Poster presentations and abstracts

It has been a long-standing challenge to reconcile the perceived similarities of Uranus and Neptune with their highly different intrinsic heat fluxes. Previous evolution calculations using the conventional assumption of an adiabatic interior yield too high present-day luminosities or - equivalently - too long cooling times for Uranus  (e.g. [1,2]). For Neptune, however, we found that similar assumptions yield too short cooling times [3].
One proposed mechanism for reproducing the observed brightness is a conducting interface between the hydrogen- and helium-rich outer part and the ice-rich inner part that would inhibit efficient energy transport across it [4]. In this work, we use our recently developed tool for modelling giant planets based on the Henyey-method for stellar 
evolutions [5] to investigate such a conducting interface in the planet's interior, examining the influence of parameters such as assumed layer thickness and thermal conductivity on the cooling behaviour. 
We find that even a thin conductive interface of a few kilometers has significant influence on the planetary cooling. Initially, the presence of such a boundary layer speeds up cooling, while after about 0.1-0.5 Gyr the cooling is slowed down drastically compared to the adiabatic case, similar to what was found for Saturn previously [6]. Our preferred solutions for Uranus suggest equilibrium evolution with the solar incident flux, while for Neptune, we find that plateaus in T_eff(t) near its observed value require fine-tuned combinations of layer thickness and thermal conducitivity. 

[1] Fortney, Ikoma, Nettelmann, Guillot, and Marley (2011). ApJ 729, 32
[2] Nettelmann, Helled, Fortney, and Redmer (2013). Planet. Space Sci. 77, 143
[3] Scheibe, Nettelmann, Redmer (2019). A&A 632, A70
[4] Nettelmann, Wang, Fortney, Hamel, Yellamilli, Bethkenhagen, and Redmer (2016). Icarus 275, 107
[5] Henyey, Forbes, and Gould (1964). ApJ 139, 306
[6] Leconte and Chabrier (2013): Nat Geosci. 6, 023007

How to cite: Scheibe, L., Nettelmann, N., and Redmer, R.: Influence of a thermal boundary layer on the thermal evolution of Uranus and Neptune, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-640, https://doi.org/10.5194/epsc2020-640, 2020.

The outermost planet Neptune is known to have a giant storm system as Jupiter's Great Red Spot (GRS). However, there are only a few observations of Neptune's storm, and the structure, formation mechanism, and lifecycle of these giant storms are poorly understood. Voyager 2 observed Neptune on May 24, 1989, and discovered the Earth-sized Great Dark Spot (GDS) with 13,000 km. GDS was located in the southern hemisphere, but GDS became extinct when the Hubble Space Telescope observed it in 1994 (Hammel et al., 1995). It is unknown whether it is a sudden thing or storms such as GDS always occur in Neptune. A huge storm of 9,000 km at the equator was observed on July 2 and June 26, 2017, by Keck observatory (Edward et al., 2019). It's considered that Neptune storms occur at mid-latitudes in the north and south that an ascending air occurs. However, this huge storm occurred near the equator. A rotation axis of Neptune is 29.6°, and the storm possibly occurred near the equator because of seasonal change. Neptune is a great distance away from the Earth, storms in Neptune can be resolved only by using large telescopes such as Keck observatory and the Hubble Space Telescope. However, it is not easy to use those telescopes for long-term continuous monitoring. In order to investigate the temporal evolution of GDSs and storms in Neptune, we developed the technique to estimate the drift rate and intensity of storms by observing Neptune's whole spectrum in this study. When seeing is bad, it's possible to observe and acquire Neptune's observation data for a long-term on a short time scale. The purpose of this study is to understand the atmosphere convection structure related to Neptune's storm and its temporal evolution. We observed Neptune by using 1.6 m Pirka telescope operated by the Faculty of Science in Hokkaido University from October 22, 2018, to November 26, 2018. The wavelength is 890, 855 nm. From this analysis, we can retrieve a weak absorption at 890 nm because the altitude of storms is higher than the surrounding areas. In addition, the apparent size of storms from the observation point changes by the rotation of Neptune, so an 890 nm flux changes by the rotation. We took the ratio of an 890 nm flux and an 855 nm flux to correct the effect of the earth atmosphere and calculated the relative intensity's theoretical values by the rotation. The bottom left figure shows the profile of the theoretical line. Here, we defined the longitude difference between the observer longitude and the storm's longitude. We assumed the storm's area and longitude of the storm at the start of our observation, and fit the observed values with the theoretical values in the method of least squares to estimate the drift rate and 890 nm albedo inside the storm. The fitting result is shown in the bottom right figure. We estimated that the drift rate of the storm is 24.6°/ day, and the 890 nm albedo is 0.055.                                                                                                                                   

