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Europa, one of Jupiter's Galilean moons, has twice as much liquid water as Earth's oceans, if not more. An ocean estimated to be anywhere from 40 to 100 miles (60 to 150 kilometers) deep spans the entire moon, locked beneath an icy surface over a dozen kilometers thick. The only direct evidence for this ocean is the plumes of water that occasionally erupt through cracks in the ice, jetting as high as 200 km above the surface.

This review of miniaturised instrumentation is motivated by the ongoing and forthcoming exploration of the confirmed, or candidate ocean worlds of the Solar System. It begins with a section on the evolution of instrumentation itself, ranging from the early efforts up to the current rich-heritage miniaturised mass spectrometers approved for missions to the Jovian system. The geochemistry of sulphur stable isotopes was introduced for life detection at the beginning of the present century. Miniaturised instruments allow the measurement of geochemical biosignatures with their underlying biogenic coding, which are more robust after death than cellular organic molecules. The role of known stable sulphur isotope fractionation by sulphate-reducing bacteria is discussed. Habitable ocean worlds are discussed, beginning with analogies from the first ocean world known in the Solar System that has always being available for scientific exploration, our own. Instrumentation can allow the search for biosignatures, not only on the icy Galilean moons, but also beyond. Observed sulphur fractionation on Earth suggests a testable “Sulphur Hypothesis”, namely throughout the Solar System chemoautotrophy, past or present, has left, or are leaving biosignatures codified in sulphur fractionations. A preliminary feasible test is provided with a discussion of a previously formulated “Sulphur Dilemma”: It was the Galileo mission that forced it upon us, when the Europan sulphur patches of non-ice surficial elements were discovered. Biogenic fractionations up to and beyond δ34S = −70‰ denote biogenic, rather than inorganic processes, which are measurable with the available high sensitivity miniaturised mass spectrometers. Finally, we comment on the long-term exploration of ocean worlds in the neighbourhood of the gas and ice giants.

The Jovian Infrared Auroral Mapper (JIRAM) on board Juno is a spectro-imager which is observing theatmosphere of Jupiter and its auroral emission using its two imagers in the L (3.3-3.6μm) and M bands (4.5-5.0μm) and a spectrometer (2-5 μm spectral range).The highly elliptic orbit of Juno and the unprecedented resolution of the JIRAM imager allowed to retrievewealth of details about the morphology of moon-related aurorae. This phenomenon is due to the jovian magneticfield sweeping past the Galiean moons, which generate Alfven waves travelling towards the ionosphere and setup field aligned currents. When the associated electrons reach the ionosphere, they interact with the hydrogenand make it to glow. In particular, the tails of the footprints showed a spot-like substructure consistently, whichwere investigated using the L-band of the imager from perijove 4 to perijove 30. This feature was observed closeto the footprints, where the the typical distance between spots lies between 250km and 500km. This distancedecreases to 150km in a group of three observations in the northern emisphere when each moon is close to 250 ◦west longitude. No correlation with orbital parameters such as the longitude of the moons was found so far,which suggests that such morphology is almost purely due to ionospheric processes.Moreover, during PJ 13 a long sequence of images of the Io footprint was shot and it revealed that thesecondary spots appears to corotate with Jupiter. This behaviour is observed also during orbits 14 and 26.During these sequences JIRAM clearly observed the Io footprint leaving behind a trail of ”footsteps” as brightspots.The characteristics of these spots are incompatible with multiple reflection of Alfven waves between the twoemispheres. Instead, we are currently investigating ionospheric processes like the feedback instability (FI) as apotential candidate to explain the generation of the observed small scale structure. This process relies on localenhacement of conductivity in the ionosphere, which is affected by electron precipitation. Order of magnitudeestimates from the FI are compatible with the inter-spot distance and the stillness of the spots.

Io is the most volcanically-active object in the solar system. The moon ejects a tonne per second of sulphur-rich gases that fill the vast magnetosphere of Jupiter and drives million-amp electrical currents that excite strong auroral emissions. We present the case for including a detailed study of Io within Voyage 2050 either as a standalone mission or as a contribution to a NASA New Frontiers mission, possibly within a Solar System theme centred around current evolutionary or dynamical processes. A comprehensive investigation will provide answers to many outstanding questions and will simultaneously provide information on processes that have formed the landscapes of several other objects in the past. A mission investigating Io will also study processes that have shaped the Earth, Moon, terrestrial planets, outer planet moons, and potentially extrasolar planets. The aim would be simple – tracing the mass and energy flows in the Io-Jupiter system.

