Jupiter’s moon Ganymede possesses a tenuous atmosphere, as part of which atomic hydrogen has been detected in an extended exosphere by the Galileo spacecraft and the Hubble Space Telescope (HST). Here, we analyse temporally resolved Lyman-α observations from HST’s Space Telescope Imaging Spectrograph (STIS) of Ganymede in eclipse and after egress to investigate the behaviour hydrogen bearing species in the atmosphere. Atomic hydrogen is probed via resonant scattering of solar Lyman-α radiation at 121.6 nm and can therefore be observed immediately after but not during eclipse, when possible auroral Lyman-α emission is instead examined. We use an observation beginning at egress to determine whether the H component underwent depletion or collapse while Ganymede was in Jupiter’s shadow. The H surface number densities inferred from radial Lymanα brightness profiles recorded directly after eclipse match the values derived from subsequent exposures and are similar to previously reported densities. This absence of depletion suggests that Ganymede’s hydrogen exosphere remains largely unaffected by eclipse. These findings support hydrogen production mechanisms that operate independently of sunlight. Finally, unlike the bright electron-impact–driven 1356 Å oxygen aurora, we find no conclusive evidence for localized Lyman-α auroral emission from hydrogen-bearing species in eclipse.

We analysed far-ultraviolet (FUV) spectra of Uranus obtained by the HST STIS and COS instruments in 2012 and 2014, respectively, to determine the brightness of Raman-scattered Lyman-alpha (Ly α ) emissions centred at 1280 Å (hereafter, the Raman feature). The Raman feature is unique among the Solar System’s giant planets and forms in Uranus’ atmosphere due to weak vertical mixing of hydrocarbons with H 2 , leading to efficient Rayleigh–Raman scattering. Methane is the dominant hydrocarbon species on Uranus, and since it absorbs FUV radiation, it affects the Rayleigh–Raman scattering of Ly α photons by H 2 and, eventually, the brightness of the Raman feature. We derive a brightness of 20 −6 +1 R from the STIS data, which is similar to the brightness measured by Voyager 2 UVS during the 1986 flyby of Uranus, when considering the suggested recalibration of UVS measurements by a factor of ∼0.5. Based on the observed brightness, we constrain the upper altitude (pressure) level for the abundance of methane in the upper atmosphere using radiative transfer simulations that include resonant scattering by H, Rayleigh–Raman scattering by H 2 , and absorption by CH 4 . We considered the solar Ly α flux as the source of Ly α radiation at Uranus. We find that resonant scattering by H significantly affects Rayleigh–Raman scattering by H 2 and thus the modelled brightness of the Raman feature. We derive methane profiles by obtaining the simultaneous fit to the observed Ly α , as well as the 1280 Å brightness of Uranus. Methane appears to be depleted (number density becomes less than 1 cm −3 ) above the altitude (pressure) range of ∼478–515 km (4 × 10 −3 –2.4 × 10 −3 mbar), while the Ly α absorption optical depth reaches unity for methane in the altitude (pressure) range of ∼237–257 km (2.54 × 10 −1 –1.65 × 10 −1 mbar). When neglecting resonant scattering by H, the methane depletion must be deeper in the atmosphere at an altitude (pressure) of ∼395 km (1.4 × 10 −2 mbar), similar to previous findings based on Voyager 2 observations of the feature. The analysis of the Raman feature provides independent CH 4 constraints in the upper atmosphere for detailed photochemistry modelling and highlights the importance of UV instruments for the future Uranus Orbiter and Probe (UOP) mission.

