(JUICE) an ESA Mission to Orbit Ganymede and to Characterise the Jupiter System

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Planetary and Space Science 78 (2013) 1–21

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Planetary and Space Science

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Planetary Pioneers Series JUpiter ICy moons Explorer (JUICE): An ESA mission to orbit Ganymede and to characterise the Jupiter system

O. Grasset a,n, M.K. Dougherty b,n, A. Coustenis c, E.J. Bunce d, C. Erd e, D. Titov e, M. Blanc f, A. Coates g, P. Drossart c, L.N. Fletcher h, H. Hussmann i, R. Jaumann i, N. Krupp j, J.-P. Lebreton k, O. Prieto-Ballesteros l, P. Tortora m, F. Tosi n, T. Van Hoolst o a Planetology ad Geodynamics, University of Nantes, CNRS, France b Imperial College, United Kingdom c LESIA-Observatoire de Paris, CNRS, UPMC Univ Paris 06, Univ. Paris-Diderot, France d University of Leicester, United Kingdom e ESA/ESTEC, Netherlands f IRAP—Observatoire Midi-Pyreńeés, France g University College London, United Kingdom h Atmospheric, Oceanic and Planetary Physics, University of Oxford, Clarendon Lab., Oxford, OX1 3PU, United Kingdom i DLR, Institute of Planetary Research, Germany j Max-Planck-Institut fur¨ Sonnensystemforschung, 37191 Katlenburg-Lindau, Germany k LPC2E, CNRS Orleáns, France l Centro de Astrobiologıá-INTA-CSIC, Spain m Universityof Bologna, Italy n National Institute for Astrophysics, Institute for Space Astrophysics and Planetology, Italy o Roy. Obs. of Belgium, Belgium article info abstract

Article history: Past exploration of Jupiter’s diverse satellite system has forever changed our understanding of the Received 6 September 2012 unique environments to be found around gas giants, both in our solar system and beyond. The detailed Received in revised form investigation of three of Jupiter’s Galilean satellites (Ganymede, Europa, and Callisto), which are 7 December 2012 believed to harbour subsurface water oceans, is central to elucidating the conditions for habitability of Accepted 7 December 2012 icy worlds in planetary systems in general. The study of the Jupiter system and the possible existence of Available online 19 December 2012 habitable environments offer the best opportunity for understanding the origins and formation of the Keywords: gas giants and their satellite systems. The JUpiter ICy moons Explorer (JUICE) mission, selected by ESA Space exploration in May 2012 to be the ﬁrst large mission within the Cosmic Vision Program 2015–2025, will perform Ganymede detailed investigations of Jupiter and its system in all their inter-relations and complexity with Jupiter particular emphasis on Ganymede as a planetary body and potential habitat. The investigations of the Europa Callisto neighbouring moons, Europa and Callisto, will complete a comparative picture of the Galilean moons Jovian system and their potential habitability. Here we describe the scientiﬁc motivation for this exciting new European-led exploration of the Jupiter system in the context of our current knowledge and future aspirations for exploration, and the paradigm it will bring in the study of giant (exo) planets in general. & 2012 Elsevier Ltd. All rights reserved.

Contents

1. Introduction ...... 2 2. Context and science themes ...... 2 3. Investigation of habitable worlds beyond the snow line ...... 3 3.1. Ganymede as a planetary object and possible habitat ...... 3 3.1.1. The ocean and its relation to the deep interior ...... 4 3.1.2. Formation of surface features and search for past and present activity ...... 5

n Corresponding authors. Tel.: þ33251125469. E-mail addresses: [email protected] (O. Grasset), [email protected] (M.K. Dougherty).

3.1.3. Determine global composition, distribution and evolution of surface materials ...... 6 3.1.4. Local environment and its interaction with the Jovian magnetosphere...... 6 3.2. Europa’s recent active zones ...... 7 3.2.1. Recently active processes...... 7 3.2.2. Composition of the non-ice material, especially as related to habitability ...... 8 3.2.3. Searching for liquid water under the most active sites ...... 8 3.3. Callisto as a remnant of the early Jovian system ...... 8 3.3.1. Past activity ...... 8 3.3.2. The outer shells, including the ocean ...... 9 3.3.3. Composition of the non-ice material...... 9 4. Investigations of Jupiter’s environment: The planet, its magnetosphere and the interactions with the moons ...... 10 4.1. Jupiter, the giant planet and how to better understand it ...... 10 4.1.1. Atmospheric dynamics and circulation ...... 10 4.1.2. Composition and chemistry ...... 11 4.1.3. Vertical structure of the atmosphere and interior ...... 12 4.2. The Jovian magnetosphere...... 12 4.2.1. A fast magnetic rotator ...... 13 4.2.2. A giant accelerator ...... 13 4.2.3. Sources and sinks of magnetospheric plasma ...... 14 4.3. Coupling processes in the Jovian system ...... 14 4.3.1. Gravitational coupling—The Laplace resonance ...... 14 4.3.2. Magnetospheric coupling...... 14 4.4. The formation of the Jovian system ...... 15 5. The mission proﬁle of JUICE ...... 16 5.1. The Jupiter tour ...... 16 5.2. The Ganymede tour ...... 18 6. Conclusion ...... 19 Acknowledgements ...... 19 References ...... 19

1. Introduction in-depth study of the electromagnetic coupling processes between the magnetosphere, ionosphere and thermosphere. Aurora and radio The science incentive for the JUpiter ICy moons Explorer emissions and their response to the solar wind will be elucidated. The (JUICE) mission is to study the largest giant planet, its extensive moons’ interactions with the magnetosphere and the gravitational magnetosphere, its giant icy moon Ganymede, and to a lesser coupling and long-term tidal evolution of the Galilean satellites will extent Callisto and Europa, as well as the interactions occurring in be studied. the environment. The JUICE mission will perform: a detailed In Section 1, the context in which the JUICE mission has been characterisation of the ocean layers; a detection of putative designed and selected for implementation is described in detail, subsurface water reservoirs; a study of the Ganymede’s intrinsic especially regarding the consistency with the ESA Cosmic Vision magnetic field; topographical, geological and compositional map- 2015–2025 program. Section 2 is devoted to the exploration of the ping; an analysis of the physical properties of the icy crusts; the habitable zone, i.e., the three icy Galilean moons, with a special characterisation of the internal mass distribution; a study of the emphasis on Ganymede, around which JUICE will orbit in the final dynamics and evolution of the interiors; an investigation of the stages of the mission. Section 3 presents our current understanding moons’ exospheres/ionospheres. For Europa, where two targeted of the Jovian system, including the giant planet itself, its magneto- flybys are foreseen, the focus will be on the chemistry essential to sphere and the coupling processes at work. The objectives of the life, including organic molecules, and on understanding the JUICE mission in this context are described for each part of the formation of surface features and the composition of the non- system. Finally, the baseline mission profile, which has been water-ice material, leading to the identification and characterisa- conceived during the assessment phase of the project as a result tion of candidate sites for future in situ exploration. Furthermore, of the defined science objectives, is detailed in Section 4. JUICE should provide the first subsurface observations of this icy moon, including the first determination of the minimal thickness of the icy crust over the most recently active regions. 2. Context and science themes JUICE will study Jupiter’s circulation, meteorology, chemistry and structure from the cloud tops to the thermosphere over the There have been numerous ground and space-based observa- long 3 year duration of the mission. The giant planet is the best tions of the Jupiter and Saturn systems; flybys by the Pioneer, and closest example of a gas giant atmosphere, yet several key Voyager, Ulysses, Cassini and New Horizons spacecraft; the questions about the physicochemical processes at work on Jupiter orbital tour by the Galileo spacecraft at Jupiter; and the ongoing remain unresolved. JUICE observations will serve to enhance our orbital tour by the Cassini spacecraft at Saturn. Ground based understanding of this archetypal planet as a template for giant observing facilities (such as the Very Large Telescope) and Earth planets beyond our solar system. These observations will be orbiting telescopes (such as the Hubble Space Telescope) have attained over a sufficiently long temporal baseline to investigate enabled remote studies of the Jovian atmosphere and related evolving weather systems and the mechanisms transporting emissions. Galileo made new discoveries in the Jovian system, energy, momentum and material between the different layers. especially as concerns the four Galilean satellites, which have The focus in Jupiter’s magnetosphere will include an investigation driven the identification of the next generation of key scientific of the three dimensional properties of the magnetodisc and an questions. Many of these relate to our quest for a better O. Grasset et al. / Planetary and Space Science 78 (2013) 1–21 3 understanding of the Jupiter system as a whole; its components JUICE will also address in theme 1 the sub-theme ‘‘From gas and their interactions, their origin, formation, evolution, and, and dust to stars and planets’’ by studying the composition of ultimately, their habitability. Similar key outstanding science Jupiter and its satellites, which are essential in order to under- questions are resulting at the Saturn system from the NASA- stand the origin of the system and its relation to other regions of ESA-ASI Cassini–Huygens mission. These two missions clearly planet formation in our galaxy. From the analysis of the cratering demonstrate the need for orbiting spacecraft at the gas giant record on the satellites’ surfaces, it will provide constraints on the systems, in order to globally monitor and resolve spatial and surface ages, and the period of the ‘‘late heavy bombardment’’ of temporal variations. the early Solar System (Gomes et al., 2005). It will contribute The Cosmic Vision 2015–2025 call for proposals, issued in further to sub-theme ‘‘From exoplanets to biomarkers’’ by studying October 2005, describes the top-priority science questions that Jupiter and its potentially habitable satellite system as an analo- should be addressed by space missions in the coming decade, as gue to Jupiter-like planets and their as yet undetected satellite was done previously with the Horizon 2000 (1984) and Horizon systems around other stars. 2000 Plus (1994–1995) plans. The solar system and astronomy communities identified four themes to be the key science drivers for future missions in the 2015–2025 program: 3. Investigation of habitable worlds beyond the snow line

