non-seasonal variability of these planets, as is evident in the atmospheres of both Jupiter and Saturn. The latter would include mid-infrared observations from the James Webb Space Telescope.

How to cite: Orton, G., Sinclair, J., Fletcher, L., Rowe-Gurney, N., Roman, M., Irwin, P., and Hammel, H.: Recent Mid-Infrared Through Submillimeter Observations of Uranus and Neptune, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3382, https://doi.org/10.5194/egusphere-egu21-3382, 2021.

In the pursuit of deciphering the formation of our solar system, the exploration of the compositional and dynamical structure of planetary atmospheres with entry probes plays a crucial role. A probe's measurements provide insight into an atmosphere's deeper composition and dynamical processes not accessible via remote sensing, providing key information on the origin and possible migration of planets during early formation phase. A planetary entry probe mission has been in discussion in several Planetary Science Decadal Surveys, one to Saturn has been identified as a mission of highest priority in the current one 2013-2022, and a mission to Uranus and/or Neptune carrying a probe is being considered as a Flagship mission in the next one spanning 2023-2032.

In the development of such missions, the probe approach and delivery trajectory is a critical element to mission success, including ring avoidance, and targeting of highly desirable regions in the atmosphere, while balancing other requirements such as providing an optimal communication geometry between the probe and the relay spacecraft while meeting the mission's science objectives. Due to the complexity of the problem, mission concept studies are usually limited to the investigation of a limited number of specific trajectories and probe delivery opportunities to a very small, pre-defined range of latitudes while leaving a huge trade space unexplored.

The tool VAPRE (Visualization of Atmospheric PRobe Entry conditions) has been developed to enable a fast and wide-range evaluation of entry conditions for planetary probes, spanning the complete range of latitudes for each of the three planets. VAPRE allows a rapid assessment of feasible entry sites by evaluating a large number of arrival trajectories based on their hyperbolic arrival velocities with respect to parameters such as the flight path angle and the relative entry velocity of the probe at the entry interface point. VAPRE facilitates the mission design process by combining the evaluation of technical feasibility and science value for the investigated scenarios to assess potential entry sites. VAPRE is developed in the framework of IPED (Impact of the Probe Entry zone on the trajectory and probe Design), which is a two- to three-year research study to investigate both the impact of interplanetary and approach trajectories on the feasible range of entry sites as well as on probe design, considering Saturn, Uranus, and Neptune as target bodies.

In this paper we fully demonstrate the functionalities of the VAPRE tool on a case scenario for a mission to the Ice Giants.

The presented research was supported by an appointment to the NASA Postdoctoral Program (NPP) at the Jet Propulsion Laboratory (JPL), California Institute of Technology, administered by Universities Space Research Association (USRA) under contract with National Aeronautics and Space Association (NASA). © 2020 All rights reserved.

How to cite: Probst, A., Spilker, L., Spilker, T., Atkinson, D. H., Mousis, O., Hofstadter, M., and Simon, A.: VAPRE: Facilitating Planetary Probe Mission Design and Entry Site Selection, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3286, https://doi.org/10.5194/egusphere-egu21-3286, 2021.