The rotation period of Uranus was estimated to be 17.24 +/- 0.01 h in 1986 from radio auroral measurements during the brief Voyager 2 flyby. This value is the basis for the Uranian SIII longitude system still in use. However, the poor period uncertainty limited its validity to a few years, after which the orientation of the magnetic axis was lost. Alternate, conflicting, rotation periods have also been proposed since then. Here we use the long-term tracking of Uranus' magnetic poles between 2011 and 2022 from Hubble Space Telescope images of its ultraviolet aurorae to achieve an updated, independent, extremely precise rotation period of 17.247864 +/- 0.000010 h, only consistent with the Voyager 2 estimate. Its 28-s-longer value and improved accuracy yields a new longitude model now valid over decades, up to the arrival of any future Uranus mission, which will allow the reanalysis of the whole set of Uranus observations. In addition, it has strong direct implications for formation scenarios, interior models, dynamo theories and studies of the magnetosphere. This approach stands as a new method to determine the rotation rate of any object hosting a magnetosphere and a rotationally modulated aurorae, in our Solar System and beyond.

We observed Io with the James Webb Space Telescope (JWST) NIRSpec/Integral Field Unit (1.0–5.3 μm, (Formula presented.)) in August 2023 while the satellite was in eclipse. Thermal emission from Kanehekili Fluctus is consistent with the cooling of lava flows after a vigorous eruption in November 2022. At Loki Patera, after a new brightening event was detected in November 2022, the lava lake was in a quiescent state, as expected from previous analyses. We mapped the SO emission band at 1.707 μm, and detected, for the first time, [SI] emission lines at 1.082 and 1.131 μm. The SO emissions are concentrated above Kanehekili Fluctus, and in two regions in the northern hemisphere. The disk-averaged brightness is 14.5 kR. The emissions are sourced from SO molecules ejected from 1,500 to 1,700 K vents in an excited state, with a typical SO column density above the northern hemisphere of (Formula presented.) (Formula presented.). Sulfur emissions are distributed homogeneously across a band in the northern hemisphere. The disk-averaged total brightness is 5.6 kR, versus 9.65 kR in the north. The emissions are produced through direct electron impact excitation by (Formula presented.) 4 eV electrons in the torus (density 2,500 (Formula presented.)) penetrating the atmosphere, and require the atmosphere to be hot ((Formula presented.) 1,700 K) to populate the upper levels before excitation. The sulfur column density over the northern hemisphere is (Formula presented.) (Formula presented.). These same parameters can explain recent 0.7725- (Formula presented.) m observations, as well as the 147.9-nm multiplet emissions observed with HST-STIS (Formula presented.) 20 years earlier. This suggests a quite stable system over decades-long timescales.

Europa has been modified by a variety of geologic processes, exposing internally derived materials that are heavily irradiated by charged particles trapped in Jupiter’s magnetosphere. Prior spectral analysis of H 2 O ice on Europa relied on low signal-to-noise data at wavelengths >2.5 μ m, limiting assessment of a 3.1 μ m Fresnel peak that is diagnostic of exposed crystalline ice. We report new measurements of H 2 O ice spectral features using high signal-to-noise data collected by the NIRSpec spectrograph (1.48–5.35 μ m) on the James Webb Space Telescope. These data reveal a narrow 3.1 μ m crystalline H 2 O ice Fresnel peak, which is primarily located at southern latitudes in Tara and Powys Regiones. Our analysis indicates that crystalline ice exposed in these low-latitude regiones is likely sustained by ongoing thermal (re)crystallization outpacing charged particle amorphization of the top ∼10 μ m of Europa’s regolith over short timescales (<15 days). We also measured H 2 O ice features centered near 1.5, 1.65, and 2.0 μ m, and a broad 3.6 μ m H 2 O continuum peak, which are all stronger at northern latitudes, in contrast to the 3.1 μ m Fresnel peak identified at southern latitudes. These results support the hypothesis that H 2 O ice in Europa’s regolith is vertically stratified, with amorphous ice grains dominating its exposed surface, except in Tara and Powys Regiones. We also find that a previously detected 4.38 μ m 13 CO 2 feature is present almost exclusively at southern latitudes in Tara and Powys Regiones, likely derived from an internal source of carbon-bearing material.