Theme 1: What are the conditions for planet formation and the Habitability is commonly understood as ‘‘the potential of an emergence of life? environment (past or present) to support life of any kind’’ (Steele Theme 2: How does the Solar System work? et al., 2008). The concept does not relate to whether life actually Theme 3: What are the fundamental physical laws of the exists, has existed, or could exist in the future. It refers instead to Universe? whether environmental conditions are available that could sup- Theme 4: How did the Universe originate and what is it made of? port life. The minimum requirement of habitability (e.g., Kasting et al., 1993) is the presence and stability of liquid water on a The JUICE mission was designed to address wide-ranging, planet or moon. Water is an abundant compound in our galaxy cross-disciplinary scientific questions at the heart of Europe’s and it can be found in many places, from cold dense molecular vision for planetary and space science, specifically focusing on the clouds to the innermost layers of hot circumstellar envelopes first two of these four themes. In May 2012, JUICE was selected as (e.g., Cernicharo and Crovisier 2005). However, life will probably the first large mission of the Cosmic Vision programme, due to be never spontaneously originate and evolve in bodies of pure water launched in 2022. Here, we will describe how JUICE will address because life also requires the supply of chemical blocks made of in depth the first two of the four themes of ESA’s Cosmic Vision (C, H, O, N, P, S) to drive biochemical reactions. Habitability programme. Within these themes, exploration priorities are therefore relies on the fulfilment of four conditions: liquid water, identified as being fully addressed by JUICE: elements (nutrients), energy (for the metabolism), and time Theme 1: Life and habitability in the Solar System: Explore in situ (stability of the system). The Galilean satellites provide a con- the surface and subsurface of the solid bodies in the Solar System ceptual basis within which new theories for understanding most likely to host – or have hosted – life. Explore the environmental habitability can be constructed (Fig. 1). Large satellites of gas conditions that make life possible. JUICE will fully address this goal giants, at orbits beyond the snowline, can contain a large amount by exploring the surface and subsurface of Ganymede (through of water up to 45% in mass (Schubert et al., 1996; Anderson et al., flybys and an orbital tour) and to a lesser extent Callisto (through 2001). Liquid water reservoirs have been proposed by now in the flybys), including their subsurface water oceans and their envir- interiors of several icy moons and in particular of Ganymede, onments in the Jupiter system. Measurements towards these Europa, and Callisto from geophysical models, based on Galileo science aims shall also be made for Europa with two planned observations. Here, tidal dissipation and radiogenic energy keep flybys. the water liquid (e.g., Spohn and Schubert, 2003; Hussmann et al., Theme 2: From the Sun to the edge of the Solar System: Study the 2006). Finally, icy and liquid layers cannot be solely constituted of plasma and magnetic field environment of the Sun, the Earth, the pure H2O. It is likely that salty materials such as salt hydrates are Jovian system (as a Solar System in miniature), and out to the trapped within the moons (Kargel et al., 2000). Many other heliopause where the solar wind meets the interstellar medium. compounds such as CO2 have been observed on the surfaces JUICE will address this sub-theme by studying the plasma and and may emerge from the deep interiors of the moons. This magnetic field environment in the Jovian system, as well as the discovery has changed the habitability paradigm and included magnetosphere of Ganymede. The radiation environment and its ‘‘deep habitats’’, i.e., the moons with habitability conditions below implications for habitability in particular will be investigated at sub-surface. Europa and Ganymede. The study of habitable worlds around gas giants will be Theme 2: Gaseous giants and their moons: Study Jupiter in situ, its addressed by JUICE, because it will constrain the volume of liquid atmosphere and internal structure. Giant planets with their rings, water in the Jovian system, realise an inventory of biologically diverse satellites and complex environments constitute systems that essential elements on the surfaces of the icy moons, and deter- play a key role in the evolution of planetary systems. JUICE will mine the magnitude of their transport among the moons which explore the atmosphere and environment of Jupiter as the arche- exchange material as a result of volcanism, sputtering, and type for giant planet systems. The broad wavelength coverage and impacts. JUICE should also investigate the effects of radiation on long temporal baseline of the mission will permit a long-term the detectability of surface organics. remote sensing investigation of the plethora of physical and chemical processes at work within the jovian system, comple- 3.1. Ganymede as a planetary object and possible habitat menting the more focused aims of the Juno mission (to arrive in 2016), which will study Jupiter’s internal structure and inner Voyager and Galileo data indicate that Ganymede possesses magnetosphere, but will not explore Jupiter’s lower latitudes nor important prerequisites to be considered habitable. Galileo’s the extensive satellite system. The Galilean satellites, along with detection of induced magnetic fields (Kivelson et al., 2000, Jupiter’s magnetosphere and atmosphere above the cloud tops, 2002) combined with imaged surface characteristics will be the focus of the JUICE mission. (Pappalardo et al., 2004) and thermal modelling of the moons’ 4 O. Grasset et al. / Planetary and Space Science 78 (2013) 1–21

Fig. 1. The habitable zone in our galaxy. Habitability should not be restricted to the planets and moons where liquid water may exist on the surface. A much larger domain exists beyond the snow-line, where very large liquid reservoirs can exist below the icy crusts of these bodies (Credits: Neal Powell, Imperial College, London). evolution (Spohn and Schubert, 2003), advocate the presence of 400 liquid water oceans below the icy crust. However, the depth and Melting curve composition of the ocean, as well as the dynamics and exchange 350 constraints Libration processes between the ocean and the deep interior or the upper 300 amplitude ice shell, remain unclear. Furthermore, it is unknown whether liquid water reservoirs or compositional boundaries exist in the 250 Magnetic induction shallow subsurface ice and how the dynamics of the outermost ice shell is related to geologic features and surface composition. In 200 addition, a tenuous neutral O2 atmosphere was detected with Hypothetical range column densities of (0.3–5) 1014 cm2 (Feldman et al., 2000) 150 compatible with

Ocean thickness (km) measurements probably created through sputtering from impinging energetic k2 (gravity) 100 and particles from the Jovian magnetosphere. The composition of the h2 (altimetry) atmosphere includes O, O2, and possibly ozone (O3)(Hall et al., 50 1998; Noll et al., 1996). Additional evidence of the oxygen atmo- 0 sphere comes from spectral detection of gases trapped in the ice 10 20 40 6080 100 200 at the surface of Ganymede (Coustenis et al., 2010). Ice-I shell thickness (km)

