Jupiter exploration
Jupiter has the most complex and energetic radiation belts in our Solar System and one of the most challenging space environments to measure and characterize in-depth. Their hazardous environment is also a reason why so many spacecraft avoid flying directly through their most intense regions, thus explaining how Jupiter’s radiation belts have kept many of their secrets so well hidden, despite having been studied for decades. In this paper we argue why these secrets are worth unveiling. Jupiter’s radiation belts and the vast magnetosphere that encloses them constitute an unprecedented physical laboratory, suitable for interdisciplinary and novel scientific investigations: from studying fundamental high energy plasma physics processes which operate throughout the Universe, such as adiabatic charged particle acceleration and nonlinear wave-particle interactions, to exploiting the astrobiological consequences of energetic particle radiation. The in-situ exploration of the uninviting environment of Jupiter’s radiation belts presents us with many challenges in mission design, science planning, instrumentation, and technology. We address these challenges by reviewing the different options that exist for direct and indirect observations of this unique system. We stress the need for new instruments, the value of synergistic Earth and Jupiter-based remote sensing and in-situ investigations, and the vital importance of multi-spacecraft in-situ measurements. While simultaneous, multi-point in-situ observations have long become the standard for exploring electromagnetic interactions in the inner Solar System, they have never taken place at Jupiter or any strongly magnetized planet besides Earth. We conclude that a dedicated multi-spacecraft mission to Jupiter is an essential and obvious way forward for exploring the planet’s radiation belts. Besides guaranteeing numerous discoveries and huge leaps in our understanding of radiation belt systems, such a mission would also enable us to view Jupiter, its extended magnetosphere, moons, and rings under new light, with great benefits for space, planetary, and astrophysical sciences. For all these reasons, in-situ investigations of Jupiter’s radiation belts deserve to be given a high priority in the future exploration of our Solar System. This article is based on a White Paper submitted in response to the European Space Agency’s call for science themes for its Voyage 2050 programme.
Introduction
Why explore planetary radiation belts?
Radiation belts are the regions of a magnetosphere where high energy charged particles, such as electrons, protons, and heavier ions, are trapped in large numbers. All planets in our Solar System that are sufficiently magnetized (Earth, Jupiter, Saturn, Uranus, and Neptune) host radiation belts Radiation belts are not the only regions that high energy particles can be observed; they can be found throughout a planetary magnetosphere, in the heliosphere, or the astrospheres of stars, in astrophysical objects such as brown dwarfs, and in the interstellar and intergalactic medium. Many of the environments where energetic particles are found cannot be replicated in the laboratory. Even measuring particle radiation in space is not by itself sufficient to understand its origins: For instance, while we have constrained many properties of Galactic Cosmic Rays (GCRs), the highest energy particles that we can measure, their acceleration sites are inaccessible for in-situ studies.
Radiation belts offer the opportunity to perform ground truth measurements for a variety of high energy physics processes. Apart from containing the energetic particles which we can measure in-situ, they also host most mechanisms that accelerate these particles from low to high energies in a small enough region and over time scales that can be fully monitored with space missions. These processes are explored in conjunction with additional in-situ particle and fields measurements (plasma, magnetic, electric fields, electromagnetic waves) or close-proximity remote sensing observations, such as Energetic Neutral Atom (ENA) imaging These observations, which are critical for understanding the production and dynamics of particle radiation , are similarly challenging to constrain for astrophysical systems. In that sense, planetary radiation belts can be seen as laboratories for in-situ, high energy astrophysics.
The strong links between radiation belts and their host planet further advocate their exploration. Radiation belt particles are modified by the accumulated effects of the planetary neutral environment with which they interact: The properties of planetary exospheres, rings and moon-generated neutral torii are regularly studied through energetic particle measurements, particularly in extraterrestrial systems . The reverse path, i.e. the impact of the radiation belts on different components of a planetary system, is also important: Surface sputtering or physical and chemical alteration of moon surfaces are among several fundamental consequences of such an interaction .