The Jupiter Icy Moons Explorer (JUICE) is a European Space Agency mission to explore Jupiter and its three icy Galilean moons: Europa, Ganymede, and Callisto. Numerous JUICE investigations concern the magnetised space environments containing low-density populations of charged particles that surround each of these bodies. In the case of both Jupiter and Ganymede, the magnetic field generated internally produces a surrounding volume of space known as a magnetosphere. All these regions are natural laboratories where we can test and further our understanding of how such systems work, and improved knowledge of the environments around the moons of interest is important for probing sub-surface oceans that may be habitable. Here we review the magnetosphere and plasma science that will be enabled by JUICE from arrival at Jupiter in July 2031. We focus on the specific topics where the mission will push forward the boundaries of our understanding through a combination of the spacecraft trajectory through the system and the measurements that will be made by its suite of scientific instruments. Advances during the initial orbits around Jupiter will include construction of a comprehensive picture of the poorly understood region of Jupiter's magnetosphere where rigid plasma rotation with the planet breaks down, and new perspectives on how Jupiter's magnetosphere interacts with both Europa and Callisto. The later orbits around Ganymede will dramatically improve knowledge of this moon's smaller magnetosphere embedded within the larger magnetosphere of Jupiter. We conclude by outlining the high-level operational strategy that will support this broad science return.

Since the Voyager mission flybys in 1979, we have known the moon Io to be both volcanically active and the main source of plasma in the vast magnetosphere of Jupiter. Material lost from Io forms neutral clouds, the Io plasma torus and ultimately the extended plasma sheet. This material is supplied from Io's upper atmosphere and atmospheric loss is likely driven by plasma-interaction effects with possible contributions from thermal escape and photochemistry-driven escape. Direct volcanic escape is negligible. The supply of material to maintain the plasma torus has been estimated from various methods at roughly one ton per second. Most of the time the magnetospheric plasma environment of Io is stable on timescales from days to months. Similarly, Io's atmosphere was found to have a stable average density on the dayside, although it exhibits lateral (longitudinal and latitudinal) and temporal (both diurnal and seasonal) variations. There is a potential positive feedback in the Io torus supply: collisions of torus plasma with atmospheric neutrals are probably a significant loss process, which increases with torus density. The stability of the torus environment may be maintained by limiting mechanisms of either torus supply from Io or the loss from the torus by centrifugal interchange in the middle magnetosphere. Various observations suggest that occasionally (roughly 1 to 2 detections per decade) the plasma torus undergoes major transient changes over a period of several weeks, apparently overcoming possible stabilizing mechanisms. Such events (as well as more frequent minor changes) are commonly explained by some kind of change in volcanic activity that triggers a chain of reactions which modify the plasma torus state via a net change in supply of new mass. However, it remains unknown what kind of volcanic event (if any) can trigger events in torus and magnetosphere, whether Io's atmosphere undergoes a general change before or during such events, and what processes could enable such a change in the otherwise stable torus. Alternative explanations, which are not invoking volcanic activity, have not been put forward. We review the current knowledge on Io's volcanic activity, atmosphere, and the magnetospheric neutral and plasma environment and their roles in mass transfer from Io to the plasma torus and magnetosphere. We provide an overview of the recorded events of transient changes in the torus, address several contradictions and inconsistencies, and point out gaps in our current understanding. Lastly, we provide a list of relevant terms and their definitions.

The presence of cryovolcanic activity in the form of geyser-like plumes at Jupiter’s moon Europa is a much-debated topic. As an active plume could allow direct sampling by a passing spacecraft of a potentially habitable interior environment, the detection and analysis of ongoing plume activity would be of the highest scientific value. In the past decade, several studies have interpreted different remote and in situ observations as providing evidence for large gaseous plumes at different locations on Europa. However, definitive proof is elusive, and visible imaging data taken during spacecraft flybys do not reveal clear indications of ongoing activity. After arrival at Jupiter in 2030, the NASA Europa Clipper spacecraft will systematically search for and constrain plume activity at Europa utilizing a variety of investigations and methods during, before, and after close flybys. Given the lack of a confirmed plume detection to date, the Europa Clipper science team has adopted a global plume search strategy, not focusing on any specific geographical area or any specific type of observation. This global search strategy assigns enhanced value to data obtained early in the mission, which allows time for further observations and characterization of any observed plume at later times. Here we describe the current state of knowledge on plume activity, the Europa Clipper search strategy, and the role of various instruments on the Europa Clipper payload in this search.

