Ice Giant Systems: New Results and Future Exploration
MSO: Leigh Fletcher (Univ. of Leicester)
DSO: Amy Simon (Goddard Spaceflight Center)
COSPAR sub-commission B5 hosted several symposia focussing on the exploration of the outer solar system at the 2018 COSPAR meeting in Pasadena, ranging from the highlights of past missions (Cassini), active missions (Juno), and future missions (exploration of ocean worlds). The ice giant community met on Wednesday afternoon to focus on the exploration of Uranus and Neptune, from their interiors, to their atmospheres, magnetospheres, rings and satellite systems. Unusual for the COSPAR symposia, the organisers split the time between standard oral presentations and a workshop-style discussion forum on international collaboration for a future ice giant mission. Both sections were well received and fostered interesting discussions between conference delegates for the remainder of the COSPAR meeting.
New Scientific Results
The ice giants, Uranus and Neptune, have been visited only once by a robotic spacecraft (Voyager 2 in 1986 and 1989, respectively). Many of the most recent insights into how these unusual systems work are therefore the result of Earth-based observations, both from ground-based observatories and space telescopes. Hoftstadter et al. contrasted radio-wave observations of Saturn and Uranus from facilities like the Very Large Array (VLA), explaining how spectral models are used to explore the deep abundances of ammonia, hydrogen sulphide and (potentially) water within these worlds. The existing Uranus data sensing the atmospheric composition can be explained by approximately solar abundances of these heavy materials, raising the question of how the materials might be trapped within their deeper interiors. Wong et al. described Hubble Space Telescope observations of a new dark oval that formed on Neptune in 2015, not dissimilar from the Great Dark Spot observed by Voyager in 1989. This oval has been re-observed several times since 2015 as it drifted southwards, whereas most drift equatorward. Its bright companion clouds have become more centred within the vortex, and Wong’s team plans to track its continued evolution in the coming years.
Observations at mid-infrared wavelengths, sensing the atmospheric temperatures and composition of Uranus and Neptune, featured significantly for the rest of the symposium, largely as a result of preparation for the expected slew of data from the James Webb Space Telescope, due for launch in 2021. Orton et al. presented images of Uranus at wavelengths sensing stratospheric emission from acetylene gas. This provides a capability inaccessible to Voyager – the ability to probe the circulation of Uranus’ stratosphere. The results, from the Very Large Telescope (VLT) and Gemini Observatory, indicate an unusual circulation pattern that localises the stratospheric emission to Uranus’ poles at the time of the 2007 equinox. Fletcher et al. followed this up in a subsequent talk using spatially-resolved mid-infrared spectroscopy of Uranus, which is extremely challenging from Earth but whets the appetite for future data from the MIRI 5-28 µm instrument on JWST, which will provide the first
spatially-resolved spectra at these wavelengths. Fletcher also presented VLT observations of Neptune, characterising stratospheric temperatures within its warm polar vortex during Neptune’s southern summertime conditions. Sinclair et al. presented further stratospheric imaging of Neptune from the VLT, correlating diffuse warm regions in the stratosphere with cloud activity observed in the troposphere. Finally, a poster by Rowe-Gurney et al. explored longitudinal variability observed in Spitzer Space Telescope disc-averaged spectra of Uranus, but not Neptune, hinting at unexpected dynamic variability on a world usually thought of as stagnant and inactive.
The scientific presentations continued with Masters et al. explaining how the processes governing ice giant magnetospheres might be rather different from those at work on the gas giants, presenting models indicating the important role of a viscous-like interaction between the solar wind and the magnetosphere. Kirchoff et al. contrasted impact cratering size distributions on the Uranian satellites with those on Jupiter and Saturn, advocating future exploration of these terrains. And Mandt et al. concluded the scientific discussion by exploring how measurements of the deuterium and nitrogen content of Triton’s atmosphere could prove crucial in understanding the origin of volatiles on distant worlds, including Pluto.
The final hour of the symposium shifted into a discussion of future international collaboration on missions to the ice giants, hosted by Hoftstadter, Simon and Fletcher. Hofstadter reported the primary outcomes of the 2016-17 NASA-ESA Science Definition Team study, which concluded in 2017 with an extensive report (https://www.lpi.usra.edu/icegiants/mission_study/). Simon described a recent white paper advocating a two-mission concept, combining a Uranus flyby with a KBO mission, alongside a dedicated Neptune-Triton orbital mission (https://arxiv.org/abs/1807.08769). It was stressed that this was one of many potential ice giant exploration strategies, and concepts for Uranus orbital exploration were also discussed. There was discussion of the criticality of planetary entry probes (https://arxiv.org/abs/1708.00235), combined with orbital remote sensing in the infrared and microwave, in order to explore the chemical composition of Uranus and Neptune for comparisons with other targets in the solar system.
Several ideas were raised during the wide-ranging discussion, including the need to identify and develop key enabling technologies for future ice giant missions, and how to ensure that ice giant science remained at the top of the agenda in the US Decadal Survey, ESA’s Cosmic Vision, and within the sites of other space agencies. Particular attention focussed on how to open up the US medium-class missions (New Frontiers) to allow for ice giant exploration and on starting a new large-class mission before the next Decadal review. Given the breadth of ideas, and the timescale for the next US decadal and future potential ESA studies, it was suggested that a dedicated ice giant workshop be held in 2019, to be used as a focal point for the development of topical white papers. The discussion section was warmly received, and we would encourage similar events be worked into the programme for future COSPAR symposia.