The science case for an orbital mission to Uranus: Exploring the origins and evolution of ice giant planets
Introduction
Giant planets account for more than 99% of the mass of the Sun’s planetary system, and helped to shape the conditions we see in the Solar System today. The Ice Giants (Uranus and Neptune) are fundamentally different from the Gas Giants (Jupiter and Saturn) in a number of ways and Uranus in particular is the most challenging to our understanding of planetary formation and evolution (e.g., Lissauer, 2005, Dodson-Robinson and Bodenheimer, 2010). Our Solar System provides the only local laboratory in which we can perform studies that help us to understand the nature of planetary systems in general. The fact that Kepler observations have shown that Uranus/Neptune class planets are a common class of exoplanet (Fressin et al., 2013) makes it all the more timely and compelling to better explore these fascinating systems.
The Ice Giants are fundamentally different from the Gas Giants (Jupiter and Saturn) in a number of ways and Uranus in particular is the most challenging to our understanding of planetary formation and evolution, with its puzzling interior structure, unclear energy balance and internal energy transport mechanisms, and its high obliquity. Yet our exploration of the Ice Giants in our own Solar System remains incomplete, with several fundamental questions unanswered. Voyager 2 remains the only spacecraft to have returned data from the uranian environment, see for example papers in Science 233(4759) from the Voyager 2 Uranus encounter, with an introduction given by Stone and Miner (1986), and the current authoritative book on the Voyager 2 encounter science (Matthews et al., 1991).
A mission to Uranus will provide observations and measurements that are vital for understanding the origin and evolution of Uranus as an Ice Giant planet, answer the fundamental question of why some giant planets become icy and other so gas rich, and provide a missing link between our Solar System and planets around other stars. Observations of Uranus’ rings and satellite system will also bring new perspective on the origin of giant planet systems and will help validate the models proposed for the origin and evolution of Jupiter’s and Saturn’s systems. The cruise phase will also offer the possibility of testing the law of gravitation in a dynamic environment, still poorly probed, and study the outer heliosphere and its connection to the Sun. Such a mission to the uranian system would open a new window on the origin and evolution of the Solar System and directly addresses two of European Space Agency׳s (ESA) Cosmic Vision themes “What are the conditions for Planet Formation and the Emergence of Life?” and “How Does the Solar System Work?”. The fundamental processes occurring within the uranian system confirm that the exploration of Uranus is essential in meeting ESA׳s Cosmic Vision goals. A mission to Uranus is also highlighted in the NASA Planetary and Heliophysics Decadal Surveys (Squyres et al., 2011, Baker et al., 2013).
In 2013 ESA issued a call for science themes for its large-class (L-class) mission programme. This paper represents the white paper on the scientific case for the exploration of Uranus that was submitted to this call (a compilation of these white papers can be found at http://sci.esa.int/science-e/www/object/doc.cfm?fobjectid=52029) and looks forward to future missions. This white paper followed the Uranus Pathfinder mission proposal that was submitted to ESA׳s medium-class (M-class) mission programme in 2010 which is described in Arridge et al. (2012). In September 2013 a Uranus-focused workshop “Uranus beyond Voyager 2, from recent advances to future missions”, was held at the Observatory of Paris (Meudon, France) and was attended by 90 scientists and engineers from 12 countries, interested in the scientific exploration of this unique planetary system (the detailed programme, abstracts and lists of participants is available at http://uranus.sciencesconf.org).
In Section 2 of this paper the science case for a Uranus mission is presented and arranged into three key themes: (1) Uranus as an Ice Giant Planet, (2) An Ice Giant Planetary System, and (3) Uranus׳ Aeronomy, Aurorae and Highly Asymmetrical Magnetosphere. In addition, a mission to Uranus naturally provides a unique opportunity to study the outer heliosphere, fundamental gravitational physics, and Solar System bodies such as Centaurs near the orbit of Uranus and so in this paper we also describe the science case associated with a cruise phase in the outer Solar System. The short mission concept that was described in the white paper is presented in Section 3 along with a discussion of the critical enabling technologies.
Section snippets
Uranus as an ice giant planet: The interior and atmosphere of Uranus
Fig. 1 indicates the bulk composition for various Solar System objects and shows how different the Ice Giants are from the Gas Giants. Jupiter is an H/He planet with an ice and rock mass fraction of 4–12% as inferred from standard interior models (Saumon and Guillot, 2004). Uranus and Neptune seem to consist mostly of ices and rocks, but current observations are only able to provide an upper limit of 85% on the ice and rock mass fraction (Fortney and Nettelmann, 2010). The self-luminosity of
Strawman mission concept
In terms of mission options, the primary trade space is between an orbiter and a flyby mission. Some goals can be partially satisfied with a flyby mission but to fully answer the questions laid out in Section 2 requires an orbiting platform to make repeated observations of Uranus and its planetary system. There exists an additional trade space between enhanced remote sensing instrumentation and an entry probe. But some science questions (2.1.1 What is the internal structure and composition of
Acknowledgements
CSA and LNF were supported by Royal Society University Research Fellowships. C.S.A. thanks O. Bedworth, B. Jacobson, and J.-P. Lebreton for useful discussions and comments on the manuscript.
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