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Future exploration of the ice giants

Discussion meeting

Starts:

January
202020

09:00

Ends:

January
212020

17:00

Location

The Royal Society, London, 6-9 Carlton House Terrace, London, SW1Y 5AG

Overview

Scientific discussion meeting organised by Dr Leigh Fletcher, Dr Adam Masters, Dr Athena Coustenis, Dr Kathleen Mandt, Dr Ian Cohen, Dr Christopher Arridge and Dr Amy Simon.

Neptune viewed from beyond the orbit of its largest moon, Triton

Uranus and Neptune are our closest representatives of a class of planet that may be commonplace in our universe, and yet our exploration and understanding of these icy worlds is in its infancy. This meeting aimed to shape the key questions, motivations, and concepts for future collaborative missions to these tantalising destinations.

Recorded audio of the presentations will be available on this page soon. Meeting papers will be published in a future issue of Philosophical Transactions of the Royal Society A.

Enquiries: contact the Scientific Programmes team

Splinter meeting

A splinter meeting was held following this meeting, details of which can be found on the organisers' website.

Event organisers

Select an organiser for more information

Schedule of talks

20 January

Session 1 09:00-12:30

Ice giant planets

9 talks Show detail Hide detail

Chairs

Dr Leigh Fletcher, University of Leicester, UK

09:00-09:05 Welcome by the Royal Society and Leigh Fletcher

Abstract

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.

09:05-09:15 Meeting objectives

Dr Leigh Fletcher, University of Leicester, UK
Dr Amy Simon, NASA Goddard Spaceflight Center, USA
Dr Mark Hofstadter, JPL/Caltech, USA

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09:15-09:45 Origin, evolution, and internal structure of the ice giants

Professor Ravit Helled, University of Zurich, Switzerland

Abstract

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.

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09:45-10:15 Interior structure and energy balance of Jupiter and Saturn

Dr Jonathan Fortney, University of California, Santa Cruz, USA

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10:15-10:30 Discussion - interiors objectives

10:30-11:00 Coffee break

11:00-11:25 Atmospheric dynamics and cloud structure of the ice giants

Dr Ricardo Hueso, University of Bilbao, Spain

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11:25-11:50 Photochemistry in the atmospheres of Uranus and Neptune

Dr Julianne Moses, Space Science Institute, USA

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11:50-12:15 The upper atmospheres of the ice giants

Dr Henrik Melin, University of Leicester, UK

Abstract

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, we discuss how future facilities, including dedicated spacecraft missions, can enhance our understanding of the ionosphere of Uranus and Neptune.

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12:15-12:30 Discussion 2 - Atmospheric objectives

Session 2 13:30-17:00

Ice giant systems

7 talks Show detail Hide detail

Chairs

Dr Amy Simon, NASA Goddard Spaceflight Center, USA

13:30-13:55 Dynamos of ice giant planets

Dr Krista Soderlund, University of Texas at Austin, USA

Abstract

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.

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13:55-14:20 Ice giant magnetospheres

Dr Carol Paty, University of Oregon, USA

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14:20-14:45 Ice giant aurora

Dr Laurent Lamy, Observatoire de Paris, PSL, CNRS, France

Abstract

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.

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14:45-15:00 Discussion - magnetic field objectives

15:00-15:30 Tea break

15:30-16:00 US Perspectives on ice giant missions

Dr Mark Hofstadter, JPL/Caltech, USA

Abstract

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.   

Show speakers

16:00-16:30 ESA perspectives on Ice Giant Missions

16:30-17:00 Discussion - individual agencies and mission proposals

21 January

Session 3 09:00-12:30

Cross-disciplinary perspectives

8 talks Show detail Hide detail

Chairs

Dr Mark Hofstadter, JPL/Caltech, USA

09:00-09:25 Lessons learned from (and since) the Voyager 2 flybys of Uranus and Neptune

Dr Heidi Hammel, Association of Universities for Research in Astronomy, USA

Abstract

More than 30 years have passed since the Voyager 2 fly-bys of Uranus and Neptune. What lessons can we glean from the intervening decades? I discuss a range of topics that can be broadly grouped into process, planning, and people. PROCESS. We must be open to new concepts. Reliance on existing instrument technologies, propulsion systems, and operational modes is inherently limiting. I’ll cite examples of each during recent decades that could open new vistas in exploration of the deep outer Solar System. PLANNING. Mission gaps stretching over three decades leaves much scope for evolution both in mission development and in the targets themselves. I touch only briefly on the planetary system science, as that will likely be covered in other talks. The role of the decadal surveys will be examined in this section. I also sketch out how coordinating distinct and divergent international planning time-lines yields both challenges and opportunity. PEOPLE. Finally, with generational length gaps between missions, ensuring continuity in knowledge and skills requires careful attention to people. As the youngest participants in the Voyager missions (myself included) near retirement age, we and our elders reflect on lessons learned to pass on to the next generations. In preparation for this talk, I reached out to fellow Voyager alums for their lesson ideas for preparing the next generation of voyagers. Some lesson titles include: flexibility (embrace your inner naïveté); empower the youth; to build a cathedral; and outreach matters.