Fig. 2. Schematic view of the JUICE strategy to characterise Ganymede’s icy crust 3.1.1. The ocean and its relation to the deep interior and liquid layer by using combined techniques. The parameter space (ice-I shell thickness and ocean thickness) is bounded by the domain of stability of ices (red A unique characteristic of Ganymede is its intrinsic magnetic curves), but not fully constrained due to our poor knowledge of the temperature field generated in the satellite’s metallic core, and comparable to profile and the volatile content. JUICE will provide the required additional dynamo-activity in the Earth and Mercury (Kivelson et al., 2002). constraints (resulting black area) by determining (a) the Love numbers h2 and Ganymede is so far the only moon known in the Solar System to k2 (main ambiguity: rigidity of ice-I), (b) the libration amplitude (main ambiguity: density contrast between ice-I and ocean), (c) the magnetic induction signal (main possess its own intrinsic magnetic field. This results in a mini- ambiguity: electrical conductivity of the ocean). In this schematic view very magnetosphere embedded within the Jovian magnetosphere. generous error bars have been assumed. (For interpretation of the references to Observational evidence for the presence of a global water ocean color in this figure legend, the reader is referred to the web version of this article.) has been indirectly obtained by the Galileo mission with the detection of an induced magnetic field generated at shallow depth The peak-to-peak amplitudes of periodic surface deformation on in response to the time-variable rotating magnetosphere of Ganymede are in a range of 7 to 8 m (ocean) and a few tens of cm Jupiter. However, the available data are inconclusive because of (no ocean), which will be measured by JUICE from orbit. Indeed, the complex interaction of the induced field, Ganymede’s intrinsic the tidal deformations of the icy crust will be monitored by field, Jupiter’s magnetosphere and the plasma environment ranging the spacecraft distance to the moon’s surface at crossover (Kivelson et al., 2002, 2004). In order to explore the liquid ocean, points which are globally distributed. Along with the tidal surface and particularly to constrain its thickness (Fig. 2), the magnetic displacements, there is a time variability of the gravitational induction response from the ocean must be characterised by potential of the satellite because of the formation of the tidal measuring the magnetic field vector continuously at multiple bulge. Precise radio tracking of the JUICE spacecraft will yield frequencies and high accuracy. Due to the complexity of the precise determination of gravity fields up to degree 12. Both system, these measurements must be supported by plasma, surface displacements and variations of the gravitational poten- particle and wave observations in order to constrain the con- tial will be measured in order to estimate the thickness of the tribution from currents not related to the subsurface ocean. crust above the ocean (Fig. 2). In addition, the Galilean moons are The tidal response of the satellites’ icy shells depends on locked in a stable 1:1 spin–orbit resonance. However, slight the presence of oceans (e.g., Moore and Schubert 2000, 2003). periodic variations in the rotation rate (physical librations) and O. Grasset et al. / Planetary and Space Science 78 (2013) 1–21 5 the amplitudes associated with these librations will provide further constraints for subsurface oceans. JUICE will measure precisely the rotation rate, pole-position, obliquity, and libration amplitude of Ganymede. This will further constrain the dynamical history of the satellite, e.g., despinning, resonance capture, non- synchronous rotation of the icy shell, besides yielding information on the subsurface ocean and deeper interior. Ganymede is a highly condensed object (Schubert et al., 2004). Interior structure models are currently based on degree-2 measurements of the gravity fields using an a priori hydrostatic assumption (Schubert et al., 2004). Using the orbit phases at Ganymede, JUICE will improve the degree-2 fields without relying on the assumption of hydrostatic equilibrium. For Ganymede, the estimates on the degree-2 gravitational coefficients J2 and C22 will be three orders of magnitude better than the current value. JUICE will also determine time-dependent variations of J2 and C22, and thus the satellite’s response to tidal forcing for the first time. High-order fields and deviations from hydrostatic equilibrium will also be detected. These measurements will improve our understanding of the degree of differentiation of the satellite. Measuring the high-order fields, JUICE will also quantify mass anomalies, asymmetries in the mass distribution and other non- hydrostatic contributions to the gravity field. The amount of knowledge will ultimately depend on the degree of precision that will be achieved on each measurement. JUICE will also study the icy shell of Ganymede, which represents an entirely new science field. No investigation of the subsurface has ever been carried out on any icy moons below the few micrometres investigated by remote sensing techniques. Fig. 3. Ganymede’s surface is characterised by old, dark densely cratered plains, JUICE will investigate the crustal structure and its physical and by younger, bright and more water ice-rich, tectonically resurfaced terrain. properties, the interactions with the ocean, and the correlation Bright terrain formed at the expense of dark terrain, mostly through extensional tectonism, (lower panel; Galileo SSI target area C9GSSULCUS, 900 m/px). Bright, between the surface features and the subsurface. This will require smooth bands (upper right panel; Galileo SSI target area 28GSARBELA02, 130 m/px) mapping of regions of high interest by a radar sounder, which has indicate lithospheric spreading, involving extension as well as strike-slip move- the ability to penetrate the surface and to perform a subsurface ments, as, e.g., in Arbela Sulcus (AS). analysis down to a few kilometres (maximum depth from 1 to 9 km depending on the crust properties), and with vertical (e.g., Pappalardo et al., 1998, 2004). Several caldera-like, scalloped resolution of some tens of meters. depressions termed paterae found in the bright terrain represent probable volcanic vents, and ridged deposits in one of the largest of such paterae were interpreted as viscous cryovolcanic flows 3.1.2. Formation of surface features and search for past and present (Head et al., 1998). Smooth units which embay other surface units activity such as crater rims are thought (a) either to represent cryovolca- With its mix of old and young terrains, ancient impact basins nic flows, extruded as icy slushes (Pappalardo et al., 2004), or and fresh craters, and landscapes dominated by tectonism, (b) to be issued from mass wasting processes along slopes possible icy volcanism, or slow-rate degradation by space weath- (Prockter et al., 1998, 2010). Although the ultimate driving ering (Fig. 3), Ganymede serves as an archetypal body for under- mechanism for the formation of bright grooved and smooth standing many icy satellite processes throughout the outer Solar terrain is uncertain, there is a substantiated possibility that it System and how this entire class of worlds evolved differently may be tied to the internal evolution of Ganymede and to the from the terrestrial planets (e.g., Pappalardo et al., 2004,; Prockter history of orbital evolution of the Galilean satellite system, et al., 2010; Stephan et al., 2012). Ganymede’s surface is sub- involving tidal interactions (Showman et al., 1997). divided into (i) dark, densely cratered ancient plains (perhaps Impact features on Ganymede exhibit a wider range of essentially primordial and grossly similar to the surface of diversity than those on any other planetary surface. They include Callisto), covering about 1/3 of its total surface; and (ii) bright, vast multi-ring structures, low-relief ancient impact scars called less densely cratered, heavily deformed, grooved terrain. Dark palimpsests, craters with central pits and domes, pedestal craters, terrains also display hemispheric-scale sets of concentric troughs dark floor craters, and craters with dark or bright rays (e.g., Passey – termed furrows – which are probably the remnants of vast and Shoemaker, 1982; Schenk et al., 2004). The subdued character multi-ring impact basins, now broken up by subsequent bright of Ganymede’s oldest impact craters implies a steep thermal terrain tectonism. These terrains are dark due to the addition of gradient in Ganymede’s early history, with more recent impact non-water ice contaminants concentrated on the surface by a structures reflecting a thicker and stiffer elastic lithosphere (e.g., variety of processes including sublimation, sputtering and mass Shoemaker et al., 1982). Such an interpretation indicates a much wasting (Prockter et al., 1998). Bright terrains subdivide the dark warmer shallow subsurface ocean early in Ganymede’s history units into broad, up to several hundred kilometres wide, linear or than at present. The size-frequency distribution of impact craters curved parallel, closely spaced grooves, termed sulci. The bright also provides an important tool to derive relative and absolute terrain units formed predominantly at the expense of dark terrain ages of Ganymede’s geologic units (e.g., Neukum et al., 1998; through a process termed tectonic resurfacing, generally char- Zahnle et al., 1998, 2003). acterised by extensional rifting, causing the partial or total The Galileo SSI provided enough data to describe the global transformation of dark terrain into bright terrain by tectonism geology of Ganymede. However, it was not possible, except in a 6 O. Grasset et al. / Planetary and Space Science 78 (2013) 1–21 few cases, to study regional and local geology in extent, most of the data being at low or medium resolution (10 km/px). Less than 1% of the surface was studied at resolutions better than 100 m/px. In order to improve our understanding of geological processes on the moon, a better coverage by higher resolution data is required. Combined with spectral mapping, these observations will contribute to a comprehensive picture of the geological evolution, constrain the role of cryovolcanism and tectonics in the geological evolution, and help us to understand the origin of this body. JUICE will acquire detailed topographic profiles of tectonic features, grooved terrain, impact forms and cryovolcanic features. This will enable the identification of dynamical processes that cause internal evolution and near-surface tectonics. JUICE will provide a breakthrough in the geology of Ganymede because it will investigate its surface from orbit by global imaging with Fig. 4. Spatial coverage of Galileo/NIMS on Ganymede superimposed on the regional spatial resolution ( 400 m/px) and high-resolution mosaic obtained from the Galileo/SSI optical images. The low spatial resolution o (mostly 420 km/px) of Galileo data did not allow for a proper investigation of imaging (o5 m/px) of selected targets. JUICE will significantly composition and spatial distribution of non-water-ice compounds on the surfaces improve the current estimates of surface ages by measuring crater of the moons. (Credit: K. Stephan, R. Jaumann, DLR). distributions with nearly global image coverage at 200–1000 m/px resolutions, and sufficient high-resolution target areas (5–50 m/px), and by monitoring Ganymede’s surface on a time-scale of the order Imaging spectroscopy in a wide spectral range from ultraviolet of hundreds of days up to years to identify potentially newly to infrared will be the main remote sensing technique of JUICE to formed craters. This will allow for the establishment of a compre- study the surface composition. The mission goals require that at hensive stratigraphy of the moon. JUICE sounding of the subsurface least 50% of the surface be covered with resolutions between down to a depth of a few km will in turn provide a third dimension 2 and 3 km/px and the mapping of selected target sites with a to the surface investigations. resolution of about 100 m/px. Spectral resolution will be high enough to resolve characteristic features of surface ices/minerals. Remote sensing will be complemented by ion and neutral mass- 3.1.3. Determine global composition, distribution and evolution of spectrometry and particle/plasma analysis of the moon’s exo- surface materials sphere issued from sputtering and sublimation of surface mate- On Ganymede, bright terrains are water ice-rich compared to rial. This latter technique should allow us to measure major dark terrains. The composition of the non-water-ice material volatiles (H2O, CH4,NH3, CO, N2,CO2,SO2, etc.), stable isotopes ranges from heavily hydrated at high latitudes to only slightly of C, H, O, as well as the noble gases Ar, Kr, and Xe. hydrated material associated with dark ray ejecta. However, most of the non-water-ice material is a moderately hydrated material—possibly salt. It is worth noting that carbon dioxide, the 3.1.4. Local environment and its interaction with the Jovian most abundant of the trace materials, is concentrated in dark magnetosphere terrains, while neither leading/trailing hemispheric asymmetry in Ganymede has earned a unique place within the Solar the distribution of CO2 exists nor the impact craters tend to be System because of its internally generated magnetic field and CO2-rich (Hibbitts et al., 2002). It is also occasionally enriched in hence its miniature magnetosphere (about the same size as terrain containing larger-grained ice in comparison with adjacent Mercury’s) within the larger Jovian magnetosphere. This mini- terrain of similar morphology and ice abundance. Various non- magnetosphere constantly interacts with the corotational plasma water-ice materials have been suggested from Galileo data and flow and electromagnetic fields of the rapidly rotating Jovian ground-based spectra: carbon dioxide, sulphur dioxide, molecular magnetosphere, producing a dynamic interaction region, which oxygen, ozone and possibly cyanogen, hydrogen sulphate and has some parallels with the Earth’s magnetosphere (e.g., driving various organic compounds (e.g., McCord et al., 1998). The source of the system through magnetic reconnection). The ‘‘opening’’ of of the organic material could be formed in situ from radiolysis (co- Ganymede’s magnetic field lines allows direct access of the Jovian product of radiolysis is O2 gas, Hall et al., 1998). So the detection plasma onto the surface of Ganymede near the poles in the open of O2 at mid-latitudes due to exogenic material falling onto field region, which may result in a subsequent alteration of the Ganymede’s surface (Calvin et al., 1996; Spencer et al., 1995)is surface properties and brightening of the polar caps (Fig. 5; the signature of the probable presence of organic material. A Khurana et al., 2007). JUICE will observe the system over multiple reliable identification of all non-water-ice compounds is still frequencies (e.g., hours, days, and weeks) both from elliptical missing, due to the lack of high spatial resolution data with good (magnetosphere) and circular orbits (internal and induced field), signal-to-noise ratio (Fig. 4), and to low spectral resolution. thus characterising the interaction region both close to Ganymede Surface composition can also be inferred by measuring mate- and at the boundaries of the magnetosphere. JUICE will investi- rials sputtered or ejected from the surface into the atmosphere gate Ganymede’s intrinsic magnetic field in detail and character- using direct sampling, which is not affected by the physical ise the interplay between this intrinsic field, induced magnetic properties of the material. Models predict that large molecules, fields generated in the subsurface ocean, and the Jovian magneto- such as hydrated Mg and Na sulphates and organics, may be sphere. It will establish the dimensions of Ganymede’s magneto- sputtered to orbital altitudes at detectable levels for an orbiting sphere and will determine the regions of open and closed spacecraft (e.g., Leblanc et al., 2002; Cassidy et al., 2009). These Ganymede magnetic field where particles are either trapped, observations, however, are limited in spatial resolution to transported, or field-aligned. approximately the height at which the measurement is made JUICE will also identify particle precipitation along the open and by the need of inferring the surface composition from the field lines at the poles through remote auroral observations at measured derived products through the processes of sputtering multiple wavelengths and through in-situ detection of sputtered and radiation-induced chemistry. charged particles and remotely detected radiolytically-produced O. Grasset et al. / Planetary and Space Science 78 (2013) 1–21 7

Fig. 5. Surface alteration associated with magnetosphere-surface interactions at Ganymede (from Khurana et al., 2007 and credit to X.Jia (University of Michigan) for the Ganymede magnetosphere diagram). energetic neutral atoms. It will take measurements in the regions 3.2. Europa’s recent active zones between the atmosphere/exosphere of Jupiter and Ganymede (where the particles are mainly produced through sputtering 3.2.1. Recently active processes and radiolysis)—along the flux tube connecting both bodies. JUICE Europa’s surface (Fig. 6) can be subdivided into bright (bluish will finally uncover the nature of the time-varying interaction of colour) plains, featuring numerous parallel ridges in a wide range Ganymede’s magnetic field and plasma environment with the of orientations, and darker, brownish mottled terrain (Lucchitta surrounding Jovian magnetosphere—separating the various and Soderblom, 1982; Greeley et al., 2004; Prockter et al., 2010; sources of magnetic fields. Long-term changes in the internal Stephan et al., 2012). Linear ridges are the most widespread and induced magnetic field may also be detected by comparison landforms on Europa; the most common type of ridges are double with the Galileo data. ridges, consisting of a pair of ridges with a medial trough. They Ganymede possesses an exosphere and an ionosphere. The are thought to have originated through a variety of mechanisms, tenuous exosphere is produced by sputtering processes, as the including, e.g., tectonism, cryovolcanism, or diapirism, and surface is bombarded by particles from Jupiter’s radiation belt require either the presence of liquid water in the shallow subsur- magnetosphere, and sublimation of the surface materials face, or warm mobile ice underlain by an ocean at depth (Greeley (McGrath et al., 2004). The exospheric properties are thus indi- et al., 2004, and references therein, Schmidt et al. (2011)). cative of such sputtering and sublimation processes. Ganymede Bright plains are separated by dark bands, which are possible shows evidence for the presence of oxygen species. In particular, indications of crustal spreading, with brittle plates moving on a solid O2 and O3 have been detected in the trailing hemisphere of warmer, mobile substrate (e.g., Greeley et al., 2004; Prockter Ganymede, consistent with the preferential orientation of that et al., 2010). Chaos regions are characterised by broken plates of side of the satellite toward the upstream flow of Jupiter’s pre-existing terrain, such as ridged plains, which have been magnetosphere (Noll et al., 1996; Hendrix et al., 1999). Both of translated, rotated and tilted in a matrix of predominantly these species appear to be trapped within the ice matrix, and hummocky terrain which in turn could be comprised of, or has probably originate from ionic bombardment of the icy surface altered, pre-existing terrain (e.g., Greeley et al., 2004; Prockter

(the presence of CO2 should also produce monomeric or poly- et al., 2010). Widespread abundance of erosional or degradation merised H2CO and an H2CO3 residue, two species that have not features are absent and craters, especially those larger than been yet identified). The abundance of ozone varies with latitude, 10 km, are rare. Europa’s surface is characterised by a very low with the strongest concentration measured at higher latitudes. density in impact craters (only 16 craters with diameters of This was interpreted as being the result of plasma bombardment 3–27 km could be identified) that suggests a young surface age creating O3 in the ice matrix and photodissociation destroying it, (e.g., Greeley et al., 2004; Schenk et al., 2004). on a continual basis. JUICE will study the neutrals produced by Europa also has a tenuous mainly-O2 atmosphere (Hall et al., plasma-surface interaction over an energy range from a few eV to a 1995) produced by intense radiation bombardment (though occa- few keV, and provide 2D imaging of impacting plasma. It will also sional venting, never detected so far, cannot be ruled out). Na and K search for products of ionic bombardment on Ganymede and will have also been measured from ground-based observations (Coustenis allow a detailed mapping of the oxygen species over its surface. It et al., 2010 and references therein). O2 is seen on Europa’s surface will significantly enhance our understanding of ion bombardment (Hand et al., 2006). The evidence for trapped O2 indicates that the processes and the dynamical response of the surface. Moreover, it radiolytically-produced O2 may be supplied to the subsurface ocean, will closely explore the physical processes involved in the cycling where it could be a source of energy for life (Chyba, 2000). of oxygen species and the availability of oxidants for biological With its two Europa flybys, JUICE will enable, with a suite of processes. JUICE should also identify the particle populations near imaging instruments covering a broad range of parameters (field of Ganymede and its interaction with Jupiter’s magnetosphere by view, spatial resolution), high resolution (few m/px) observations measuring the velocity-space distribution of thermal plasma and of selected high priority targets and it will place them in the global energetic particles from eV to MeV, plasma and radio waves, and context of distant imaging. The imaging will be complemented by neutral imaging from eV to keV of the impacting plasma and topography studies and sub-surface sounding. These observations ejected material. Due to the JUICE orbit evolution, the exosphere will provide a geological context to the high priority composition will be studied at almost all local solar times. mapping (see next section). They will also constrain global and 8 O. Grasset et al. / Planetary and Space Science 78 (2013) 1–21