Radiation belt measurements have been performed at all the planets hosting them. The terrestrial radiation belts, studied since the beginning of the space age, are the best understood in terms of structure, origin, and dynamical evolution . However, detailed observations of other planetary radiation belts show us that not one of them can be used as a prototype for all others . Generalizing our understanding of how radiation belts work requires that we realize the different ways through which particle acceleration and loss processes can be coupled in any particular magnetospheric environment.
In that respect, measurements in the radiation belts of Uranus and Neptune, sampled only once by the Voyager 2 spacecraft, should definitely be part of any future attempt to explore the two planets . Saturn’s radiation belts were surveyed in depth thanks to the 13-year Cassini mission at the Kronian system . In comparison, Jupiter’s radiation belts, while visited by numerous missions and monitored for decades through their synchrotron emission , still hold onto many of their secrets. No single mission, payload, or observation campaign was ever designed to capture and/or cope with their full scale, complexity, dynamics, and energetics, as argued in the two follow-up subsections.
The uniqueness of Jupiter’s radiation belts
Jupiter’s radiation belts are contained within the planet’s magnetosphere, formed by a magnetic field that is 20000 times stronger than Earth’s. Jupiter’s fast rotation and material from Io’s volcanoes that fills the system aid the magnetic field to push against the solar wind even further leading to a magnetosphere of enormous dimensions . Within this giant system, the radiation belts grow into one of the most hazardous regions of our Solar System, trapping charged particles of extreme fluxes and energies that are typical for GCRs (Fig. 1). Unlike the radiation belts of Earth and Saturn that are limited in their extent , substantial fluxes of energetic particles fill Jupiter’s magnetosphere until the magnetopause . Energetic electrons leaking into the solar wind are so intense that they overwhelm the < 10 MeV GCR electrons inside ∼10 AU from the Sun, despite Jupiter being a point source in the vast heliosphere
Jupiter’s magnetic field is so strong that even ultrarelativistic, ∼100 GeV protons can be trapped near the planet, over 50 times higher in energy than at Earth . Most importantly, observations and theory dictate that processes which may populate the radiation belts with ultrarelativistic particles do exist: Jupiter’s inner radiation belts contain electrons with energies in excess of 70 MeV , possibly even above 100 MeV based on model predictions . These electrons emit intense synchrotron radiation which can be detected with radio telescopes. This is one of Jupiter’s most useful and unique qualities from an observational perspective, since a global picture of the most intense part of the belts can be seen remotely. Furthermore, data from the Juno spacecraft, currently orbiting the planet, reveal sites at locations remote from the radiation belts, where electron acceleration to MeV energies is regular and impulsive , indicating that charged particles can gain significant energy well before reaching low jovian altitudes.
Jupiter’s belts have a distinctively large variety of ions in comparable abundance to protons (Fig. 1, right). At other planets, ions at many MeV/n are typically trace elements. Heavy energetic ions at Jupiter, such as oxygen and sulfur, originate primarily from its moons through volcanism or particle sputtering. At lower abundances, ions like helium, sodium, magnesium, carbon, and iron, have also been measured. Furthermore, some of the ions have a range of charge (ionization) states . This zoo of particle species and charge states evolving across very broad energy ranges, plasma, magnetic field, and wave environments within the magnetosphere, render Jupiter into an unparalleled physics laboratory for testing theories of charged particle transport and non-adiabatic acceleration and losses .
Jupiter’s magnetosphere is peppered with moons and rings which sculpt the radiation belts by absorbing energetic particles, thereby obstructing their transport and energization. Energetic particle scattering into the atmosphere through waves generated by a moon’s electrodynamic interaction with the magnetosphere, as also seen at Saturn , can have similar effects . Under certain conditions, moons may instead drive charged particle acceleration, mimicking the coupling that may exist between exoplanets and their host astrosphere . The presence of moons within the radiation belts is thus all the more interesting. Studying how the perturbations they generate on energetic particle distributions evolve within the magnetosphere and why moons seem to affect certain species and energies more than others (Fig. 1), we can gain valuable insights into the belts’ complex dynamics . Radiation belt measurements probing physical properties of Jupiter’s faint rings, that are challenging to obtain by other means, is another important application . Jupiter is also considered as the closest analogue for pulsar and brown dwarf magnetospheres that could host radiation belts
Observational challenges and missing links
From the missions that have been or are being used to gain insights into Jupiter’s radiation belts (Table 1), none was designed to investigate all elements of this multi-component system in a comprehensive way. To minimize radiation exposure, spacecraft orbits or operations are often planned in ways that avoid the belts’ core region where extreme levels of particle radiation are expected . This region is roughly located inward of Io’s orbit at ∼5.9 RJ and at low magnetic latitudes (1 RJ = 71492 km, a jovian radius). Pioneer 10, Pioneer 11, and Voyager 1 reached deeper than Io, offering just brief snapshots of the belts from their flyby trajectories. Only few of Galileo’s 34 orbits had their periapsis inward of Io’s distance. Future missions JUICE and Europa Clipper will not reach deeper than Europa’s orbit at ∼9.4 RJ.