Core accretion is the conventional model of the formation of gas giants, Jupiter and Saturn. According to this model, a core of 10-15 Earth-mass forms in 1-5 Myr from non-gravitational collisions between submicron size grains of dust − ice, rock, metals, and trapped gases. Most volatile of the gases, hydrogen, helium, and neon, can then be gravitationally captured, completing the planetary formation. Unlike gas giants, formation timescale of the icy giant planets (IGPs), Uranus, and Neptune by core accretion at their present orbital distance exceed the typical lifetime of the protoplanetary nebula. Thus, there are two alternatives: IGPs begin their formation also in the neighborhood of Jupiter and Saturn (5-10 AU) and then migrate out to their present orbital distances (20 and 30 AU), or they form by a fast process, called the gravitational instability model that requires only 1000’s of years for to form them from clumps in massive protoplanetary disks at their present orbital distances. Core accretion followed by migration is still the favored scenario for the IGPs, considering the latter model does not satisfactorily explain the measured elemental abundances in the giant planets. Moreover, the exoplanet observations also support the core accretion theory. The heavy elements are key constraints to formation and migration models. Those found in the condensible, reactive, and disequilibrium species (C, N, S, O) require measurements in the deep well-mixed atmosphere, which is below kilobar levels at the IGPs, according to our thermochemical models. Extension of the models deeper shows formation of alkali metal and rock clouds at several kilobars and greater. These cloud aerosols provide extensive sites for adsorption of volatiles, irrespective of any volatile loss by sequestration or clustering in a purported water ocean or ionic-superionic ocean proposed previously [1]. Fortunately, abundances and isotopic ratios of the noble gases, He, Ne, Ar, Kr and Xe, will provide necessary constraints to the formation and evolution models of the IGPs [1,2], and entry probes deployed to only a few bars can measure them precisely. In addition, complementary measurements of gravity, magnetic field, stratospheric composition, and depth profiles of certain condensible gases from an orbiter are important to make [1,3]. Atmospheric temperature vs. pressure from exosphere to the probe depth of 5-10 bars is essential also for the interpretation of the measurements. An orbiter-probe mission that makes use of a Jupiter gravity-assisted trajectory to deliver affordable payload mass requires launch between 2030-2034 for Uranus and 2029-2031 to Neptune [1]. Such a mission requires no new technology. This presentation will discuss the new models mentioned above and possible mission scenarios. The US Astrobiology and Planetary Science Decadal Survey committee is presently reviewing the White Papers submitted in support of a mission to the icy giants in the 2023-2032 decade [e.g., 4], and would make a recommendation of mission priorities for NASA in 2022. [1]Atreya et al. Space Sci. Rev. 216:18; [2]Mousis et al. Space Sci. Rev. 216:77, 2020; [3] Fletcher et al. Trans. R. Soc. A 378: 20190473, 2020; [4]Beddingfield et al. arXiv.2007.11063, 2020.

How to cite: Gupta, P., Atreya, S., Kumar, T., Li, C., Mousis, O., and Reh, K.: Measurements by Probe and Orbiter Critical for Models of Formation and Evolution of Uranus and Neptune, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1403, https://doi.org/10.5194/egusphere-egu21-1403, 2021.

We explore the performance of the Gibbs-ensemble Monte Carlo simulation method by calculating the miscibility gap of H₂-He mixtures with analytical exponential-six potentials [1]. We calculate demixing curves for pressures up to 500 kbar and temperatures up to 1800 K. Our results are in good agreement with ab initio simulations in the non-dissociated region of the phase diagram. Next, we determine new parameters for the Stockmayer potential [2] to model the interactions in the H₂O-H₂O system for temperatures of 1000 K < T < 2000 K. The corresponding miscibility gap of H₂-H₂O mixtures was determined and we calculated demixing curves for pressures up to 150 kbar and temperatures up to 2000 K. Our results show reasonable agreement with previous experimental data of Bali et al. [3]. These results are important for interior and evolution models for ice giant planets [4].

References
[1] A. Bergermann, M. French, M. Schöttler and R. Redmer, Phys. Rev. E, 103 (2021)
[2] W. Stockmayer, The Journal of Chemical Physics 9, S. 398-402 (1941)
[3] E. Bali, A. Audétat and H. Keppler, Nature, 495, 7440 (2013)
[4] R. Helled, N. Nettelmann and T. Guillot, Space Science Reviews, 216 (2020)

How to cite: Bergermann, A., French, M., and Redmer, R.: Gibbs-ensemble Monte Carlo simulations for binary mixtures, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8847, https://doi.org/10.5194/egusphere-egu21-8847, 2021.

