Future exploration of the ice giants

The Ice Giants, Uranus and Neptune, have never had a dedicated robotic explorer. What little we know of these worlds comes from a brief flyby more than three decades ago, combined with challenging remote observations from ground- and space-based facilities. Missions during the first decades of the 21st century have returned to Jupiter (Galileo, Juno, and JUICE) and Saturn (Cassini), meaning that a journey to the “frozen frontier” is the next logical step in our exploration of the Solar System. This meeting occurs at a crucial moment in our quest for missions to Uranus and/or Neptune – Ice Giant science may become a cornerstone of both ESA’s Voyage 2035-2050 programme, and the US planetary decadal survey beyond 2022. Meanwhile, celestial mechanics moves Jupiter into the perfect location for gravity-assist trajectories to Uranus and Neptune in the 2029-34 period. If we are to rise to the technological and scientific challenge of exploring these remote worlds, we must continue the spirit of international collaboration, and raise the maturity of our ambitious mission concepts. This three-day workshop will review our knowledge of the Ice Giants at the start of the 2020s, and hopes to set in motion a coordinated international effort to realise an ambitious mission to these worlds before the decade is out. Such a paradigm-shifting mission would define Solar System exploration for a whole generation of planetary scientists, just as Cassini did during the previous decade.

There are many open questions regarding the nature of Uranus and Neptune, the outermost planets in our Solar System. This presentation will briefly summarise the current-knowledge about Uranus and Neptune with a focus on their formation, thermal evolution, and internal structure. According to standard planet formation theories, the masses of Uranus and Neptune lie in a regime where planets are expected to accrete gas rapidly and become giant planets. Therefore, the termination of gas accretion at the mass of the ice giants needs to be explained. This presentation will present various formation scenarios that can explain their observed properties and discuss the connection to planet formation theory and for our understanding of exoplanets. It will also briefly discuss the possibility and outcomes of giant impacts on Uranus and Neptune and the connection to their observed differences. Finally, it will present non-standard internal structure models of the planets, suggesting that they are likely to have non-adiabatic interiors with no-distinct layers, and question whether they are indeed “icy" planets.

The upper atmosphere of a giant planet plays an important role as the interface layer between the lower atmosphere below and the magnetosphere beyond. It has two basic components: the neutral thermosphere and the charged particle ionosphere. The ionosphere senses the magnetic field whilst the thermosphere is affected by dynamical processes in the lower atmosphere. Understanding momentum transfer between these systems becomes critical in understanding the energy balance of the atmosphere and magnetosphere as a coupled system. The ionosphere can be sensed remotely from the ground via spectral observations of the molecular ion H₃⁺ which reveal the temperature of the upper atmosphere and ion density of this region. H₃⁺ was discovered at Uranus in 1992 and we have a 27 year baseline of observations that reveal that the upper atmosphere is subject to long-term cooling that may be related to the extreme seasons of Uranus. However, the 2007 equinox did not offer a reversal in this trend, and as of 2018 continued cooling has been observed. Surprisingly, H₃⁺ remains undetected at Neptune, with the upper limit of the ionospheric density being much lower than predicted by models. Finally, Dr Melin will discuss how future facilities, including dedicated spacecraft missions, can enhance our understanding of the ionosphere of Uranus and Neptune.

The magnetic fields of Uranus and Neptune are non-axisymmetric and multipolar with quadrupole and octupole components that are comparable to or greater than the dipole. At present, spherical harmonics greater than degree four are below the limits of spatial resolution, and there is no information about any secular variations. Because of these unique characteristics, the ice giants serve as excellent laboratories for determining the fundamental dynamical and chemical processes responsible for generating all planetary magnetic fields.

Magnetic fields originate in the electrically conducting fluid regions of planetary bodies and likely result from convectively driven dynamo action. Consequently, an understanding of the dynamo processes that control the magnetic field strength, morphology, and temporal evolution of ice giant planets is critically dependent on their poorly constrained interior structures, bulk compositions, heat balance, and dynamics. It is typically assumed that the magnetic fields are generated in the planets’ “watery” oceans. Moreover, if the transition between the watery ocean and the overlying atmosphere is continuous, these two regions may be dynamically coupled, linking the dynamo to atmospheric dynamics and its thermal emissions.

A variety of competing numerical dynamo models have been developed to explain the ice giants’ magnetic fields. Additional constraints derived from a second mission to Uranus and/or Neptune would test these hypotheses, aid in development of more realistic models, and yield better predictions about the evolution of planetary magnetic fields.

Uranus and Neptune possess internal magnetic fields with similar characteristics (largely tilted, offset and multipolar) which interact with the solar wind form to ‘twin’  asymmetric magnetospheres unique in the solar system. Our current knowledge of their auroral processes, and of the underlying solar wind-magnetosphere-ionosphere interaction, still mostly relies on Voyager 2 radio/UV and in situ observations obtained during the flyby of each planet, respectively in 1986 and in 1989. These observations were acquired at epochs corresponding to specific solar wind/magnetosphere configurations, expected to vary significantly for both planets along their revolution around the Sun. Fortunately, while waiting for future in depth exploration missions of ice giant planets, some additional clues were in-between obtained from remote long-term Earth-based UV/IR observations which sampled different seasons and reveal strongly different solar wind/magnetosphere interactions.

In two years, the US Planetary Science Decadal Survey will lay out its vision for the next phase of NASA's activities in the Solar System. Part of that vision will define the first dedicated mission to an Ice Giant planet. The Survey could call for a Flagship-class mission that explores all aspects of an ice giant system. Alternatively, by not prioritizing Ice Giants, it might set the stage for a small Discovery-class competed mission that focuses on more limited objectives. Or perhaps it will lay out a path in between those extremes or one not yet considered. While we do not know what direction will be taken, the importance of Ice Giants to the scientific community has already triggered extensive work to define the broad parameters of possible missions, and to explore ways in which other space agencies could contribute to a NASA-led mission. This talk will review recent and ongoing mission studies and preparations for the Decadal Survey, and discuss some of the programmatic and science trade-offs likely to factor in to the Survey's deliberations. 

This talk will include a pre-recorded presentation from Dr Lori Glaze, Director of NASA's Planetary Science Division.