How to cite: Sato, Y., Takahashi, Y., Sato, M., Takagi, S., Imai, M., and Ono, T.: Estimation of the drift rate and intensity of Neptune's storm by Pirika telescope, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-80, https://doi.org/10.5194/epsc2020-80, 2020.

Uranus and Neptune’s atmospheres are active worlds, with vigorous meteorological activity and strong zonal winds occurring despite small absorbed solar radiation and internal heat fluxes. A few 3-D General Circulation Models (GCM) of their atmospheres exist in the literature, focusing mostly on understanding their zonal jet structure [1,2] or the evolution of large disturbances [3,4].

Building a complete and realistic GCM is a challenging task, given the long orbital and radiative timescales involved, along with the rather high spatial and temporal resolution needed when solving the atmospheric equations of motion on the rotating sphere. For this reason, existing GCMs include crude representation of radiative transfer (a simple relaxation scheme to an equilibrium temperature profile) and/or neglect seasonal variations.

We are currently developing a GCM for Uranus and Neptune’s atmospheres, building on our existing expertise on Jupiter and Saturn GCMs [5,6]. Compared to other existing GCMs for ice giants, our model includes state-of-the art parametrization of radiative transfer. The radiation scheme is a full radiative transfer using correlated-k distributions. Seasonal variations of the incoming solar flux are taken into account. Opacity sources include gaseous opacity from methane, ethane, acetylene, H2-H2, H2-He continua along with opacity from two aerosol layers: one optically thick cloud with a base at the 2-bar level and one optically thin haze layer with a base at 300 mbar. These layers are consistent with the putative H2S and CH4 clouds reported by many observational studies (eg [7,8]).

Simulations at radiative-equilibrium are discussed in a companion abstract [9] ; in this one we focus on dynamical aspects. We will present results from first 3D GCM simulations performed at a horizontal resolution up to 256x192 in longitude x latitude (corresponding to 1.4°x0.9°), extending from 3 bars to 0.3 mbar. A broad equatorial retrograde jet develop on both Uranus and Neptune and two prograde jets emerge near 50° latitude in the Neptune simulation. This is in qualitative agreement with the observed zonal wind structure on Neptune, although the zonal jet wind speeds are much smaller than the observed ones. We are able to show that acceleration by eddies is an important contributor to the two prograde jets in the Neptune simulation.

However, the Uranus simulation does not exhibit high-latitude prograde jets that have been reported by cloud-tracking observations. In other words, the zonal jet structure currently obtained in our simulations differs significantly between the two planets, which is puzzling and at odds with their qualitatively similar observed zonal wind structures. This might indicate that important processes governing the atmospheric circulation of ice giants is missing in our GCM.

Another outcome of these simulations is that all tropospheric zonal jets are slowed down to near zero wind speeds in the lower stratosphere. The reason behind this behaviour is under investigation, as is the associated meridional circulation.

Next steps will include the study of the role of Uranus and Neptune respective axial tilts and internal heat fluxes (or lack thereof) on their circulation. Furthermore, our GCM is still lacking important processes, such as latent heat release from water and other condensing species, and is lacking a realistic parametrization for convective processes. This might explain the observation-model mismatches in their zonal wind structure and will be the subject of future developments.