Plumes spewing water high above Europa's surface have been inferred from several observation campaigns in the past decade. Whereas the occasional existence of plumes on Europa has thus been confirmed, the origin of the plumes remains uncertain. Most notably, it is still unclear whether the inferred plumes are of oceanic origin, possibly containing information about the ocean's habitability, or if they are of surficial nature, offering information on the highly processed surface instead. In this study, we use a Monte-Carlo model to analyze three different plume models, two of which are surficial (near-surface liquid inclusion and diapir), and one of which originates in the sub-surface ocean (oceanic plume). We analyze all relevant Lyman- (Formula presented.) and OI 1304 (Formula presented.) emission mechanisms to determine the three models' emission profiles. These profiles are then compared to the Hubble Space Telescope/Space Telescope Imaging Spectograph measurements presented by Roth, Retherford, et al. (2014, https://doi.org/10.1073/pnas.1416671111) and Roth, Saur, et al. (2014, https://doi.org/10.1126/science.1247051) with the goal of determining which ab initio model fits these measurements best. Our analyses show that all three models investigated produce similar Lyman- (Formula presented.) and OI 130.4 nm emission profiles, with differences being perceivable only on scales well below the HST resolution and sensitivity. Since none of the models contradict the Roth, Retherford, et al. (2014, https://doi.org/10.1073/pnas.1416671111) and Roth, Saur, et al. (2014, https://doi.org/10.1126/science.1247051) observations, none of them can be ruled out as being the acting force behind the observed plumes. The currently available optical measurements are thus not sufficient to determine the physical nature of Europa's plumes, identification of which can only be achieved through higher resolution images or in situ measurements.

Although mass and energy in Jupiter’s magnetosphere mostly come from the innermost Galilean moon Io’s volcanic activity, solar wind perturbations can play crucial roles in releasing the magnetosphe...