Our team is using radio observations of Uranus, collected with the Very Large Array (VLA) telescope, to track seasonal changes in the deep troposphere of Uranus between 1981 and the present.  We previously reported on changes between 1981 and 1994, as the Southern Hemisphere moved from mid- to late-summer (Hofstadter and Butler 2003, Icarus 165, https://doi.org/10.1016/S0019-1035(03)00174-X).  During that time, the distribution of opacity sources in the atmosphere (now thought primarily to be H₂S) changed in such a way as to suggest an increase in the strength of the planetary-scale circulation pattern in the 5 to 50 bar region of the atmosphere.  More specifically, using wavelengths from 1 to 20 cm, we found that regions poleward of 45 degrees latitude in the Southern Hemisphere are significantly depleted in absorbers compared to more equatorial latitudes, down to a pressure of about 50 bars (which is near the top of where a liquid water cloud is expected to form).  This opacity distribution could be explained by a planetary-scale circulation pattern, with absorber rich air parcels moving upward in equatorial regions, being depleted in absorbers by condensation at higher altitudes, and then moving poleward and descending, keeping the pole depleted in absorbers.  We found that the opacity difference between the pole and equator increased between the 1980's and the 1990's, suggesting a strengthening of the assumed circulation pattern.  Radio observations by our group and others since 1994 have shown that the Northern Hemisphere is roughly symmetric with the Southern, and that smaller-scale latitudinal banding exists (e.g., Molter et al. 2020 https://arxiv.org/abs/2010.11154).  

We are currently analyzing additional Uranus data collected at the VLA, and will present results from a subset of those observations taken in 2012 (during Southern Fall).  We will also discuss plans for extending the time line to the present.  The complete data set will span half a uranian year, allowing all seasons to be observed.  We will also discuss how the composition and chemistry of the ice giant planets (Uranus and Neptune) differ from those of the gas giants (Jupiter and Saturn).

How to cite: Hofstadter, M., Akins, A., and Butler, B.: Seasonal Change in the Deep Atmosphere of Uranus, 1981 to 2012, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1374, https://doi.org/10.5194/egusphere-egu21-1374, 2021.

The current composition of giant planet atmospheres provides information on how such planets formed, and on the origin of the solid building blocks that contributed to their formation. Noble gas abundances and their isotope ratios are among the most valuable pieces of evidence for tracing the origin of the materials from which the giant planets formed. In this review we first outline the current state of knowledge for heavy element abundances in the giant planets and explain what is currently understood about the reservoirs of icy building blocks that could have contributed to the formation of the Ice Giants. We then outline how noble gas isotope ratios have provided details on the original sources of noble gases in various materials throughout the solar system. We follow this with a discussion on how noble gases are trapped in ice and rock that later became the building blocks for the giant planets and how the heavy element abundances could have been locally enriched in the protosolar nebula. We then provide a review of the current state of knowledge of noble gas abundances and isotope ratios in various solar system reservoirs, and discuss measurements needed to understand the origin of the ice giants. Finally, we outline how formation and interior evolution will influence the noble gas abundances and isotope ratios observed in the ice giants today. Measurements that a future atmospheric probe will need to make include (1) the ³He/⁴He isotope ratio to help constrain the protosolar D/H and ³He/⁴He; (2) the ²⁰Ne/²²Ne and ²¹Ne/²²Ne to separate primordial noble gas reservoirs similar to the approach used in studying meteorites; (3) the Kr/Ar and Xe/Ar to determine if the building blocks were Jupiter-like or similar to 67P/C-G and Chondrites; (4) the krypton isotope ratios for the first giant planet observations of these isotopes; and (5) the xenon isotopes for comparison with the wide range of values represented by solar system reservoirs.

Mandt, K. E., Mousis, O., Lunine, J., Marty, B., Smith, T., Luspay-Kuti, A., & Aguichine, A. (2020). Tracing the origins of the ice giants through noble gas isotopic composition. Space Science Reviews, 216(5), 1-37.

How to cite: Mandt, K., Mousis, O., Lunine, J., Marty, B., Smith, T., Luspay-Kuti, A., and Aguichine, A.: Tracing the Origins of the Ice Giants through Noble Gas Isotopic Composition, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7798, https://doi.org/10.5194/egusphere-egu21-7798, 2021.

Uranus and Neptune present unique challenges to planetary modelers. The
composition of the so-called ice giants is very uncertain, even more so than the
composition of the gas giants. For instance, it is far from clear that either
planet's composition is dominated by water. Instead, the composition of Uranus and
Neptune likely includes water and other refractory elements in large quantities as
well as a substantial H/He envelope. Furthermore, formation models also predict
that composition gradients are likely in the interiors of these planets, rather
than a neat differentiation into layers of homogeneous composition. (See Helled
and Fortney 2020 and references within.)