Fig. 6. Europa’s surface shows the widest range in colours of the three icy Galilean satellites (left) and exhibits two major surface units: bright, bluish plains, and dark, brown, mottled terrain. Bright plains consist of numerous parallel ridges and troughs (RP) superposed by mottled terrain (M) which at higher resolution (centre of the right panel) is revealed as chaotic terrains. Most features cutting plains and mottled terrain are double ridges, either linear (d) or cycloidal (cr) (middle panel), and bands (b). Very few impact craters (c) are observed. regional surface ages, and should allow local investigation of the Hand et al., 2007). If oxidants can be delivered to the internal processes of erosion and deposition. Finally, this exploration of liquid water reservoirs, they can be a source of free energy specific sites at very high resolution may provide the necessary available for biology. JUICE will provide information on the inputs for identifying future landing sites on Europa. contamination processes acting on the surface of Europa. At medium spatial resolution in the range 5–10 km/px, it will map large areas to reveal leading/trailing asymmetries due to contam- 3.2.2. Composition of the non-ice material, especially as related to ination by exogenic material. These data should be complemen- habitability ted by a study of the exospheric composition. Galileo’s spectra have distortions in several water ice absorption bands between 1 and 3 mm, indicating the presence of hydrate 3.2.3. Searching for liquid water under the most active sites compounds concentrated in the visually dark and reddish regions. It One of the main objectives of JUICE is to explore for the first has been hypothesised (e.g., McCord et al., 1998, 1999; Dalton et al., time the subsurface in the most recent active regions to under- 2005) that this material may be made up of hydrated salt minerals stand the exchange processes from the subsurface to the surface enriched in Mg and Na sulphates that form by the crystallisation of and also to constrain the minimal thickness of the ice shell. By brines erupted from the subsurface. Alternatively, this material was using a subsurface radar sounding experiment, JUICE will inves- proposed to be due to hydrated sulphuric acid (H2SO4 nH2O), tigate the subsurface down to a few kilometres depth (maximum formed by the radiolysis of water and of a sulphur-bearing species, depth from 1 to 9 km depending on the crust properties) with a or by the decomposition of sulphate salts (Carlson et al., 1999, b). vertical resolution of few tens of meters. At the closest approach, Later, Dalton (2007) reported that the Europa non-water-ice spectra JUICE will probe the crust possibly down to the ice-ocean inter- would be actually best matched by mixtures of sulphuric acid face if the ice shell is only a few km thick as expected in a few hydrates together with hydrated salts, so both these chemical models (Greenberg and Geissler, 2002). With its two flybys, JUICE classes may be present on the surface with variable concentrations. will help to solve the controversy concerning the depth of the Other non-water-ice species, like CO2 and H2O2,werealsofoundin ocean below the active regions, and will determine whether or the leading hemisphere at equatorial to mid-latitudes, while SO2 not the liquid material can ascend through cracks up to the was reported on the trailing hemisphere (Dalton et al., 2010a). surface, an active process which is possible only if the icy crust is With its two close flybys of the moon, JUICE will perform high- very thin (Pappalardo, 1999). resolution multi-wavelength spectral imaging of selected targets Hydrated compounds are concentrated at the lineaments and (see Fig. 14). Imaging spectroscopy in the broad spectral range chaotic terrains. Some young cryovolcanic flow and deposit units from UV to IR will be the main remote sensing technique to study exhibit high proportions of hydrated salts and low abundance of the surface composition. The mission goals require spectral sulphuric acid hydrate when compared to older surface units of mapping of top priority sites with spatial resolution of at least the same type, or to surface units of different geologic origin. 1 km/px and spectral resolution high enough to resolve charac- This suggests that for some units we are observing an intermedi- teristic features of non-water-ice materials expected to exist on ate stage of the conversion of endogenically-produced sodium the surface. The imaging spectroscopy will emphasise composi- and magnesium sulphate salts into sulphuric acid hydrate by tional differences between geologic features (bands, chaos, exogenically-driven radiolysis. The presence of large quantities of domes, or ridges) and the surrounding areas. brine and sulphate salts in certain deposits may reflect the If the non-water-ice material is made from mixtures of composition of subsurface liquid source reservoirs (Dalton et al., sulphuric acid hydrate and hydrated salts, one mechanism might 2010b). JUICE will correlate distribution of non-water-ice mate- be that Na associated with some salts could be easily sputtered rial with geologic units in a wide range of spatial scales, up to very þ away and abundant H could take its place, forming sulphuric high spatial resolution (1 km and possibly better) over regions acid. Thus sulphuric acid hydrate abundance is linked to the of very high interest targeted at closest approach. The combina- magnetospheric charged particle energy flux, and could result tion of high spatial and spectral resolutions data with unprece- from radiolytic processing of implanted sulphur from Io, or of dented subsurface exploration will be the key to unveil the sulphur emplaced as part of the surface deposits that came from exchange processes occurring between surface and subsurface. the interior. Destruction of large molecules by the same radiation however suggests that there may be equilibrium between crea- 3.3. Callisto as a remnant of the early Jovian system tion and destruction that varies based on sulphur content and radiation flux. O3 is not as obvious in Europa as in Ganymede, 3.3.1. Past activity but signatures of O2 and H2O2 are evident (Hall et al., 1998; Callisto is characterised by globally abundant dark, densely Fanale et al., 1999; Carlson et al., 1999a; Johnson et al., 2003; cratered plains (Fig. 7). It is the geologically least evolved Galilean O. Grasset et al. / Planetary and Space Science 78 (2013) 1–21 9

investigation of the processes of erosion and deposition. Callisto will be imaged during several ﬂybys at envisaged resolutions of 300–800 m/px in order to complete coverage of its densely cratered plains, and at high resolution (5 100 m/px) of selected target areas in order to perform detailed studies of its unique erosion and degradation processes.

3.3.2. The outer shells, including the ocean The internal structure of Callisto remains a mystery. The very old surface indicates that this moon has not been active for some billion years, which may imply a total lack of dynamics in its deep interior. But surprisingly, Galileo detected an induced magnetic field as for the two other icy moons, suggesting the presence of a large liquid reservoir within the crust. Thus, among the key questions to be solved by future missions to this moon, there is the need to characterise the structure of the icy shells including the possible detection of shallow subsurface water, to verify whether hydrostatic states are actually obtained, and to improve our understanding of Callisto’s degree of differentiation. JUICE will add new constraints on the evidence for a subsurface ocean by measuring the induced magnetic field during the flyby Fig. 7. Galileo images of Callisto. Upper left: old densely cratered surface of campaign. The magnetic induction response from the ocean will be Callisto with large multi-ring structures, such as Valhalla (V). Lower panel: SSI characterised by precise measurements of 3 axis magnetic and medium resolution image of a cratered plain including dome craters (d) and ring electric field vectors with high sampling frequency, combined with arcs of old, degraded multi-ring structures (arrows). Right panel: SSI high resolution image revealing the high state of surface degradation driven by plasma and wave observations over a broad range of distances sublimation. from the moon. Determination of the hydrostatic state of the moon, and study of the deep structure, will require Doppler tracking during an equatorial as well as a highly inclined flyby. satellite and therefore represents an end-member body (e.g., The latter was not possible with Galileo. This will be carried out Moore et al., 2004; Prockter et al., 2010; Stephan et al., 2012). by radio tracking of the spacecraft with sufficient range-rate Its surface is dominated by various impact features, similar to accuracy using close to polar flybys. It will be possible to verify those which occur on Ganymede, and by landforms indicative of whether Callisto is in a hydrostatic state by measuring the low- intense surface erosion and degradation. Callisto and its cratered order static gravity field, J and C , independently from each landscape, including crater size-frequency distributions, has a 2 22 other. JUICE will also improve the determination of the low- specific place as a window into the early history of the Jovian order gravity field and the moment of inertia and thus, constrain system. Similar crater forms on Callisto and Ganymede indicate further the deep interior structure of the moon, especially similar rheological properties and subsurface layering but degra- regarding the possible separation of ice and rock components dation states and ages of craters with a specific morphology, e.g., in the outer part of the icy crust. Several strategies will also be palimpsests, infer different rates of change of these properties used to investigate the surface and shallow subsurface of the icy with time. A process called sublimation degradation, triggered by crust, the ocean, as well as the deep interior. Subsurface the presence of CO , caused the degradation of bright high- 2 exploration will enable a search for water reservoirs, the study standing terrain (e.g., crater rims) and the formation of a globally of the dynamical processes leading to the crater morphologies, abundant dark, smooth blanket but the time-scale of this dark lag and the investigation of relaxation processes. Combining these formation is not known (Moore et al., 1999, 2004; Schenk et al., complementary datasets and measurements for the same tar- 2004). Unlike Ganymede or Europa, tectonism on Callisto is not geted regions, knowledge will be gained as to how internal, widespread but systems of furrows and albedo lineaments do subsurface and surface processes act together. occur. Some of these features are caused by impacts, others could originate from not impact-related stresses active at early times (e.g., Moore et al., 2004, Prockter et al., 2010). 3.3.3. Composition of the non-ice material

A thin atmosphere of CO2 (Carlson et al., 1999a), and perhaps As for the two other icy moons, a reliable identiﬁcation of all O2 (Liang et al., 2005), as well as an ionosphere (Kliore et al., non-water-ice compounds is still missing on Callisto, due to the 2002) were also detected around Callisto by the Near-Infrared lack of high spatial resolution data with good signal-to-noise Mapping Spectrometer (NIMS) during the 1999 Galileo ﬂybys. The ratio, and to low spectral resolution. Callisto’s surface composi- surface pressure is estimated to be 7.5 1012 bar and particle tion is thought to be broadly similar to its bulk composition. Non- density 4 108 cm3. It is thought to be constantly replenished water-ice compounds include Mg- and Fe-bearing hydrated by slow sublimation of carbon dioxide ice from the satellite’s icy silicates, CO2,SO2, and possibly ammonia and various organic crust (Carlson et al., 1999a), which would be compatible with the compounds (Moore et al., 2004; Showman and Malhotra, 1999), sublimation hypothesis for the formation of the bright surface with abundances greater than those reported on Ganymede knobs (Coustenis et al., 2010). and Europa, and with an extreme heterogeneity at the small scale

The twelve JUICE flybys of Callisto will enable near to global (1–10 km). Superficial CO2 is concentrated on the trailing hemi- mapping at regional scale, and high resolution [up to a few m/px] sphere (Hibbitts et al., 2000), leading to a slightly larger atmosphere observations of selected high priority targets that will be placed on that side of the satellite (Johnson et al., 2004), consistent with a in the global geological context. Similarly to Europa, a suite slightly more robust ionosphere (Kliore et al., 2002). Therefore, of imaging instruments covering a broad range of parameters surface alteration due to radiolysis and photolysis of many organic (field of view, spatial resolution) is required. These observations molecules that may be intrinsic or delivered by comets and will constrain global and regional surface ages, and enable the meteoroids to the surface (Bernstein et al., 1995; Ehrenfreund 10 O. Grasset et al. / Planetary and Space Science 78 (2013) 1–21 et al., 2001) are likely to also be important on Callisto. Since the Jupiter’s giant magnetosphere, the largest in the Solar System, impactor bodies cannot be the source of CO2 as this compound within which all other objects are embedded including two would rapidly sublimate at the temperatures typical of the unique components: Io, the main source of material, and satellite’s equator at noon, trapping structures (e.g., ice clathrates, Ganymede with its mini-magnetosphere embedded within physisorption) that can form a stable underground reservoir of that of Jupiter’s.