[1] Lian and Showman, Icarus, Vol. 207, Issue 1, p. 373-393, 2010.

[2] Liu and Schneider, Journal of the Atmospheric Sciences, Vol. 67, issue 11, pp. 3652-3672, 2010.

[3] Lebeau and Dowling, Icarus, Vol. 132, Issue 2, pp. 239-265, 1998.

[4] Hammel et al., Icarus, Vol. 201, Issue 1, p. 257-271, 2006.

[5] Spiga et al., Icarus, Vol. 335, article id. 113377, 2020.

[6] Guerlet et al., accepted in Icarus, 2020.

[7] Irwin et al., Icarus, Vol. 227, p. 37-48, 2014.

[8] Sromovsky et al., Icarus, Vol. 317, p. 266-306, 2018.

[9] Vatant d’Ollone et al., EPSC 2020

How to cite: Milcareck, G., Guerlet, S., Vatant d'Ollone, J., Spiga, A., and Millour, E.: First GCM simulations of Uranus and Neptune atmospheres with realistic radiative transfer, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-297, https://doi.org/10.5194/epsc2020-297, 2020.

Abstract

 This molecular ion H₃⁺ provides a really powerful tool with which we can remotely measure the ionospheric density and thermospheric temperature of the gas giants using near-infrared ground-based telescopes. These measurements show that the upper atmosphere of all four giant planets in our solar system are much hotter than solar input along can produce - this is known as the ‘energy crisis’ - and it is currently unclear from where this additional energy is sourced from. The two major candidates are the re-distribution of auroral Joule heating and the breaking of gravity waves generated in the turbulent lower atmosphere. Emission from H₃⁺ has been observed from Uranus on a semi-regular intervals since 1992. We have previously shown that the upper atmosphere is subject to a remarkable long-term cooling, and has cooled from ~750 K in 1992 to 500 K in 2018, at a rate of ~8 K/year. This has important implications for the drivers of the elevated temperatures in the upper atmosphere, and may be related to the ever changing geometry between the offset magnetic field of Uranus the incoming  solar-wind. Here, we present the latest observations of H₃⁺ emission from Uranus, obtained using the iSHELL instrument at the NASA Infrared Telescope Facility (IRTF) in October and November 2019. We explore any potential changes in the observed cooling, and discuss how these new observations fit within our current theories  of how the upper atmosphere of Uranus is heated.

 Figure 1: The long-term temperature (a) and density (b) evolution of the upper atmosphere of Uranus between 1992 and 2018. 

How to cite: Melin, H., Fletcher, L., Stallard, T., Trafton, L., Thomas, E., Moore, L., Chowdhury, N., and O'Donoghue, J.: The long-term cooling of the upper atmosphere of Uranus , Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-268, https://doi.org/10.5194/epsc2020-268, 2020.

Within the solar system, X-ray emissions have been detected from every planet except the Ice Giants: Uranus and Neptune. Here, we present three Chandra X-ray Observations of Uranus (each 24-30 ks duration): an Advanced CCD Imaging Spectrometer (ACIS) observation during solar maximum on 7 August 2002 and two High Resolution Camera (HRC) observations during solar minimum on 11 and 12 November 2017. The ACIS observation from 2002 shows a low signal but statistically significant detection of X-rays from Uranus. The measured Uranus X-ray fluxes of 10^-15-10^-16 erg/cm²/s from this detection are consistent with upper limits and modelling predictions in previous work (Ness & Schmidt. 2000; Cravens et al. 2006).  The photon energy distribution from this observation is consistent with an X-ray emission from charge exchange or scattering of solar photons, as observed for Jupiter and Saturn. The two HRC observations from 2017 constitute non-detections. For 11 Nov 2017, the X-ray emission coincident with Uranus’ location is dimmer than 98% of the Field of View. 12 November 2017, was also a non-detection, but with tentative hints of planetary X-ray signatures: Uranus was 4 times brighter than the previous day, and brighter than 94% of the Field of View (1.6 standard deviations > Field of View mean). At this time, the Uranus coincident X-ray signature also exhibited timing variation distinct from the field of view. Further and longer observations will be required to better characterise the nature of the X-ray emissions from Uranus.