The four larger satellites of Jupiter, with roughly comparable mass, are termed Galilean moons. They display (i) a water fraction that increases with dis- tance to Jupiter (ranging from no-water for volcanic innermost Io, to approximately half-half ratio of water and rocks for outermost Callisto) as well as (ii) various degrees of differentiation (two end-members corresponding to the two outer moons of similar radius: highly differentiated Ganymede, including a metallic core, locus of the dynamo observed at present, and much-less differentiated Callisto possibly harboring large volumes of ice-rock mixtures). A plausible cause for these variations can be related to the formation of the moons, although their later evolution could also affect both properties. Owing to the orbits of the four moons (compact, prograde, coplanar and nearly circular), a contemporaneous formation in a circum-jovian disk orbiting in the equatorial plane of the proto-planet is put forward, similar to the formation of planets albeit with significantly different time- and length-scales. The latter is not a minor difference: in practice, classical scenarios [1, 2] cope with the problem of too-much heating associated to large impacts as well as the migration of satellitesimals onto the proto-planet. A new paradigm for the formation of giant planets has emerged re- cently. It has been termed ’pebble accretion’ and highlights the efficiency of accretion of small particles in a context where gas drag dissipates energy as the pebble passes the protoplanet [3]. In this framework, formation of the Galilean moons is envisioned to occur naturally and contemporaneously to the growth of the giant planet. Details are nevertheless controversial: two re- cent scenarios propose either a fast [4] or a slow [5] accretion. Here, we reproduce these two end-member scenar- ios and specifically focus on the thermal evolution of the growing moons. A simple description of the heat equation is adopted assuming a spherical sym- metry. Heat deposition by pebbles as well as radiogenic heat sources are accounted for during a 30 Myr period that encompasses the formation of all moons. We systematically investigate the influence of timing (start/duration) and consider a range for the concentra- tion in of short-lived radio-isotopes (26Al,60Fe,53Mn) that reproduces the composition of LL (high enery content) and CI (low enery content) chondrites. Our results demonstrate that, whatever the scenario, the formation process involves little collisional energy, as can be expected owing to the small size of impactors. As a consequence, after 30 Myr of evolution, moons formed via pebble accretion show a small degree of differenciation or none at all. Either subsequent heat- ing via radioactive decay of long-lived isotopes or tidal heating possibly associated to the moons entrance in the Laplace resonance must be considered to explain the highly differentiated state observed at present for Ganymede. This includes the formation of the hydrosphere, the dehydration of rocks and the melting of the metallic component in order to form the moon’s core. Interestingly, the main heat sources differ depend- ing on the scenario: in the case of slow accretion, the decay of short-lived radio-isotopes is predominant when accretion of the moons starts sufficiently early; in the case of a fast accretion, the main heat source is associated to viscous dissipation in the disk that heats up the surface of the protosatellites. In the former case, the interior might reach the melting point of water in a limited fraction of the parameter space but only in the innermost region of the moons. Conversely, in the latter case, the melting point can be reached in the outer envelope, as predicted for large moons in the classical formation scenarios [6], but to a much lesser extent. The subsequent evolutionary path leading to the differentiation observed at present for Ganymede thus necessarily involve distinct dynami- cal phenomena: Rayleigh-Taylor type instabilities of the thick rock-ice mixture into the hydrosphere [7] if accretion is slow; if accretion is rapid, a possible delay in solid-state segregation of the rock component if double-diffusive convection occurs owing to a stabilizing density gradient [8]. [1] Estrada, P. R., Mosqueira, I., Lissauer, J. J., D’Angelo, G., and Cruikshank, D. P. (2009). Formation of Jupiter and conditions for accretion of the Galilean satellites. Eu- ropa, edited by RT Pappalardo, WB McKinnon, and K. Khurana, University of Arizona Press, Tucson, 27-58. [2] Canup, R. M., and Ward, W. R. (2009). Origin of Europa and the Galilean satellites. Europa, edited by RT Pappalardo, WB McKinnon, and K. Khurana, University of Arizona Press, Tucson, 59-83. [3] Johansen, A., and Lambrechts, M. (2017). Forming planets via pebble accretion. Annual Review of Earth and Planetary Sciences, 45, 359-387. [4] Ronnet,T., and Johansen,A.(2020). Formation of moon systems around giant planets-Capture and ablation of planetesimals as foundation for a pebble accretion scenario. Astronomy & Astrophysics, 633, A93. [5] Shibaike, Y., Ormel, C. W., Ida, S., Okuzumi, S., and Sasaki, T. (2019). The Galilean Satellites Formed Slowly from Pebbles. The Astrophysical Journal, 885(1), 79. [6] Monteux, J., Tobie, G., Choblet, G., and Le Feuvre, M. (2014). Can large icy moons accrete undifferentiated?. Icarus, 237, 377-387. [7] Rubin, M. E., Desch, S. J., and Neveu, M. (2014). The effect of Rayleigh-Taylor instabilities on the thickness of undifferentiated crust on Kuiper Belt Objects. Icarus, 236, 122-135. [8] O’Rourke, J. G., and Stevenson, D. J. (2014). Stability of ice/rock mixtures with application to a partially differentiated Titan. Icarus, 227, 67-77.

The current lattice configuration of the water ice on the surfaces of the inner satellites of Jupiter and Saturn is likely shaped by many factors. But laboratory experiments have found that energetic proton irradiation can cause a transition in the structure of pure water ice from crystalline to amorphous. It is not known to what extent this process is competitive with other processes in solar system contexts. For example, surface regions that are rich in water ice may be too warm for this effect to be important, even if the energetic proton bombardment rate is very high. In this paper, we make predictions, based on particle flux levels and other considerations, about where in the magnetospheres of Jupiter and Saturn the ∼MeV proton irradiation mechanism should be most relevant. Our results support the conclusions of Hansen and McCord (2004), who related relative level of radiation on the three outer Galilean satellites to the amorphous ice content within the top 1 mm of surface. We argue here that if magnetospheric effects are considered more carefully, the correlation is even more compelling. Crystalline ice is by far the dominant ice state detected on the inner Saturnian satellites and, as we show here, the flux of bombarding energetic protons onto these bodies is much smaller than at the inner Jovian satellites. Therefore, the ice on the Saturnian satellites also corroborates the correlation.

The Jovian high-latitude radio emissions produced by Jupiter's magnetosphere extend from a few kilohertz to 40 MHz. Part of the decametric (DAM) emissions are driven by the Galilean moon Io (Io-DAM). As UV aurorae have been detected at the footprint of Europa and Ganymede, we expect that these moons drive Jovian radio emissions as well. To check this assumption, we used the ExPRES simulation code (Exoplanetary and Planetary Radio Emissions Simulator) to predict dynamic spectrum (time-frequency spectograms) of the radio emissions controlled by the four Galilean moons. Then we compared the simulations to the Voyager/PRA and Cassini/RPWS radio observations of Jupiter (1979, and between 2000 and 2003, respectively). We present the first clear evidence for the existence of decametric emissions controlled by Europa and Ganymede. Their statistical analysis allows us to describe the average properties of the Europa-DAM and Ganymede-DAM emissions such as their spectrum, temporal variability, and occurrence as a function of moon phase and subobserver's longitude.

