This blog post has been written in preparation for the 2020 Europlanet Society Congress, which will be held virtually from September 21st. Contributors were asked to submit pre-recorded video presentations or 6-slide virtual posters, which would be viewed asynchronously at a time convenient for the audience. Each presentation and poster was accompanied by a text-based chat for questions, allowing for asynchronous discussion of the various presentations. However, in an effort to advertise the individual sessions and to ensure a lively discussion, the EPSC 2020 organisers scheduled short 20-minute science showcases, to be delivered by the session conveners or a selected presenter. This blog represents a summary of the Ice Giant science being presented at EPSC2020, grouping the abstracts into themes. We urge you to visit and participate in the discussion of the individual contributions, and hope for a lively meeting.
1. Ice Giant Atmospheres
Guillot (514) discusses how the Ice Giants can be used as a laboratory for methane-driven convection and storms, providing insights into how moist convection works in atmospheres where the condensates are heavier than the surrounding air. They point out that the methane clouds are more readily accessible than the water clouds of Jupiter and Saturn, making this a fascinating regime to study via future missions.
Sticking with this theme, Hueso et al. (354) describe a programme of Earth-based observations of Neptune in 2019, using contributions from amateur observers to fill time-gaps between large-class facilities like Hubble. The data suggest more variability but less cloud activity in 2019 than previously, and the team uses the observed cloud features to reassess Neptune’s zonal winds. In addition, Sato et al. (080) study the evolution of dark spots and storms on Neptune via the 1.6-m Pirka telescope of Hokkaido University in 2018, using the spectrum in the near-infrared to estimate the drift rate of a storm.
The nature of the clouds themselves are explored by Toledo et al. (593), who present constraints on the formation of hazes and clouds from a coupled cloud-haze microphysical model, used to understand the vertical structure and evolutionary timescales of ice giant aerosols. Irwin et al. (241) describe observational constraints on clouds and methane in Neptune’s atmosphere, via fitting visible-light spectroscopy acquired by the VLT MUSE instrument in 2018, using the observed limb-darkening to discriminate between methane and aerosol contributions to the spectrum, and confirming the strong equator-to-pole gradient in methane gas on Neptune.
Moving into the stratosphere, Rowe-Gurney et al. (244) use Spitzer observations to reveal the properties of Uranus’ stratosphere, discovering a variation in emission as a function of longitude as the planet rotated in 2007. They demonstrate that these changes are the result of stratospheric temperature variations, and relate it to small-scale structures in bands of warm mid-latitude emission on Uranus. Roman et al. (471) use thermal infrared observations of Neptune over the past two decades to reveal surprising changes in emission, implying that processes occurring much faster than a Neptunian season are significantly modifying the temperature and/or chemistry of the stratosphere, with a substantial change occurring between 2006 and 2008.
Finally, Milcareck et al. (297) describe the development of a general circulation model for Uranus and Neptune, using state-of-the-art radiative-equilibrium models (Vatant d’Ollone, (292)) and reproducing the broad structure of Ice Giant wind fields, but generating a number of open questions concerning differences between the two planets.
2. Planetary Origins and Interiors
Turning to the bulk composition of the Ice Giants, Cavalie et al. (053) explore Ice Giant chemistry via a combination of chemical modelling and Earth-based compositional measurements, concluding that a combination of in situ atmospheric probes and remote sensing are required to understand Ice Giant composition. Mousis et al. (613) describe a model of the outer region of the proto-solar nebula to understand how materials might have been delivered to the Ice Giants to explain the observed elemental enrichments of Uranus and Neptune. They suggest that these planets accreted from building blocks of grains and pebbles that condensed in the vicinity of the N2 and CO ice lines.
Scheibe et al. (640) attempt to reconcile thermal evolution models with the extreme differences in intrinsic heat flux between Uranus and Neptune, proposing and studying a conducting interface that inhibits energy transport between the ice-rich interior and the outer atmosphere.
Moving from the planets to the satellites, Li & Christou (769) revisit the idea that Triton and Nereid, by virtue of their peculiar orbits, must have been captured by Neptune. By assuming instead that the satellites formed in situ around Neptune, but were then perturbed during the period of instability suggested by the Nice Model, a small number of satellites survive the encounter and can gain Triton-like or Nereid-like orbits, challenging the conventional capture model.
3. Magnetospheres and Upper Atmosphere
Gershman & DiBraccio (258) study how the solar wind couples to the Ice Giant magnetospheres, showing how solar wind magnetic pressure changes influence the strength of the coupling, and therefore hinting that magnetospheric dynamics may have a strong dependence on the solar cycle, being strongest at solar maximum.
Two techniques are explored for studying the upper atmospheric emission. Melin et al. (268) continue to monitor the long-term cooling trend of Uranus’ upper atmosphere using near-infrared emission from the H3+ ion, showing cooling by 8 K per year since records began in 1992 that they relate to the changing geometry of Uranus’ magnetic field and the solar wind over the past three decades. And Thomas et al. (797) report on the ongoing attempts to detect Uranian auroras via emission from H3+, to supplement previous detections in the ultraviolet. They used NIRSPEC on Keck in 2006 to identify fast changes in intensity that are suggestive of auroral processes, potentially the first spatially resolved infrared images of Uranian auroras.
Finally, Dunn et al. (1028) also continue to search for X-ray emissions from the Ice Giants, using Chandra observations of Uranus to show two non-detections during solar minimum and one statistically significant detection during solar maximum consistent with an X-ray emission from charge exchange or scattering of solar photons.
4. Future Missions and Instruments
The final theme of this EPSC session concerns a look ahead to future missions. Blanc et al. (984) describe the outcomes of the “Horizon 2061” foresight exercise, suggesting questions that could be addressed via a long-term plan for Ice Giant exploration, potentially with one or more spacecraft and making use of gravity assists in the 2030s. Costa Sitjà et al. (878) consider the opportunities that a long cruise to Uranus and Neptune might provide, by developing a tool to assess potential flyby encounters enroute to Neptune, searching for Jupiter Trojans, Centaurs, Trans Neptunian Objects and Jupiter Family Comets, with the aim of ultimately optimising trajectory designs.
Probst et al. (435) introduces a tool that investigates the feasibility for planetary entry probes accessing different latitudes on Saturn, Uranus and Neptune, looking at how different trajectories, approach angles, and probe designs impact the available probe sites, which will provide crucial insights for mission development. For such an atmospheric probe, Irwin et al. (306) describe the needs for a net flux radiometer instrument, able to measure the upward and downward fluxes of solar and thermal radiation as the probe descends. They evaluate how such an instrument would constrain both the radiation energy budget and the properties of the clouds and haze layers.
In addition, Molina-Cuberos et al. (523) discuss the potential for in situ probes measuring electrical properties of Ice Giant atmospheres during the probe descent. Using a model developed for Mars and Titan, the authors show how important aerosol properties are to the observed ionisation and electrical conductivity.
Tortora et al. (1042) describe the proposed Discovery-class Trident mission to Triton, specifically focussing on the capabilities of the radio science instrument to study electron densities, neutral atmospheric temperatures, and the thickness of Triton’s hydrosphere. And Lamy et al. (941) discuss the variety of radio emissions from the “radio twins” Uranus and Neptune, and how a digital high-frequency receiver could provide a low-resource instrument to study auroral and atmospheric radio and plasma waves or dust impacts.
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