How to cite: Dunn, W., Ness, J.-U., Lamy, L., Tremblay, G., Branduard-Raymont, G., Snios, B., Kraft, R., Wibisono, A., and Yao, Z.: In Search of X-rays from Uranus, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-1028, https://doi.org/10.5194/epsc2020-1028, 2020.

How to cite: Lamy, L., Cecconi, B., and Dekkali, M. and the LESIA plasma physics group: A modern digital High Frequency Receiver to explore Uranus and Neptune radio emitters, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-941, https://doi.org/10.5194/epsc2020-941, 2020.

The comparative study of the different planetary systems accessible to our observations is a unique source of new scientific insight: it can reveal to us the diversity of these systems and of the objects within them, help us better understand their origins and how they work, find and characterize habitable worlds, and ultimately, search for alien life in our galactic neighborhood. But, in the solar system itself, two of its secondary planetary systems, the ice giant systems, as well as their two main objects, Uranus and Neptune, remain poorly explored. We will present an analysis of our current limited knowledge of these systems in the light of six key science questions about planetary systems formulated in the “Planetary Exploration, Horizon 2061” long-term foresight exercise: (Q1) What is the diversity of planetary systems objects? (Q2) What is the diversity of their architectures? (Q3) What do we know of their origins and formation scenarios? (Q4) How do they work? (Q5) Do they host potential habitats? (Q6) Where and how to search for life?

We will show that a long-term plan for the space exploration of ice giants and their systems, complemented by the combination of Earth and space-based observations, will provide major contributions to answers to these six questions. In order to do so, we identify the measurements that must be performed in priority to address each of these questions, the destinations to choose (Uranus, Neptune, Triton or a subset of them), and the combinations of space platform(s) (orbiter, atmospheric entry probe(s), lander…) and of  flight sequences needed.

Based on this analysis, we look at the different launch windows available until 2061, using a Jupiter fly-by, to send a mission to Uranus or Neptune and find that:

(1) a single mission to one of the Ice giants, combining an atmospheric entry probe and an orbiter tour starting on a high-inclination, low-periapse orbit, followed by a sequence of lower- inclination orbits, at least at one of the planets, will make it possible to address a broad range of these key questions;

(2) a combination of two well-designed missions to each of the ice giant systems, to be flown in parallel or in sequence, will make it possible to address five out of the six key questions, and to establish the prerequisites for addressing the sixth one. The 2032 Jupiter fly-by window offers a unique opportunity to achieve this goal;

(3) if this window cannot be met, using the 2036 Jupiter fly-by window to send a mission to Uranus first, and then the 2045 window for a mission to Neptune, will achieve the same goals. As a back-up option, the feasibility of sending an orbiter + probe mission to one of the planets and using the opportunity of a mission on its way to the interstellar medium to execute a close fly-by of the other planet and deliver a probe into its atmosphere should be studied carefully;

(4) based on the expected science return of the first two missions, a third mission focusing on the search for life at a promising moon, namely Triton based on our current knowledge, or perhaps one of the active moons of Uranus after due characterization, can be properly designed.

By the 2061 horizon, the first two missions of this plan can be implemented and the design of a third mission focusing on the search for life can be consolidated. Given the likelihood that such a plan may be out of reach of a single national agency, international collaboration is the most promising way to implement it.

How to cite: Blanc, M., Mandt, K., Mousis, O., Andre, N., Bouquet, A., Charnoz, S., Craft, K., Deleuil, M., Griton, L., Helled, R., Hueso, R., Lamy, L., Lunine, J., Ronnet, T., Schmidt, J., Soderlund, K., Turrini, D., Turtle, E., Vernazza, P., and Witasse, O.: Science Goals and Mission Objectives for the Future Exploration of Ice Giants Systems - A Horizon 2061 Perspective, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-984, https://doi.org/10.5194/epsc2020-984, 2020.