For many past spacecraft that passed through the inner radiation belts (even at distances outward of Io’s orbit), their instruments suffered from saturation and/or radiation damage, rendering part of their data unusable or very challenging to calibrate . Juno, the only spacecraft currently passing through the inner belts repeatedly, has instruments responding to < 1 MeV electrons and < 10 MeV/n ions . At higher energies (e.g. > 10 MeV electrons), particles are identified through the noise they create on instruments like cameras . These measurements offer unique insights into the structure of the planet’s high-latitude radiation belts but are limited in energy or angular resolution. Energetic particle detectors on Galileo or earlier missions (e.g. , were similarly constrained in terms of energy resolution above a few MeV for electrons or ≳40 MeV/n for ions.
Energy and direction-resolved measurements are key for radiation belt studies as demonstrated by many published works for Earth and Saturn or Jupiter (e.g. below 1 MeV for electrons) . Much of the electron physics at Earth and Saturn, for instance, are contained below few MeV (EG. At Jupiter, much higher energies, which are hard to resolve, are equally important. Measurements of wave properties (e.g. chorus, Z-mode) and plasma distribution functions which impact the belts’ dynamics are also limited, particularly close to the planet , although the Juno mission is already filling some gaps from its high-latitude orbits that would also enable several deep radiation belt crossings until 2025.
Table 1 lists Earth-based observation methods that offer context information for Jupiter’s radiation belts. Synchrotron emissions, currently available also from the vantage point of Juno , offer global views for Jupiter’s electron belts, but achieve little in terms of energy resolution and provide no data on energetic ions. Monitoring of the Io plasma torus, a product of the moon’s volcanic activity, offers insights about magnetospheric flow fields which control the circulation of radiation belt particles , but only where the torus is present.
It is clear that remote sensing observations may capture only a small fraction of the big picture. In-situ (ground-truth) observations of the radiation belts are the best way to study them, link their smallest scales to the largest, and understand their structure, origin, and dynamics. However, even with the highly anticipated scientific measurements by ongoing and future Jupiter missions in mind, a follow-up mission that aims for a dedicated and detailed in-situ investigation of the planet’s radiation belts is necessary to offer closure to existing or emerging open questions. The outstanding science that can be performed in Jupiter’s radiation belts, discussed in Section 2, justifies why such an endeavor deserves to be assigned as a high priority target in ESA’s Voyage 2050 programme.
Outstanding science in Jupiter’s radiation belts
Scientific investigations in Jupiter’s radiation belts are well linked to three of the four overarching themes of ESA’s ongoing Cosmic Vision programme, which should be relevant also for the Voyage 2050 cycle. These links are traced in Table 2 and exemplified in
Adiabatic electron heating vs local electron radiation belt sources and losses
State of the art and open questions
Despite decades of research on how low energy electrons are accelerated to the very high energies observed in Jupiter’s radiation belts, many fundamental questions remain open. Similar to Earth , two main modes of acceleration are considered: Adiabatic heating and local acceleration of electrons by nonlinear interactions with electromagnetic waves. The challenges involved in separating the two contributions arise from the fact that they overlap in time, space, and energy. In addition, the same processes may induce particle and energy losses. For example, whistler-mode chorus waves can either energize electrons through energy diffusion or scatter them into the atmosphere through pitch angle diffusion. Whether a process acts as a source or a sink depends on the background space environment which defines the energies, pitch angles, and regions that resonant interactions occur. Figure 2 summarizes our current view on how certain interactions in Jupiter’s electron radiation belts (e.g. scattering) are distributed in L-shell and energy and which magnetospheric mechanism is their driving force.