CO2 are envisaged. The SO2 distribution appears generally mottled, with some areas of high concentrations correlated with To explore this complex system, JUICE will first examine the ice-rich impact craters (Hibbitts et al., 2000). Large-scale patterns physical characteristics of each of the individual objects, before include the depletion of SO2 in the polar regions; and a depletion embarking on a detailed study of how they are coupled together of SO2 on the trailing side relative to the leading side is observed and how they are continuously interacting. Finally JUICE will (Dalton et al., 2010a). investigate how the system as a whole operates as a result of the JUICE will characterise the surface composition of Callisto and multiple inter-related processes described above. relate it to geology. Moreover, JUICE will investigate the intri- guing mechanism of replenishment of CO . The composition 2 4.1. Jupiter, the giant planet and how to better understand it observations will have important synergy with surface imaging and subsurface investigations that would provide geological While the thin visible atmosphere (the ‘‘weather-layer’’), the and morphological context. Imaging spectroscopy in the broad only region accessible to direct investigation by remote sensing, is spectral range from UV to IR will be the main remote sensing only a tiny fraction of Jupiter’s total mass, it provides essential technique used to study the surface composition. The Callisto insights into the interior structure, bulk composition, and forma- phase is not optimal from the point of view of remote sensing tion history of our Solar System. The exploration of Jupiter’s due to the similarity of the flybys, hence it will be necessary to dynamic atmosphere has played a pivotal role in the development achieve off-nadir observations with cameras and spectro- of our understanding of the Solar System, serving as the paradigm meters in the approach/departure phases. JUICE will perform for the interpretation of planetary systems around other stars and spectral imaging investigations at regional scale with spatial as a natural laboratory for the investigation of large-scale geo- resolution of 1–30 km/px and study selected targets by means physical fluid dynamics and physiochemical phenomena. How- of spectral imaging and in situ observations in the closest ever, our characterisation of this archetypal giant planet remains approach phase. incomplete, with many fundamental questions about its nature At Callisto, sublimation may be more significant than charged unanswered. This is largely due to (i) the sparsity of remote particle sputtering. However, the interaction with the plasma sensing observations across a broad wavelength range; and environment must be evaluated and compared to different sur- (ii) considerable differences between spatial resolutions and cover- face release processes. Current knowledge of the atmosphere is age at different wavelengths. In particular, previous missions and based on isolated observations, derived largely from observations ground-based observations have provided mere snapshots of this of spectral emission features that are reliant on the local plasma complex, evolving atmosphere, insufficient to identify the mechan- environment, which provide little information on minor species isms and processes driving the jets, eddies, vortices and plumes. chemistry and temporal variations. Therefore, combining spectro- JUICE aims to address this by long-term monitoring across a broad scopy and atmospheric/exospheric measurements, JUICE will wavelength range, simultaneously measuring visible changes (cir- enable the identification of the asymmetries of Callisto’s surface culations, colour changes, dynamic convective phenomena) and the release. JUICE measurements, including limb scans and stellar environmental conditions (composition, temperatures, cloud micro- occultations, identification of major and minor constituents of physics) to develop a three-dimensional understanding of Jupiter’s Callisto’s neutral atmosphere and mapping of low energetic atmosphere. neutral released from the surface, will reveal information about The atmospheric science objectives of JUICE fall into three the sources and sinks of the atmosphere/exosphere. categories designed to address the unresolved mysteries raised by previous missions to the outer Solar System, which are described below. JUICE will provide the first four-dimensional climate 4. Investigations of Jupiter’s environment: The planet, its database for the study of Jovian meteorology and chemistry, magnetosphere and the interactions with the moons and will investigate the atmospheric structure, clouds and composition from the thermosphere down to the lower troposphere The Jupiter system can be considered as a miniature Solar to create a global picture of the many dynamical and chemical System in its own right. It comprises a multitude of diverse processes at work in Jupiter’s atmosphere (Fig. 8). objects, which can be divided into multiple sub-systems:

Jupiter the planet, with its diverse range of atmospheric 4.1.1. Atmospheric dynamics and circulation phenomena from the deep interior, through the dynamic Jupiter is the end product of energetic accretion processes, weather layer (and its giant storms, belt/zone contrasts and thermochemistry, photochemistry, condensation processes, temporal variability) to the stratosphere, upper atmosphere planetary-scale turbulence and gravitational differentiation. Its and its coupling to the immediate planetary environment. atmosphere is characterised by distinct latitudinal bands of A huge satellite system including the four large Galilean differing cloud colours, vertical motions, temperatures and ver- satellites, Io, Europa, Ganymede, and Callisto (1000 km class tical mixing strengths separated by strong zonal winds and objects - three of which possessing subsurface oceans), the perturbed by long-lived vortices, storms, polar circulations, con- four inner satellites Metis, Adrastea, Amalthea and Thebe vective outbreaks, wave activity and variable large-scale circula- (10–100 km class objects), 55 outer irregular small satellites tion patterns (Rogers, 1995; Ingersoll et al., 2004; West et al., (1 to 100 km class objects) and by extension the Jovian ring 2004). The variety of dynamical and chemical phenomena in system located in the inner regions. Jupiter’s visible atmosphere are thought to be governed by a The tenuous atmospheres of the Galilean satellites, their balance between solar energy deposition and forcing from deeper production processes and their interactions with the sur- internal processes. Moist convection, eddies, turbulence, vertical rounding local environment. wave propagation, and frictional damping are all believed to play O. Grasset et al. / Planetary and Space Science 78 (2013) 1–21 11

Fig. 8. Examples of the Jupiter science objectives of JUICE. Each image shows Jupiter’s appearance at a range of different wavelengths, from visible coloration and wind tracking (centre, HST, credit: NASA/ESA/A. Simon-Miller/I de Pater) to cloud properties in the near-IR (left, Gemini/NIRI image, credit: Gemini observatory/AURA/L.N. Fletcher); thermal structure and chemistry in the mid-IR (right, credit: NASA/IRTF/G.S. Orton, 5 mm image) and auroral properties in the UV (top and bottom, credit: NASA/ ESA/J. Clarke). a role in atmospheric circulation, transporting and mixing energy, Quadrennial Oscillation (Leovy et al., 1991); and (c) the vertical momentum and material tracers transfer both horizontally and propagation of gravity waves in the middle and upper atmosphere, vertically (Vasavada and Showman, 2005; Salyk et al., 2006). which are thought to play an important role in energy transfer Jupiter’s atmosphere also exhibits a wealth of time-variable between different layers, and may be crucial in heating the lower phenomena, ranging from thunderstorms and lightning, forma- thermosphere to temperatures far exceeding those expected from tion and interaction of giant vortices, episodic plumes and out- the absorption of sunlight. Night side imaging will catalogue the bursts, waves, and turbulence to quasi-periodic variations in the distribution and energetics of Jovian lightning, a powerful tracer for banded cloud patterns and storms (e.g., the recent fade and atmospheric circulation. revival of Jupiter’s South Equatorial Belt and related changes The poles of the giant planets can be considered as the apex of known as a ‘global upheaval’). a planet-wide circulation pattern, surrounded by circumpolar Meteorological investigations of these phenomena will benefit vortices that separate the polar atmosphere from the lower from the long temporal baseline and broad spectral range offered latitudes. On Saturn, the poles were discovered to be the site of by JUICE, permitting global mapping at frequent intervals to unique wave activity (Saturn’s polar hexagon), dynamic convec- identify the underlying dynamical causes for Jupiter’s atmo- tive cloud activity, and warm hurricane-like polar vortices spheric variability. Through imaging, spectroscopy, and occulta- (Fletcher et al., 2008). Jupiter’s polar regions are the least- tions, JUICE will study atmospheric motion from the troposphere studied areas of the planet, having been observed only once by to the thermosphere and its relation to the deep interior by Pioneer 11. Galileo remained in a largely equatorial orbit, and measuring: vertical profiles of zonal winds and temperatures; Jupiter’s low orbital obliquity (31) means that we cannot inves- dynamical tracers of circulation (e.g., potential vorticity, disequi- tigate the atmospheric dynamics, chemistry and energy balance librium species, volatiles, cloud colours); and the distribution and at polar latitudes from Earth-based observatories. High-latitude depth of Jovian lightning. These observations will help to deter- observations are planned for JUICE during the Callisto-phase (see mine the importance of moist convection in driving Jovian Section 4). This will provide a unique survey of the dynamics and circulation, and distinguish between ‘shallow’ and ‘deep’ models chemistry of Jupiter’s polar regions across a broader spectral for the origins of eddies, vortices and zonal jets. range and longer time period than will be possible from Juno. The cloud morphology and atmospheric dynamics will be JUICE’s broad-wavelength and wide angle imaging capabilities better understood from systematic and regular imaging of Jupiter and long temporal baseline during the high-inclination phase will from UV to IR with few tens of km spatial resolution and complement the high-resolution localised imaging of Juno. repetition time from days to years in order to characterise variable phenomena like storms, waves, eddies etc. The tracking of cloud features should allow for the reconstruction of the wind 4.1.2. Composition and chemistry field (both horizontally and vertically) and assessment of the Jupiter’s atmospheric composition is the product of a myriad stability and energy sources driving zonal jets and giant vortices of thermochemical and photochemical pathways (Atreya et al., (e.g., the Great Red Spot). JUICE will also directly measure wind 2003). The atmospheric composition determines the structures of velocities in the stratosphere of Jupiter for the first time via sub- the cloud decks; radiative energy balance influences the tropo- millimetre Doppler tracking. Spectral imaging from the UV to the sphere and middle atmosphere; and condensation processes can near-IR will also allow us to monitor the distributions of minor provide the energy required for convective dynamics. Further- species in the Jupiter’s troposphere (e.g., phosphine, ammonia, more, Jupiter’s bulk composition provides a window on the arsine and possibly water) and use them as tracers of Jupiter’s formation and evolution of the gas giant, and connects it directly circulation. JUICE will study wave activity over a range of to the nature of the satellite system. Although primarily com- spatial scales, from (a) sporadic equatorial mesoscale waves; to posed of hydrogen and helium, Jupiter also contains small

(b) planetary-scale Rossby waves and the forcing of the Quasi- amounts of heavier molecules (CH4,PH3,NH3,H2S, H2O), 12 O. Grasset et al. / Planetary and Space Science 78 (2013) 1–21 providing source material for complex photochemical pathways (West et al., 2004). Submillimetre spectroscopy thanks to very powered by UV irradiation (Taylor et al., 2004, Moses et al., 2004). high resolution (l/dl–106) will provide vertical proﬁling and