A key question that impacts the science case for a potential orbiting mission to
Uranus or Neptune is how will more precise measurements of the gravitational field
better constrain either planet's interior density profile and composition.
Surprisingly, there is yet no published answer to this question.  Here, we present
new work that explores this issue, using a Bayesian framework that allows
exploration of a wide range of interior density profiles.

Our approach, which builds off our previous work for Saturn (Movshovitz et al.,
2020) and that of others  (e.g. Marley et al., 1995, Helled et al., 2011) takes a
relatively unbiased view of the interior structure by employing so-called
empirical density profiles. A parameterization is applied to the density profiles
directly (via mathematical base functions) instead of to an assumed layered
composition (H/He, water, rocks). While some of these empirical density profiles
may imply unrealistic compositions, they can also probe solutions that would be
missed by the standard layered-composition approach.

Here we will present models of Uranus and Neptune constructed with this approach,
and ask two questions: 1) How large is the space of possible solutions today? 2)
How much will it be reduced should a future mission to Uranus and Neptune improve
the precision on their gravity field measurements by several orders of magnitude,
to the level now available for Jupiter and Saturn?

How to cite: Movshovitz, N. and Fortney, J.: The promise and limitations of improved-accuracy gravity field measurements for Uranus and Neptune, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8122, https://doi.org/10.5194/egusphere-egu21-8122, 2021.

Ammonia is predicted to be one of the major components in the depths of the ice giant planets Uranus and Neptune. Their dynamics, evolution, and interior structure are insufficiently understood and models rely imperatively on data for equation of state and transport properties [1,2]. Despite its great significance, the experimentally accessed region of the ammonia phase diagram today is still very limited in pressure and temperature [3, 4].

We investigate the equation of state, the optical properties and the electrical conductivity of warm dense ammonia by combining laser-driven shock experiments and state-of-the-art density functional theory molecular dynamics (DFT-MD) simulations [5]. The equation of state is probed along the Hugoniot of liquid NH₃ up to 350 GPa and 40000 K and in very good agreement with earlier DFT-MD results [6]. Our temperature measurements show a subtle slope change at 7000 K and 90 GPa, which coincides with the gradual transition from a liquid dominated by molecules to a plasma state in our new ab initio simulations. The reflectivity data furnish the first experimental evidence of electronic conduction in high pressure ammonia and are in excellent agreement with the reflectivity computed from atomistic simulations. Corresponding electrical conductivity values are found up to one order of magnitude higher than in water in the 100 GPa regime, with possible implications on the generation of magnetic dynamos in large icy planets’ interiors.

[1] Scheibe, Nettelmann, Redmer, Astronomy & Astrophysics 632, A70 (2019).

[2] Vazan & Helled, Astronomy & Astrophysics 633, A50 (2020).

[3] Nellis, Hamilton, Holmes, Radousky, Ree, Mitchell, Nicol, Science 240, 779 (1988).

[4] Radousky, Mitchell, Nellis, Journal of Chemical Physics 93, 8235 (1990).

[5] Ravasio, Bethkenhagen, Hernandez, Benuzzi-Mounaix, Datchi, French, Guarguaglini, Lefevre, Ninet, Redmer, Vinci, Physical Review Letters 126, 025003 (2021).

[6] Bethkenhagen, French, Redmer, Journal of Chemical Physics 138, 234504 (2013).

How to cite: Bethkenhagen, M., Hernandez, J.-A., Benuzzi-Mounaix, A., Datchi, F., French, M., Guarguaglini, M., Lefevre, F., Ninet, S., Redmer, R., Vinci, T., and Ravasio, A.: Exploring the deep interior of ice giants with shock-compression experiments and ab initio simulations: The case of metallic ammonia , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12055, https://doi.org/10.5194/egusphere-egu21-12055, 2021.