Adiabatic heating, i.e. the energization of electrons through inward transport towards stronger magnetic fields, can be facilitated by at least three mechanisms: (1) energy independent radial diffusion induced by low frequency magnetic field fluctuations possibly driven by variable thermosphere winds , (2) interchange injections , and (3) transport due to variable convective electric fields, such as the dawn-dusk electric field ]. The latter refers to a strongly energy dependent mechanism, with high efficiency for 10-100 MeV electrons that drift slowly in magnetospheric local time (“corotation drift resonance”) . Existing physical radiation belt models assume an energy independent radial diffusion as available measurements cannot adequately constrain the other two processes, especially above 1 MeV. Resolving the energetic electron distribution function is necessary for separating overlapping adiabatic processes with a distinct energy dependence (Fig. 2). Transient phenomena (e.g. interplanetary shocks) may also mediate adiabatic transport, but are discussed in Section
The occurrence of resonant wave-particle interactions has been observed around Jupiter primarily by Galileo and in the extended disc of plasma outward of Io’s orbit . Inward of Io the space environment may also be favorable for wave-driven acceleration, but its properties are less constrained . Other wave types which are not as important at Earth, such as Z-mode, may have a strong impact at the giant planets, as Saturn-based research shows . The radiation belts inward of Io are a prime candidate for Z-mode acceleration of electrons . These inner belts are also the only place in the Solar System where we can investigate in-situ the impact of synchrotron energy losses . The production of synchrotron radiation not only affects the energy of the electrons but determines how far in latitude they can execute their field-aligned bounce motion, limiting in this way the wave populations they can interact with.
Key measurements and justification
Figure 2 shows how any given physical process may generate a seed electron population for another to take over. Adiabatic heating may provide electrons in the appropriate energy range and L-shell where waves would accelerate them further, and vice-versa. Processes thus complement each other and are best studied in unison. In that respect, plasma moments and composition, electromagnetic wave properties (frequency, power, polarization, wave normal angle), energetic electron spectra and their spatial distribution need to be resolved. While Juno, JUICE, and Europa Clipper will bridge several existing gaps in spatial coverage , low and mid-latitudes and a wide local time range inward of Europa would need to be surveyed. The wave frequency coverage should extend at least until up to the upper-hybrid range, such that the whistler and Z-modes are resolved at any distance, including very strong magnetic field regions near the planet. Electron measurements should be directional and energy-resolved well beyond ∼1 MeV. The signature of corotation drift resonant transport could extend up to ∼100 MeV, for instance. Observations over extended time should help define the nominal configuration of Jupiter’s electron belts.
Cosmic Ray Albedo Neutron Decay as universal proton radiation belt source
State of the art and open questions
Galactic Cosmic Rays (GCRs) with a sufficient energy to overcome the barrier of a planetary magnetic field may collide with a planet’s atmosphere and rings . The secondary neutrons produced by these collisions can travel away from their generation site until they β-decay to protons and electrons. If this happens in a material-free region of the magnetosphere, the decay products become trapped and add to the planet’s radiation belts. This process, termed Cosmic Ray Albedo Neutron Decay (CRAND – Fig. 3), supplies and maintains the radiation belts of Earth and Saturn with protons of energies between several MeV and up to the proton trapping limit of each planet in the GeV range . Earth and Saturn have weaker magnetic field strengths compared to Jupiter that allow a significant part of the GCR spectrum to reach them, and material targets that can generate considerable fluxes of secondary neutrons (Earth’s nitrogen/oxygen atmosphere, Saturn’s icy rings). Jupiter, instead, has a hydrogenous atmosphere, tenuous rings, and a much stronger magnetic field. This different parameter regime is suitable for testing whether CRAND-driven proton radiation belts are a component of any large-scale magnetosphere or if their presence depends strongly on the unique properties of a planet and its magnetosphere.