Space exploration and ground-based observations have provided outstanding evidence of the diversity and the complexity of the outer solar system. This work presents our current understanding of the nature and distribution of water and water-rich materials from the water snow line to the Kuiper Belt. This synthesis is timely, since a thorough exploration of at least one object in each region of the outer solar system has now been achieved. Next steps, starting with the Juno mission now in orbit around Jupiter, will be more focused on understanding the processes at work than on describing the general characteristics of each giant planet systems. This review is organized in three parts. First, the nature and the distribution of water and volatiles in giant and intermediary planets are described from their inner core to their outer envelopes. A special focus is given to Jupiter and Saturn, which are much better understood than the two ice giants (Uranus and Neptune) thanks to the Galileo and Cassini missions. Second, the icy moons will be discussed. Space missions and ground-based observations have revealed the variety of icy surfaces in the outer system. While Europa, Enceladus, and maybe Titan present past or even active tectonic and volcanic activities, many other moons have been dead worlds for more than 3 billion years. Ice compositions found at these bodies are also complex and it is now commonly admitted that icy surfaces are never composed of pure ices. A detailed review of the distribution of non-ice materials on the surfaces and in the tenuous atmospheres of the moons is proposed, followed by a more focused discussion on the nature and the characteristics of the liquid layers trapped below the cold icy crusts that have been suggested in the icy Galilean moons, and in Enceladus, Dione, and Titan at Saturn. Finally, the recent observations collected by Dawn at Ceres and New Horizons at Pluto, as well as the state of knowledge of other transneptunian objects, are summarized, and complete this overview of the nature and distribution of ice-rich material in the outer solar system.

Large moons such as the Galilean satellites are thought to be in an equilibrium rotation state, called a Cassini state (Peale, 1969). This state is characterized by a synchronous rotation and a precession rate of the rotation axis that is equal to the precession rate of the normal to its orbit. It also implies that the spin axis, the normal to the orbit and the normal to the Laplace plane are coplanar with a (nearly) constant obliquity.For rigid bodies, up to 4 possible Cassini states exist, but not all of them are stable. It is generally assumed that the Galilean satellites are in Cassini State I for which the obliquity is close to zero (see e.g. Baland et al. 2012). However, it is also theoretically possible that these satellites occupy or occupied another Cassini state.We here investigate how the interior structure, and in particular the presence of a subsurface ocean, influences the existence and stability of the different possible Cassini states.References :Baland, R.M., Yseboodt, M. and Van Hoolst, T. (2012). Obliquity of the Galilean satellites: The influence of a global internal liquid layer. Icarus 220, 435-448.Peale, S. (1969). Generalized Cassini’s laws. Astron. J. 74 (3), 483-489.

International audience; The final analysis of the Cassini radar observations of Saturn's icy moons presented here shows that the exchange of material between the planet's dust rings and moons, which is specific to the Saturnian system, plays a key role in the current state of the airless satellite regolith. Far from Saturn, the vast debris ring from Phoebe progressively coats the leading side of Iapetus with optically‐dark material reducing its radar brightness. On the contrary, close to the planet, the extreme radar brightness of the innermost moons Mimas, Enceladus and Tethys (that exceeds that of the Galilean satellites) is most likely related to Enceladus's geysers and the E‐ring which brings ultra‐clean water ice to their surfaces. The measured radar albedos and observed hemispheric dichotomies require at least a few decimeters thick “snow” cover and that the near‐surfaces of Saturn's innermost moons contain especially efficient backscattering structures whose nature remains an outstanding problem.