The abundances of most of these heavy elements are enriched over spatial mapping of trace gases (CO, H2O, CH4, HCN), thus adding the solar composition, providing a window into the evolution of the vertical dimension to the temperature sounding, composition primordial nebula material incorporated into the gas giants during studies as well as determination of oxygen and hydrogen isotope their formation (Lunine et al., 2004). ratios. This technique will also enable direct Doppler measure- Spectroscopic mapping from the UV to the submillimetre will ments of winds in the Jovian stratosphere for the ﬁrst time. Radio- allow JUICE to study (a) the 3D spatial distribution and variability occultation will be used to study wave activity in the neutral of stratospheric hydrocarbons to probe circulation and photo- atmosphere and the electron/ion density structure of the upper chemistry; (b) the origins and distributions of exogenic oxygen- atmosphere. JUICE will also use stellar occultations in the UV and bearing species; (c) localised and non-equilibrium compounds near-IR at a wide range of latitudes to determine the temperature,

(e.g., PH3, AsH3, GeH4) associated with discrete atmospheric density, pressure and zonal wind structure from the troposphere features in the troposphere; and (d) the spatial distribution of to the thermosphere, and the charged particle distribution in the volatiles (ammonia and water) to understand the importance of ionosphere and magnetosphere. Vertical coupling in Jupiter’s moist convection in cloud formation, lightning and chemistry. unique polar environment, which exhibits unusual composition, Previously undetected trace species in Jupiter’s stratosphere could circumpolar waves and vortices, and a poorly-understood north/ also be detected. Ammonia and water humidity within and above south asymmetry in haze properties, will be studied from JUICE’s the condensation clouds will be mapped, complementing the high inclination phase. deeper atmospheric studies of Juno. Finally, spectroscopy of Jupiter’s polar regions will be used to study the chemistry and cloud formation mechanisms at high latitudes, the location of a 4.2. The Jovian magnetosphere unique connection with the external environment and possible chemical pathways resulting from auroral energy deposition. The strong internal magnetic field of Jupiter (equatorial JUICE’s survey of Jupiter’s atmospheric composition will signifi- surface strength 50 times that of the Earth) creates the largest cantly advance our understanding of chemical processes and and fastest rotating magnetosphere in the Solar System. With transport in giant planet atmospheres. an average subsolar magnetopause distance of 75RJ,themag- netosphere rotates in less than 10 h about its rotation axis (tilted by 9.71 relative to the dipole axis). It is driven by the fast 4.1.3. Vertical structure of the atmosphere and interior rotation of its central spinning object, Jupiter. The magneto- Jupiter’s vertical atmospheric structure is governed by a sphere is heavily loaded with plasma mostly originating from delicate balance between solar, chemical and internal energy Io. Io orbits deep inside the magnetosphere, and releases about sources, and its layers are coupled by poorly-understood dyna- 1 ton/sec of oxygen and sulphur through volcanic eruptions mical processes that transport energy, momentum and material which feeds an equatorial magnetodisc extending to hundreds (Vasavada and Showman, 2005). In addition, Jupiter’s atmosphere of planetary radii. To a lesser extent the moon Europa and is intricately connected to the charged-particle environments of Jupiter’s atmosphere/ionosphere provide additional sources of the ionosphere and magnetosphere (e.g., Yelle and Miller, 2004), particles in the magnetosphere. Then, a multi-step process and the local Jovian environment of the rings and icy satellites. involving magnetic field ruptures and plasma instabilities JUICE’s broad wavelength coverage will be used to characterise accelerates ions and electrons up to very high energies. The the vertical structure and coupling processes (e.g., wave propaga- energetic particles in turn impact the moons, releasing surface tion transporting energy and momentum; ion drag and meridio- material back out in to space to form their tenuous atmo- nal transport in the upper atmosphere) from the deep interior to spheres and tori. the charged upper atmosphere (Fig. 9). Studies of clouds and The Jovian magnetosphere is also the most accessible environ- hazes at a range of observational geometries will constrain the ment for direct in situ investigations of processes regarding: global vertical structure and composition of the cloud decks and (i) the stability and dynamics of magnetodiscs, and more gen- hazes from the millibar to 5-bar level in Jupiter’s atmosphere erally, angular momentum exchange and dissipation of rotational

Fig. 9. Advanced instrumentation for global & regional observations with broad spectral coverage from UV to radio wavelengths. O. Grasset et al. / Planetary and Space Science 78 (2013) 1–21 13 energy; (ii) the electro-dynamical coupling between a central equator). In the outer magnetosphere, the azimuthal plasma body and its satellites including plasma/surface interactions, velocity lags corotation by a factor of two or more. The outer transport processes and turbulence in partly ionised media; magnetosphere on the dayside is extremely variable in size. (iii) particle acceleration processes since the Jovian inner magne- Depending on the solar wind dynamic pressure, the dayside tosphere is the most severe radiation environment in the Solar magnetopause can be found anywhere from a distance of 45

System. RJ to 100 RJ (Joy et al., 2002). An extremely disturbed region, With its suite of global imaging and in situ measurements, known as the ‘‘cushion region’’, with a radial extent of 15 RJ was JUICE will for the first time unveil the global machinery of the discovered adjacent to the noon magnetopause in the magnetic Jovian magnetosphere. It will investigate the high-latitude field observations from Pioneer and Voyager spacecraft. It is not middle magnetosphere up to 301 above the equatorial plane, yet known whether this region is a permanent or a temporal enabling exploration of the three dimensional structure of the feature of the magnetosphere. Finally, in the night side, an Jovian magnetodisc, including in situ measurements outside additional current system exists that connects the magnetodisc the magnetodisc to determine plasma heating and accelera- current to the magnetopause currents. This current system tion processes, and will provide remote sensing measure- creates a long magnetotail (length47000 RJ), which extends all ments of Jupiter’s ring current and aurora. JUICE will also the way to the orbit of Saturn. The chain of processes involved in study the magnetospheric parameters in the vicinity of this giant magnetosphere, most likely common to any magnetised Europa, Ganymede (and its magnetosphere), and Callisto in systems combining fast rotation and radial transport is still not detail. quantified. JUICE will investigate the global configuration and dynamical 4.2.1. A fast magnetic rotator behaviour of Jupiter’s magnetodisc along its trajectory inside the The magnetosphere of Jupiter has been traditionally divided system including high latitudes and the magnetospheric region between Ganymede and Europa where the neutral tori are and into inner (o10 RJ), more dipolar-like configuration, and a middle where the magnetodisc starts to form, at the transition between (10–40 RJ) to outer (440 RJ) magnetosphere with highly stretched, more radial magnetic field configuration. The inner the inner and the middle magnetosphere. This goal requires region contains the synchrotron radiation belt of Jupiter measurements of 3 axis magnetic and electric field vectors with moderate sampling frequency. Measurements of thermal plasma (1.1oro3 RJ) formed by energetic electrons gyrating in the strong magnetic field and having energies up to tens of MeV. and energetic particles will characterise three dimensional dis- The inner region is also the location of the two main plasma tribution functions of ions and electrons, as well as mass spectra sources of the magnetosphere, namely Io and Europa. The fast of ions and neutrals. They will be complemented by measure- rotation of the planet, combined with the continuous supply of ments of the plasma density, electron temperature, plasma waves ion populations principally from Io’s volcanism, lead to the and electromagnetic emissions. Imaging and spectro-imaging will formation of a neutral and plasma torus, and further out, of a also be used to monitor Io’s volcanic activity, which is the main magnetodisc (Fig. 10). In addition to the Io torus there is also a source of material in the Jupiter magnetosphere. The uniqueness neutral gas torus along Europa’s orbit, inferred from energetic of JUICE measurements will be that the torus region will be particle signatures measured onboard Galileo (Lagg et al., 2003) studied in situ during the two Europa flybys as well as remotely and imaged in energetic neutral atom emissions onboard the e.g., during the high latitude Callisto phase of the mission. Cassini spacecraft during its Jupiter flyby (Mauk et al., 2004). In Contrary to Galileo, the global imaging instrumentation of JUICE the middle magnetosphere the magnetic field becomes highly is dedicated to capturing the global dynamics of Jupiter as a space stretched as it acts to contain the plasma against the strong plasma system, rather than just being focused on parts of it. centrifugal and thermal pressure forces. The plasma temperature is quite high and it is not fully understood which processes are responsible for energising the warm plasma of the torus to such 4.2.2. A giant accelerator high values. In addition, the plasma subject to the mirror force The dominant feature of the entire Jovian magnetosphere is collects in the region of field strength minimum (magnetic the motion of the plasma in the sense of corotation in a magnetodisc configuration as described above. The corotation of the plasma is highly dependent on the distance from the planet and on local time in the Jovian system. The distance where rigid

plasma corotation breaks down, ranges from 20 RJ in the dusk sector up to 40 RJ and beyond in the dawn to predawn sector of the magnetosphere. The magnetosphere is rotating outside that distance and reaches a nearly constant velocity independent on local time of about 200 km/s in the magnetotail of Jupiter. The flow is disrupted by dynamic changes in the outer magnetosphere on various time scales with periods of hours up to several days. Especially in the predawn sector, substorm-like radial flow bursts have been observed which change the global configuration of the entire magnetosphere. One of the dynamical processes is the radial transport of the material released from Io. In this process the plasma is transported through the entire magnetosphere first radially outward where the interchange motion plays a major role; then radially inward through diffusion processes from the outer magnetosphere into the inner part violating the third adiabatic invariant and gaining energy up to MeV. Another dynamic process in the Fig. 10. Artist’s conception of the Io and Europa torus (from Mauk et al. (2004)), illustrating the neutral and charged particle distributions in the region between middle magnetosphere involves particle injections where hot Ganymede and Io, one of the key regions of the Jovian magnetosphere. plasma from the outer part is being injected into colder plasma 14 O. Grasset et al. / Planetary and Space Science 78 (2013) 1–21 further in. Finally in the outer part of the magnetosphere assumed that silicate volcanism is dominant at thermal emission reconnection of magnetic field lines and associated particle enhanced hot spots, while secondary sulphur volcanism may be acceleration takes place and influence the particle dynamic inside important at certain places (e.g., McEwen et al., 2004) and is the magnetosphere. responsible for the dominance of SO2 in Io’s atmosphere. JUICE JUICE will significantly enhance our knowledge of the trans- will monitor the volcanic activity of Io, and determine the port processes occurring in the magnetosphere in the equatorial composition of different materials on the surface at regional scale plane and at higher latitudes with better time resolution and (50–200 km/px) through remote multi-wavelength imaging spec- better directional information as for previous missions. Similarly troscopy. JUICE will significantly contribute to our understanding to the previous goal, JUICE will sample 3 axis magnetic and of the atmospheres of the icy satellites, their origin and evolution, electric field vectors, will characterise thermal plasma and ener- as well as the composition of their surfaces, by observing the getic particles, and will investigate plasma density, electron exospheres of Europa and Ganymede through remote monitoring, temperature, plasma waves and electromagnetic emissions. Jupi- imaging of the aurora, multi-wavelength limb scans and stellar ter is an enormous source of electromagnetic radiation located in occultation, supported by in situ measurements by particle the Io-torus and in the auroral high latitude regions. Some of packages from low orbits and fly-bys. those emissions are generated by field-aligned particles originating and accelerated deep in the magnetotail, travelling along field 4.3. Coupling processes in the Jovian system lines into the polar regions close to the planet. Especially during the high-latitude Callisto phase JUICE will be able to ‘‘image’’ the 4.3.1. Gravitational coupling—The Laplace resonance auroral region remotely and measure directly the source region of Io, Europa, and Ganymede are locked in a mean-motion the decametric radiation where a spacecraft orbit inclination of resonance unique in the Solar System, the so-called Laplace about 301 is required. resonance in which the orbital periods of the satellites are in the ratio 1:2:4 (Fig. 11). It is still unclear how and when the resonance formed. It might be of primordial origin (Greenberg, 4.2.3. Sources and sinks of magnetospheric plasma 1987; Peale and Lee, 2002) or formed by orbital expansion of the Many crucial parameters of the satellite/magnetosphere cou- satellites due to tides and subsequent capture into resonance as a pling have not yet been measured. During the close observation of result of the decreasing speed of orbital expansion with increas- icy satellites, plasma/surface interactions are key processes to be ing distance from Jupiter (Yoder, 1979; Yoder and Peale, 1981). investigated. This includes processes associated with sputtering The Laplace resonance plays an essential role in the redistribution of surfaces and exospheres and with resurfacing due to the of rotational and orbital energy between the Galilean moons and intense bombardment by energetic particles. Given the complex Jupiter and also determines the amount of tidal dissipation in the composition of the environment of Jupiter, including sulphur ions, satellites since it maintains finite orbital eccentricities, required the understanding of plasma resurfacing is a necessity for the for tidal interactions, on geological timescales. As tidal dissipation interpretation of the spectral signatures from the surfaces. The can be an important heat source for the satellites, and is by far the role played by charged particles in modifying the reflectance of largest energy source for Io, gravitational interactions can also the moons’ surfaces is not fully understood. It is also clear that drive the internal dynamics and the evolution of the satellite’s energetic ions and electrons are the principal chemical agents in interior and surface. Understanding the gravitational interactions layers close to the surface of moons. However, the significance of between Jupiter and the Galilean satellites is therefore essential these effects depends on the magnetic environment. JUICE will for many aspects of Jupiter system science. JUICE will provide characterise the particle populations near the moons Ganymede new constraints on the evolution of the system from a thorough and Callisto, and their interaction with Jupiter’s magnetosphere study of the tidal response of the icy moons, and especially by measuring the velocity-space distribution of thermal plasma Ganymede (see Section 2.1.1). This will complement the and energetic particles from eV to MeV, plasma and radio waves, ground-based astrometric observations to quantify tidal energy and neutral imaging from eV to keV of the impacting plasma and dissipation in the satellites and Jupiter. ejected material. As discussed earlier, the Galilean satellites are known to have tenuous atmospheres/exospheres (McGrath et al., 2004; 4.3.2. Magnetospheric coupling Coustenis et al., 2010). Their properties are indicative of processes Electromagnetic coupling processes occurring within the and composition at the surfaces. Despite its relatively small size, Jovian magnetosphere are divided into two categories (Fig. 12): Io is the most volcanically active body in the Solar System. It is (i) the processes which are the result of coupling between the

