PS5.1

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.

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.

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.