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09:25-09:50 Why Exoplanets need an Ice Giant Mission

Dr Hannah Wakeford, University of Bristol, UK

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09:50-10:15 Cross-NASA divisional relevance of an ice giant mission

Dr Abigail Rymer, JHU-APL, Maryland, USA

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10:15-10:30 Discussion - summary of knowledge gaps

10:30-11:00 Coffee

11:00-11:25 The rings and inner satellites of Uranus and Neptune

Dr Mark Showalter, SETI Institute, USA

Abstract

All four giant planets are encircled by distinctive systems of rings and small, inner satellites. These all reside within or near their central planet's Roche limit, the rough boundary within which bodies held together by self-gravity will be disrupted by tidal forces. However, the similarities of the four ring-moon systems end here; in most other regards, they are remarkably diverse. We study these systems for three key reasons: (1) for the information they reveal about the properties, history, and ongoing evolution of the planetary systems of which they are a part; (2) as dynamical analogues for other astrophysical systems such as protoplanetary disks; and (3) for the wealth of fascinating properties and origin scenarios that make them worthy of study in their own right. The inner Uranus system is characterized by ten narrow rings, some quite dense, as well as a variety of more tenuous structures. These are accompanied by thirteen known moons all orbiting interior to Miranda. Nine of these, Bianca through Perdita, comprise the most densely-packed set of moons in the solar system, with orbits so close that their interactions appear to drive chaos over time scales as short as a million years. Neptune has five named rings, all optically thin, interleaved with seven inner moons. The most notable feature is a set of arcs embedded within the Adams ring; two of these have been stable for time scales of decades. This presentation will provide a brief overview of the state of our knowledge and discuss the key scientific questions related to these systems.

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11:25-11:50 The Uranian satellite system

Dr Elizabeth Turtle, JHU-APL, Maryland, USA

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11:50-12:15 Triton and the Kuiper Belt connection

Dr Michele Bannister, Queen's University Belfast, UK

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12:15-12:30 Discussion - Satellite/rings objectives

Session 4 13:30-17:00

Enabling technologies

6 talks Show detail Hide detail

Chairs

Dr Adam Masters, Imperial College London, UK

13:30-13:55 Mission design prospects for the ice giants

John Elliot, JPL, USA

Abstract

The Ice Giants, Uranus and Neptune, are destinations that have thus far been unexplored by orbiter missions.  While trajectory and mission design work for prospective missions to these bodies has been performed for decades, new trajectory analysis tools and techniques have significantly improved our ability to evaluate a broad tradespace of potential architectures. This new capability was amply demonstrated in NASA’s recent Ice Giants Pre-Decadal Survey mission study.  The study included a broad assessment of mission design options that could enable significant science at these destinations, including dependencies on launch years, mission duration, delivered mass, etc., all of which can be used to inform trades when developing a mission concept.  This talk will address the broad findings of this mission design evaluation, as well as illustrating its impact on the four missions chosen for point design study in the report.  In addition, the results of a new study will be discussed in which the impact of incorporation of a number of new technologies was investigated to shape a high-performance Ice Giants mission.  A particular focus of this recent study was the use of drag-modulated aerocapture, a technology that was not previously incorporated in the point design options developed in the earlier Pre-Decadal study.

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13:55-14:20 Enabling technologies for ice planet exploration

Dr Thomas R Spilker, Independent Consultant, USA

Abstract

Four challenges are common to space flight missions to the outer solar system, especially to the ice giant planets: providing electric power, providing telecommunications of sufficient performance between Earth and the spacecraft, handling the thermal environment, and if the mission involves inserting into orbit at the destination, providing sufficient propulsive capability for that orbit insertion and a useful orbital tour afterward. In addition to communications challenges, missions including atmospheric entry probes involve the challenge of extreme hypersonic atmospheric entry. Fitting the responses to these challenges within the tight time and mass constraints of travel to the farthest reaches of our planetary system is a difficult engineering task involving advanced technologies and difficult trade decisions. This presentation will review appropriate technology options for addressing these challenges, including performance characteristics that influence their applicability envelopes.

Optimizing an ice giant mission's science return can require considering some clever and non-intuitive trajectory options. Both ice giants have unusual fundamental characteristics that heavily influence orbit insertion and tour designs. The Uranian system is much like a smaller-scale Jovian system, but Uranus's 98° obliquity makes orbit insertion and transition to a gravity-assist-driven orbital tour very different from a Jupiter mission, and very seasonal. The Neptunian system's single large satellite, Triton, makes it more like the Saturnian system - except that Triton's orbit is retrograde, making a tour there quite different from a Saturnian tour. This presentation will also discuss trajectory design options at both ice giants.

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14:20-14:45 The development of European radioisotope space nuclear power systems

Dr Richard Ambrosi, University of Leicester, USA

Abstract

Radioisotope power systems (RPS) have been in development in Europe as part of a European Space Agency funded programme since 2009. The programme is focused on developing all of the building blocks that will enable Europe to launch and operate RPS powered missions in deep space and in challenging environments on planetary surfaces. The maturity of the technology has now reached a level that it can be incorporated in mission studies targeting the period from the mid 2020s onwards. This presentation describes the state of the art in European radioisotope thermoelectric generators (RTGs) and radioisotope heater units (RHUs).

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14:45-15:00 Discussion - technologies

15:00-15:30 Tea

15:30-16:00 Strategy for coordination 2020+

Dr Amy Simon, NASA Goddard Spaceflight Center, USA
Dr Mark Hofstadter, JPL/Caltech, USA

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16:00-17:00 Panel discussion

Future exploration of the ice giants

20 - 21 January 2020

The Royal Society, London 6-9 Carlton House Terrace London SW1Y 5AG UK
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