We present a study on Saturn's stratospheric hazes using archived images from the Hubble Space Telescope Advanced Camera for Surveys. These observations were taken from 2005 to 2014, including the Great Storm during the years 2010 and 2011. For our research we used ultraviolet images from the Solar Blind Channel camera equipped with the F115LP and F125LP filters. At these wavelengths, the reflected spectrum is fundamentally Rayleigh-scattered, with substantial contributions from hydrocarbon absorptions and additional scattering by the aerosols in the hazes above the tropopause. The goal of this work is to analyze temporal and latitudinal changes in the characteristics of the stratospheric haze, gases and particles, analyzing the absolute reflectivity and its limb darkening. Such behavior can be reproduced using the empirical Minnaert's law. This provides nadir-viewing reflectivity and limb darkening coefficient as a function of latitude and time. This is a first approach that helps to qualitatively identify the changes occurring in the aerosol layer during this period of time, which include the massive Great White Spot of 2010. In order to quantify such aerosol changes, we use the radiative transfer code and retrieval suite NEMESIS (Non-Linear Optimal Estimator for Multivariat Spectral AnalySIS) to reproduce the observed reflectivity.  Here we will focus on the detected variations of the vertical distribution of the stratospheric particles, their integrated optical thickness and size distribution and will correlate them with the seasonal changes taken place in the atmosphere of the planet.

How to cite: Sanz Requena, J. F., Pérez Hoyos, S., Sánchez-Lavega, A., Melin, H., Fletcher, L., and Irwin, P.: Saturn´s Stratospheric Hazes From HST Ultraviolet Imaging, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10230, https://doi.org/10.5194/egusphere-egu21-10230, 2021.

The investigation of Titan’s surface chemical composition is of great importance for the understanding of the atmosphere-surface-interior system of the moon. The Cassini cameras and especially the Visual and infrared Mapping Spectrometer has provided a sequence of spectra showing the diversity of Titan’s surface spectrum from flybys performed during the 13 years of Cassini’s operation. In the 0.8-5.2 μm range, this spectro-imaging data showed that the surface consists of a multivariable geological terrain hosting complex geological processes. The data from the seven narrow methane spectral “windows” centered at 0.93, 1.08, 1.27, 1.59, 2.03, 2.8 and 5 μm provide some information on the lower atmospheric context and the surface parameters. Nevertheless, atmospheric scattering and absorption need to be clearly evaluated before we can extract the surface properties. In various studies (Solomonidou et al., 2014; 2016; 2018; 2019; 2020a, 2020b; Lopes et al., 2016; Malaska et al., 2016; 2020), we used radiative transfer modeling in order to evaluate the atmospheric scattering and absorption and securely extract the surface albedo of multiple Titan areas including the major geomorphological units. We also investigated the morphological and microwave characteristics of these features using Cassini RADAR data in their SAR and radiometry mode. Here, we present a global map for Titan’s surface showing the chemical composition constraints for the various units. The results show that Titan’s surface composition, at the depths detected by VIMS, has significant latitudinal dependence, with its equator being dominated by organic materials from the atmosphere and a very dark unknown material, while higher latitudes contain more water ice. The albedo differences and similarities among the various geomorphological units give insights on the geological processes affecting Titan’s surface and, by implication, its interior. We discuss our results in terms of origin and evolution theories.

[1] Solomonidou, A., et al. (2014), J. Geophys. Res. Planets, 119, 1729; [2] Solomonidou, A., et al. (2016), Icarus, 270, 85; [3] Solomonidou, A., et al. (2018), J. Geophys. Res. Planets, 123, 489; [4] Solomonidou, A., et al. (2020a), Icarus, 344, 113338; [5] Solomonidou, A., et al. (2020b), A&A 641, A16; [6] Lopes, R., et al. (2016) Icarus, 270, 162; [7] Malaska, M., et al. (2016), Icarus 270, 130; [8] Malaska, M., et al. (2020), Icarus, 344, 113764.

How to cite: Solomonidou, A., Coustenis, A., Lopes, R., Malaska, M., Le Gall, A., Schmitt, B., Schoenfeld, A., Wall, S., Lawrence, K., Sotin, C., Matsoukas, C., Markonis, Y., Drossart, P., and Elachi, C.: A chemical composition map for Titan’s surface, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13516, https://doi.org/10.5194/egusphere-egu21-13516, 2021.