Fig. 11. The rotational energy of Jupiter is a huge reservoir of energy for the three inner Galilean satellites. Orbital energy gained by Io due to tidal torques exerted by Jupiter is distributed among Io, Europa, and Ganymede, due to the Laplace resonance. O. Grasset et al. / Planetary and Space Science 78 (2013) 1–21 15

Fig. 12. Left: The magnetosphere—ionosphere coupling current system (After Cowley and Bunce, 2001). Right: The main auroral emissions, including the magnetically mapped moon footprints (Grodent et al., 2008). planet, its rapidly rotating magnetosphere and the satellites (e.g., ionosphere coupling system are thought to relate directly to the Io, Europa, Ganymede, and Callisto); (ii) the processes due to the main auroral emissions in Jupiter’s atmosphere (see Cowley and large-scale coupling between Jupiter and the magnetically con- Bunce, 2001), and as such the dynamics of the middle magneto- nected solar wind – magnetosphere – ionosphere system. In all sphere can be viewed through combined in situ plasma sheet and cases, the interactions result in magnetic field perturbations, remote auroral observations. Especially during the high-latitude plasma signatures, radio waves, and/or auroral emissions (at UV, Callisto phase, JUICE will be able to ‘‘image’’ the auroral region IR, visible, X-ray wavelengths). Analysis of previous data (e.g., remotely and measure directly the source region of the deca- Galileo), remote observations (e.g., Hubble Space Telescope, metric radiation where a spacecraft orbit inclination of about 301 Chandra/XMM, IRTF/UKIRT, and/or radio telescopes), and theore- is required. tical modelling and simulation studies are the main source of data in this field. However, there are major gaps in temporal coverage 4.4. The formation of the Jovian system and spatial resolution. In the first case, the Galilean moons interact with the field and The giant planets of the Solar System contain in their compo- plasma of the Jovian magnetosphere over many spatial scales. The sitional make-up signatures of the proto-planetary nebula during interactions change the plasma momentum, temperature, and the epoch of planetary formation, providing a window onto the distribution function, and generate strong electrical current sys- earliest evolutionary stages of our Solar System. The limited tems. Important intrinsic properties of the moons affect the composition information available, largely from the Galileo probe, interactions with the plasma that flows onto them, and simulta- favours the core accretion hypothesis for the formation of giant neously, the properties of the Jovian plasma at the orbit of the planets (e.g., Lunine et al., 2004), possibly with subsequent radial moon also affect the interaction. One of the most interesting migration of planetary orbits (e.g., Tsiganis et al., 2005). In such a interactions in this regard, is the one between Jupiter’s magneto- scenario, giant planets first formed a solid core of approximately sphere and Ganymede. The internally generated magnetic field of 10 Earth masses, through accretion of the primordial icy plane- Ganymede extends beyond the surface of the moon, and allows the tesimals of the outer Solar System that would act as the accretion creation of a miniature magnetosphere embedded within the centre for the gas of the Solar Nebula. The limited lifetime of the rapidly rotating Jovian magnetosphere (see Section 2.1.4). The Solar Nebula, which has been constrained to about 10 My through Galileo mission has provided new information on the above astronomical observations of circumstellar disks around near-by properties and allowed for many breakthroughs in our under- stars (Meyer et al., 2006), poses a strict constraint to the forma- standing, but there remain many open questions as we learn more tion time of the planetary core and the accumulation of the gas. about these complex interactions. JUICE will provide systematic Accretion of gas and solid material into Jupiter’s envelope actually and long-term investigations of the coupling processes described works through the formation of a sub-nebular disk, and it is above in the Jupiter magnetosphere by combining the in situ within this sub-nebular disk that formation of regular satellites measurements of the magnetic field, plasma and plasma waves by accretion of solids is believed to take place. Their further with multi-wavelength remote sensing observations including differentiation should then be completed before the complete ENA. For example combining observations of Ganymede’s magne- decay of 26Al, which is the main energy source for this process, tosphere with simultaneous remote observations of its auroral namely in a time between 2.5 and 5.0 My. Conversely, the footprint in the Jovian atmosphere would provide unique informa- irregular Jovian satellites are believed to be captured objects from tion on the dynamics and complexity of the coupled system. a population of primordial icy planetesimals. In the second case, Jupiter’s middle magnetosphere is domi- JUICE will supply new crucial information to address this topic nated by the effects of the rapid rotation of the planet. The by providing an unprecedented understanding of the surface magnetosphere–ionosphere coupling current system is set up due properties and internal structure of the icy Galilean satellites, to the sub-corotation of magnetosphere plasma, and generates a especially Ganymede. The relationship between the formation of large-scale current system which links to the ionosphere via field- the Galilean satellites and that of Jupiter will be investigated by aligned currents. The upward currents in the magnetosphere– JUICE via measurements of the abundances of the stable isotopes 16 O. Grasset et al. / Planetary and Space Science 78 (2013) 1–21 of C, H, O and N and of noble gases in the ices of these satellites, provide numerous close flybys of the giant planet and the three and via the comparative study of their internal structure. The icy moons, as compiled in Fig. 13 as a function of time. study of the impact craters, their sizes and distribution provides The model payload that was constructed for the assessment important information about the age of the surfaces of the phase was driven by the science objectives, and will provide the satellites and helps to comprehend the evolution of the early Solar measurements required to resolve the science questions. It is System, in particular the reality and the characteristics of the Late divided into four packages: a narrow and a wide angle camera Heavy Bombardment that has been suggested to be triggered by the composing the imaging part; the spectroscopic measurements are combined effects of the migration of the giant planets and their based on a sub-mm wave instrument and on ultraviolet, visible interactions with the residual planetesimal disk (Tsiganis et al., and infrared imaging spectrometers; in situ measurements of 2005). Along with a better understanding of Jupiter’s atmospheric fields and particles are based on a magnetometer, a radio and composition, all these elements will combine together to improve plasma wave instrument, and a particle and plasma package our knowledge of the environment, i.e., the Solar Nebula and the which includes an ion neutral mass spectrometer; finally soun- Jovian sub-nebula, from which Jupiter and its satellites formed. ders (a laser altimeter and ice penetrating radar) as well as radio science are part of the current model payload. The total mass should be about 100 kg. This model payload will be revised after the call for instrument proposals has reached its conclusion. 5. The mission profile of JUICE

The JUICE mission is planned to be launched in mid-2022, with 5.1. The Jupiter tour a backup opportunity in August 2023. It will arrive at Jupiter in January 2030 after 7.6-years using an Earth–Venus–Earth–Earth Beginning on approach to Jupiter, long term monitoring of gravity assist sequence. For the backup launch the transfer Jupiter’s atmosphere and magnetospheric processes and dynamics duration is 8 years, resulting in an arrival at Jupiter in August will be initiated and will be performed throughout the initial orbit 2031. The mission phases at Jupiter are identical for baseline and phases at Jupiter using the remote sensing and in situ measurement backup launches and are divided into nine phases, excluding the capability of JUICE’s instrumentation. The tour at Jupiter will include interplanetary transfer, summarised in Table 1. These phases will two targeted Europa ﬂybys observing the composition of Europa’s

Table 1 Science phases of the JUICE baseline mission.