Io is one of Jupiter’s largest moons and the most volcanically active body in the Solar system. Its very active surface has hotspots produced by volcanic eruptions popping up at seemingly random locations and times. Characterizing the complex surface of Io requires the highest angular resolution available. This work presents the analysis of aperture masking interferometric observations (at 4.3 μm) of Io taken with the Near-Infrared Imager and Slitless Spectrograph instrument on the James Webb Space Telescope. These are the first space-based infrared interferometric observations of a Solar system body ever taken. For complex extended objects like Io, the traditional visibility extraction algorithms from interferograms suffer from limitations. Here, new deconvolution methods based on neural networks allowed us to obtain reliable images from which a detailed analysis of the volcanically active surface of this moon was performed. Our study characterizes the loci and brightness of several unresolved volcanoes on the surface of Io, as well as the extended emission observed. We identified the brightest eruption (I4.3μm = 33 ± 4.3 GW μm−1), referred to as V1, within an area to the north–east of Seth Patera (129.4 ± 0.8◦ W. Longitude, 1.5 ± 0.7◦ S. Latitude). Its projected speed (VT = 86 ± 34 m s−1) is consistent with the rotational speed of Io. Additionally, six fainter volcanoes were identified and characterized. Complementary ground-based images, taken with the Keck II telescope, allowed us to benchmark the deconvolved aperture masking interferometric images, showing consistency. Finally, we highlight the importance of characterizing Io’s surface with long-term monitoring at high angular resolution.

The Radio & Plasma Wave Investigation (RPWI) onboard the ESA JUpiter ICy moons Explorer (JUICE) is described in detail. The RPWI provides an elaborate set of state-of-the-art electromagnetic fields and cold plasma instrumentation, including active sounding with the mutual impedance and Langmuir probe sweep techniques, where several different types of sensors will sample the thermal plasma properties, including electron and ion densities, electron temperature, plasma drift speed, the near DC electric fields, and electric and magnetic signals from various types of phenomena, e.g., radio and plasma waves, electrostatic acceleration structures, induction fields etc. A full wave vector, waveform, polarization, and Poynting flux determination will be achieved. RPWI will enable characterization of the Jovian radio emissions (including goniopolarimetry) up to 45 MHz, has the capability to carry out passive radio sounding of the ionospheric densities of icy moons and employ passive sub-surface radar measurements of the icy crust of these moons. RPWI can also detect micrometeorite impacts, estimate dust charging, monitor the spacecraft potential as well as the integrated EUV flux. The sensors consist of four 10 cm diameter Langmuir probes each mounted on the tip of 3 m long booms, a triaxial search coil magnetometer and a triaxial radio antenna system both mounted on the 10.6 m long MAG boom, each with radiation resistant pre-amplifiers near the sensors. There are three receiver boards, two Digital Processing Units (DPU) and two Low Voltage Power Supply (LVPS) boards in a box within a radiation vault at the centre of the JUICE spacecraft. Together, the integrated RPWI system can carry out an ambitious planetary science investigation in and around the Galilean icy moons and the Jovian space environment. Some of the most important science objectives and instrument capabilities are described here. RPWI focuses, apart from cold plasma studies, on the understanding of how, through electrodynamic and electromagnetic coupling, the momentum and energy transfer occur with the icy Galilean moons, their surfaces and salty conductive sub-surface oceans. The RPWI instrument is planned to be operational during most of the JUICE mission, during the cruise phase, in the Jovian magnetosphere, during the icy moon flybys, and in particular Ganymede orbit, and may deliver data from the near surface during the final crash orbit.