A new model is the first to simultaneously explain many of the moons’ characteristics, including their mass, orbits, and icy composition

The Galilean satellites exhibit a clear trend in composition from the rocky Io, close to Jupiter, to the icy and distant Callisto. Proposed causes of this trend can be roughly divided by when the trend developed—either as a result of the material accreted, as a byproduct of the accretion process, or due to the subsequent diverging evolution of the moons. While the first two options have been heavily favored in the existing literature and were therefore studied previously, in this work, we directly address the last of these possibilities. To do so, we determine the range of plausible tidal heating experienced by these moons and how efficiently that energy could be converted into mass loss. We find that while the total tidal energy does exceed the energy required to lose an ice shell, the loss process would have to be highly energy efficient. Examining a range of loss processes, we find that only in extreme cases could enough mass be lost from Europa and no cases where enough mass could be lost from Io. We conclude that this compositional gradient must have been in place by the end of accretion.

High-resolution mapping of Ceres, Vesta and the icy satellites of Saturn, Uranus and Pluto reveals a rich variety of well-preserved impact crater morphologies on these low gravity bodies. These objects provide a natural laboratory to study effects of composition on crater formation processes under similar surface gravity conditions (though mean impact velocities vary by several factors). Simple craters occur on all these bodies but subtle differences in morphology on Ceres and Vesta are recognized. Immature complex craters (with large floor mounds but not terraces or conical central peaks) occur on Vesta and while smaller than predicted are consistent with its silicate composition. Asymmetric simple craters (with incomplete scarp development) on all bodies are likely related to differential overburden stresses in the rim, and their occurrence is consistent with lower crustal strength on icy bodies including Ceres. Immature and mature complex craters exhibit increasing degrees of complexity, including spiral floor deformation patterns (related to failure in converging floor material), central peaks, and impact melt. Cerean crater morphologic types and simple-complex transition diameters are smaller than on Vesta but similar to those on icy satellites, indicating a much weaker rheology for Ceres' outer layers under impact conditions. These are consistent with geophysical indications of a low-density water ice and probably clathrate rich outer shell. Fluidized floor deposits (impact melt or melt-solid mixtures) are significant in craters >25 km across on Ceres but absent on Saturn satellites. Central pit craters are common on Ceres (at diameters of ~75 to 150 km consistent with gravity scaling from the larger Galilean satellites) but are absent on Saturnian satellites and Charon. The contrasting impact melt and central pit behaviors on Ceres and Saturn's moons is contrary to expectation given the higher impact velocities at Saturn but might be related to lower internal temperatures, or the higher fraction of non-ice material on Ceres. The correlation or scaling of transition diameters to surface gravity is near −0.65 rather than −1, perhaps due to increased porosity on lower gravity bodies. The fundamental similarity of crater morphologies on Ceres and icy satellites, however, indicates that the weaker rheology of water ice results in similar craters even if the non-(ice+clathrate) components are as high as ~30 vol%.

A molecular dynamics (MD) simulation is performed for the physical and chemical properties of solid and liquid Fe–S solutions using the embedded atom model (EAM) potential as applied to the internal structure of the Moon, Io, Europa, and Ganymede under the assumption that the satellites' cores can be described by a two-component iron–sulfur system. Calculated results are presented for the thermodynamic parameters including the caloric, thermal, and elastic properties (specific heat, thermal expansion, Gruneisen parameter, density, compression module, velocity of sound, and adiabatic gradient) of the Fe–S solutions at sulfur concentrations of 0–18 at %, temperatures of up to 2500 K, and pressures of up to 14 GPa. The velocity of sound, which increases as pressure rises, is weakly dependent on sulfur concentration and temperature. For the Moon’s outer Fe–S core (~5 GPa/2000 K), which contains 6–16 at % (3.5–10 wt %) sulfur, the density and the velocity of sound are estimated at 6.3–7.0 g/cm3 and 4000 ± 50 m/s, respectively. The MD calculations are compared with the interpretation of the Apollo observations (Weber et al., 2011) to show a good consistency of the velocity of P-waves in the Moon’s liquid core whereas the thermodynamic density of the Fe–S core is not consistent with the seismic models with ρ = 5.1–5.2 g/cm3 (Garcia et al., 2011; Weber et al., 2011). The revision the density values for the core leads to the revision of its size and mass. At sulfur concentrations of 3.5–10 wt %, the density of the Fe–S melt is 20–30% higher that the seismic density of the core. Therefore, the most likely radius of the Moon’s outer core must be less than 330 km (Weber et al., 2011) because, provided that the constraint on the Moon’s mass and moment of inertia is satisfied, an increase in the density of the core must lead to a reduction of its radius. For Jupiter’s Galilean moons Io, Europa, and Ganymede, constraints are obtained on the size, density, and sound velocity of the Fe–S liquid cores. The geophysical and geochemical characteristics of the internal structure of the Moon and Jupiter’s moons are compared. The calculations of the adiabatic gradient at the P–T conditions for the Fe–S cores of the Moon, Io, Europa, and Ganymede suggest the top-down crystallization of the core (Fe-snow scenario).