Phase Start End Duration Science priorities

1 Cruise/Interplanetary transfer 06.2022 01.2030 7.6 years

Jupiter Tour 2 Jupiter equatorial phase no 1/Transfer to Callisto 01.2030 01.2031 12 mon Jovian atmosphere structure, composition, and dynamics. Jovian magnetosphere as a fast magnetic rotator and giant accelerator. Remote observations of the inner Jovian system. 3 Europa flybys 02.2031 03.2031 36 days Composition of selected targets with emphasis on non-ice components Geology and subsurface of the most active areas Local plasma environment 4 Jupiter high latitude phase with Callisto 04.2031 10.2031 6 mon Jupiter atmosphere at high latitudes Plasma and fields out off equatorial plane Callisto internal structure, surface and exosphere. Remote observations of Ganymede, Europa, Io, and small moons. 5 Jupiter equatorial phase no 2/Transfer to 11.2031 08.2032 9 mon Interactions of the Ganymede magnetic field with that of Jupiter. Ganymede Jovian atmosphere and magnetosphere as in phase #2

Ganymede Tour 6 Elliptic no 1 09.2032 10.2032 30 d Global geological mapping Search for past and present activity 7 High altitude (5000 km) circular 10.2032 01.2033 90 d Global compositional mapping 8 Elliptic no 2 01.2033 02.2033 30 d Local plasma environment and its interactions with Jovian magnetosphere

9 Medium altitude (500 km) circular orbit 02.2033 06.2033 102 d Extent of the ocean and its relation to the deep interior Ice shell structure including distribution of subsurface water 10 Low altitude (200 km) circular orbit 06.2033 07.2033 30 d Geology, composition and evolution of selected targets with very high resolution Global topography Local plasma environment Sinks and sources of the ionosphere and exosphere Deep interior

Fig. 13. Illustrative timeline of the JUICE baseline mission. The markers indicate times of closest approaches, perijoves, and Jupiter Orbit Insertion (JOI). O. Grasset et al. / Planetary and Space Science 78 (2013) 1–21 17 non-water-ice material, and performing the first subsurface obser- the coverage of surface geological features, the induced magnetic vations of an icy moon. The Europa flybys will be performed from a field and the properties and structure of the top ice layer can be 4:1 resonant orbit, where Europa will be encountered at perijove. characterised. As is also indicated in Fig. 14b, the ground tracks at The flyby orbits will be initiated and terminated with Callisto gravity closest approaches for the two flybys are separated in latitude, assists, which is most efficient for propellant consumption and for demonstrating that the detailed remote sensing can be achieved radiation exposure. The planned observing sequence is optimised for at different areas of specific interest, suspected to be active both remote sensing and in situ measurements and is illustrated in regions or possibly having the thinnest ice layer. From all the Fig. 14, where JUICE’s altitude is plotted as a function of time, and possible sites with high potential for geology, chemistry and the operations of the instruments are indicated by different colours. astrobiology (see red areas in Fig. 14b), Thera and Thrace Macula The in situ instruments (plasma spectrometers, magnetometer) will and Lenticulae are currently selected for closest approach during be operated during the entire flyby (red line in Fig. 14a); the camera the two flybys. will be imaging the surface during the entire flyby, while the In the following phase, the inclination of the spacecraft’s orbit sub-nadir point is in sunlight, starting from about 10,000 km will be raised to almost 301 using repetitive Callisto flybys. The altitude (yellow line in Fig. 14a); the imaging spectrometers will observation opportunities will complement Galileo observations be operated from 10,000 to 1000 km altitudes (orange line in with respect to Callisto’s internal structure and will provide mid- Fig. 14a); during closest approach (o1000 km altitude, closest to high-latitude Jupiter atmosphere and magnetosphere measure- approach is 400 km) the ice-penetrating radar and the laser ments over an extended temporal and spatial baseline. During the altimeter will be the prime instruments (green line in Fig. 14a). Callisto flybys of this phase, a sequence similar to the one With this balance of instrument operations the achievement of presented for Europa will be performed at closest approach for main science goals can be satisfied. some of the Callisto flybys, except for those (about three) where JUICE investigations on Europa will be focused on the compo- the internal structure will be investigated using the radio- sition of the non-water-ice material, organic chemistry, and the science experiment. At the highest inclined orbit, Jupiter’s poles first subsurface observations of an icy moon, including the first will be visible providing capability for atmospheric dynamics, determination of the minimal thickness of the icy crust over the chemistry and energy balance at polar latitudes. This special most active regions. Specifically, for the chosen Europa flyby observing point allows for a better understanding of mag- observation sequences, the sampling of the tenuous atmosphere, netosphere–atmosphere coupling, as well as a more detailed

200000 Local plasma environment and interaction NIGHT SIDE with the jovian magnetosphere

100000 Global/regional geological mapping Search for past and present activity

Very HR geology and Atmosphere characterisation

Altitude (km) composition mapping 1000 Global/Regional compositional mapping 500 Ice shell structure Distribution of subsurface water Local topography

-1000 -360 -10 0 +10 + 360 +1000 Time (minutes)

90 Lenticulae

Conamara chaos

Flyby 1 0 Flyby 2

Thera and Thrace Macula Latitude (deg)

-90 0 90 180 360 East longitude (deg)

Fig. 14. An example instrument operation sequence illustrated with the Europa flybys. (a) A possible strategy of observations during the Europa flybys; see text for a more detailed description. (b) The field of view of the model instruments is indicated on the ground track of the flybys for mapping at regional and local scales: yellow (narrow angle camera), orange (spectro-imagers), green (radar sounding experiment), purple (wide angle camera). At closest approach, the field of view of the instruments for local studies are indicated in the insets on the right. Global mapping at high altitudes during approach and during departure is not shown for clarity. Areas with very high potential for geology, chemistry, and astrobiology are indicated in red. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 18 O. Grasset et al. / Planetary and Space Science 78 (2013) 1–21

Fig. 15. Sketch of the baseline scenario of the Ganymede mission. The red line shows the evolution of the b-angle for each Ganymede phase as a function of time. Grey areas mark solar eclipses corresponding to the altitude of the orbit. On top, priority investigations are displayed for each phase. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) study of atmospheric properties in both hemispheres using radio local plasma environment and its changing interaction with the science measurements. Similarly, Jovian magnetospheric science Jovian magnetosphere will be investigated during the highly elliptic will be enhanced since the mission will enable the first pro- orbits at the beginning and at the end of this period. longed studies of this region of the magnetosphere, as well as At the transfer to the medium altitude (500 km) circular orbit, provide a long temporal baseline study of Jovian aurorae. In the b-angle will be 621 (Fig. 15). JUICE will perform full mapping addition, it will provide detailed analysis of the field lines and of the moon at 1 km/px, together with high-resolution imaging field-aligned current systems well poleward of the main oval. (o10 m/px) and additional spectro-imaging of selected targets And finally, it will give access to the high latitude radio sources (100 m/px). Precise topography measurements and subsurface in the Jovian magnetosphere. exploration of regions of interest will be performed in this phase. Plasma and fields investigations close to the moon will also be 5.2. The Ganymede tour conducted in order to further disentangle the combined contribution of the fields (Jovian, intrinsic, induced fields). JUICE will be inserted into orbit around Ganymede in Septem- The JUICE spacecraft will then be transferred to a 200 km ber 2032. The mission at Ganymede consists of five phases orbit, which will be close to terminator (b 801; Fig. 15). The (Table 1) with different altitudes and different surface illumina- remainder of the nominal mission will be mainly devoted to the tions. The orbits around Ganymede are designed in a way that the geophysical observations (topography, gravity fields, subsurface Sun declination relative to the orbital plane (referred to as b-angle) exploration). Ganymede’s surface will rotate underneath the drifts towards larger values over time allowing the spacecraft to be spacecraft so that the sub-nadir point will shift by 10 km per in sunlight over the entire orbit duration, avoiding oversizing of the orbit. This phase is also essential for exosphere, plasma, and solar panels for power generation. The sequence of altitudes fields investigations. Despite the marginal illumination, this low follows the energetically most efficient steps, moving from high orbit would allow for the best resolution imaging possible: full altitudes to lower altitudes. This sequence is illustrated by the red surface coverage with 400 m/px and selected targets with few line in Fig. 15 plotting the b-angle as a function of time. The eclipse meters per pixel resolution. conditions per spacecraft altitude are indicated by grey shades. The nominal mission of JUICE will end in June 2033. If the The Ganymede mission will start with a highly elliptical orbit spacecraft health and resources permit, there is the possibility of (200 10,000km)aroundthemoonwithaperiodof12 h, inclina- extending the mission by keeping the 200 km circular orbit. As tion of 861, and Sun declination (b-angle) of 251 (Fig. 15). Due to the b-angle continuously increases, it will start decreasing again the high apocentre the gravitational perturbation by Jupiter is after having passed the terminator. The investigations started in significant, and will cause this orbit to reduce in eccentricity within the 200 km circular phase could be continued during an extended 30 days, when it will become circular with an altitude of 5000 km. mission. About 480 days after the Ganymede orbit insertion This orbit will be maintained with negligible Dv cost for about 90 (200 days after passing the terminator orbit) the b-angle days. Then the eccentricity will increase due to a similar perturbation decreases to 671 and the spacecraft starts entering eclipses. effect until a suitable point for injection into a 500 km altitude Progressing further in the extended mission would enable limited circular orbit is reached. These two elliptic and high altitude plasma and fields investigations while in eclipse and additional (5000 km) circular phases will provide for global imaging and imaging and spectro-imaging on the dayside. During these peri- spectro-imaging to study geology and surface composition. JUICE will ods, however, the spacecraft’s available power will quickly complete the surface imaging at 10 km/px resolution, and will per- deplete triggering mission end. Alternatively, at the end of the form high-resolution panchromatic and colour imaging of 16% of the mission there may be an opportunity for JUICE to probe lower surface at few hundred meters per pixel resolution. More than 50% of altitudes, as the spacecraft will spiral down towards impact on the surface will be investigated by spectral imaging at a spatial the surface of Ganymede after orbit maintenance is terminated by resolution between 2 and 3 km/px. The boundaries of Ganymede’s mission operations. O. Grasset et al. / Planetary and Space Science 78 (2013) 1–21 19

6. Conclusion carried out a study of mission concepts focusing on detailed exploration of Europa including multi-flyby, orbiter, and lander Following Galileo’s discovery 400 years ago of the Galilean scenarios. Finally, JAXA has been studying a mission to explore satellites, our knowledge of the large gas giants within our Solar Trojan asteroids, which would provide key information regarding System has continued to grow. There have been numerous ground the origin of the outer solar system. Implementation of any of and space-based observations of the Jupiter and Saturn systems; these missions would add a remarkable synergy to the JUICE flybys by the Pioneer and Voyager spacecraft; the discovery of investigations. habitable environments in deep ocean ridges on the Earth; the The addition of JUICE to the ESA science programme will orbital tour by the Galileo spacecraft at Jupiter; and the ongoing ultimately offer numerous opportunities for education and public orbital tour by the Cassini spacecraft at Saturn. JUICE is the outreach activities. JUICE will build on scientific and technological necessary step for future exploration of our outer Solar System. heritage from previous large ambitious space missions and will It is now time to characterise the potential habitable worlds pave the way for future extensive in situ endeavours to be Ganymede, Europa, and Callisto. JUICE will also provide a thor- conducted. ough investigation of the Jupiter system, which serves as a miniature Solar System in its own right, with a myriad of unique environments to explore. Jupiter serves as the archetype for Acknowledgements exoplanetary systems, it played an essential role in the development of our own habitable environment, and is the perfect The authors would like to gratefully acknowledge the essential destination for an exploration of our origins. To summarise, the contribution of the ESA Study Team: A. Wielders, ESA Payload Study observation strategy with JUICE can be described in three steps: Manager, N. Altobelli, ESA Science Operations Study Manager, A. Boutonnet, B. Garcia Gutierrez, P. Van der Plaas, J. Schoenmaekers, Conduct a comparative study of Ganymede, Callisto and P. Nieminen, J., Sorensen, G. Santin, M. Gehler, T. Voirin, P. Falkner Europa, with a special focus on Ganymede, which JUICE will and support from the ESA’s planning and coordination office: F. characterise in great detail from orbit. Favata, L. Colangeli, Ph. Escoubet. The three industrial studies were Provide a complete spatio-temporal characterisation of the conducted by Consortia led by: EADS Astrium SAS, OHB-System AG, giant, rotating magnetosphere, and of the meteorology, chem- and Thales Alenia Space. We are extremely grateful to the planetary istry and structure of Jupiter’s gaseous atmosphere. community for their permanent support and to our US colleagues Study coupling processes inside the Jupiter system, with for the joint work on the EJSM-Laplace concept. emphasis on the two key coupling processes within that It is with great sadness that we commemorate the untimely system: gravitational coupling, which ties together Jupiter departures of Angioletta Coradini, JUICE SST member, and Ronald and its satellite system, and electrodynamic interactions Greeley, the US Co-Lead of the Joint Science Definition Team for which couple Jupiter and its satellites to its atmosphere, EJSM-Laplace. magnetosphere and magnetodisc.

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