Europa, one of the Galilean satellites, has a tenuous, oxygen-dominated atmosphere that is usually referred to as a collision-less exosphere. Coupled to the neutral atmosphere and in contact with Europa's surface, a tenuous ionosphere exists on Europa, whose presence has been revealed by multiple observations. Such an ionosphere is thought to be produced by solar photoionization and electron-impact ionization of the oxygen in the atmosphere. However, a recent study showed that the maximum ionosphere coincides with intermittent water plume on Europa, suggesting that water plays an important role in the formation of the ionosphere. Based on the assumption of horizontal uniformity in the middle of the plume, we use the Caltech/Jet Propulsion Laboratory one-dimensional KINETICS model to construct profiles of neutral and ionized species near the plume region. The simulation results, which show that the ionization reactions are initiated by water electron-impact ionization and photoionization and continued by charge transfer between water and oxygen molecules, have successfully reproduced the observations. We find that H2O+ is the dominant species only above the ionopause. Below the ionopause, the density of H2O+ is orders of magnitude lower than the density of O2+, which is the major composition below the ionopause. Our model has also been used to study the dissociation processes of water molecules from the plumes, which can be regarded as an alternative source for the oxygen in the atmosphere.

The outer worlds of frosted vistas and thundering cryovolcanoes continue to beckon, like distant lighthouses in the fog. We have only penetrated that fog nine times, with Pioneers 10 and 11, Voyagers 1 and 2, Galileo, Cassini, New Horizons, Juno, and Dawn. As of this writing, New Horizons threads its way through the Kuiper Belt, and the Juno spacecraft continues to send reports from Jupiter. Juno marked the first solar-powered craft to operate long-term in the outer Solar System, and its success paves the way for future solar-powered missions. The first two of these upcoming missions are destined for Jupiter. The Europeans are training their sites on a mission to study the Galilean satellites, with a focus on Ganymede, while NASA and its international partners want to carry out an in-depth reconnaissance of Jupiter’s ocean moon Europa.

The gas and ice giant worlds each have an extensive family of satellites, but with one exception, each has only a handful of large, complex moons (Fig. 5.1). Jupiter has its four Galilean satellites, Uranus has its five major moons (Ariel, Umbriel, Titania, Oberon, Miranda, and Ariel) and Neptune has its trio of moons: Proteus, Triton, and Nereid. In each case, the rest of the moons range from a few to less than 180 km across. The exception is Saturn. Saturn’s retinue includes so many major moons that educators have come up with a mnemonic to remember them: MET DR THIP (for Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion, Iapetus, and Phoebe).

The independent ESA mission JUICE (JUpiter ICy moon Explorer) has the goal to explore Jupiter and three of its Galilean moons. The spacecraft (S/C) will launch in the year 2022 and arrives the Jovian system in the year 2030. RIME (Radar for Icy Moons Exploration) and other experiments aboard JUICE will analyse the celestial bodies. Some selected scientific objectives will be the characterization of Ganymede, Europa and Callisto as planetary objects as well as the exploration of potential habitats. Moreover, the Jovian system shall be investigated as an archetype for gas giants.

Analysis of Galileo Near Infrared Mapping Spectrometer (NIMS) observations of Marduk Fluctus, a volcano on the jovian moon Io, reveals a style of volcanic activity not previously seen there – a powerful thermal event lasting only a few minutes in 1997. The thermal emission rapidly fades, suggesting extremely rapid cooling of small clasts. The duration and evolution of the explosive eruption is akin to what might be expected from a strombolian or vulcanian explosion. The presence of such events provides an additional volcanic process that can be imaged by future missions with the intent of determining lava composition from eruption temperature, an important constraint on the internal composition of Io. These data promise to be of particular use in understanding the mechanics of explosive volcanic processes on Io.

Employing multiple gravity-assist flybys of Jupiter’s Galilean moons can save a substantial amount of \(\varDelta V\) when capturing into orbit about Jupiter. Using Callisto and Ganymede, the most massive and distant of the Galilean moons, as gravity-assist bodies reduces the Jupiter orbit insertion \(\varDelta V\) cost, while allowing the spacecraft to remain above the worst of Jupiter’s radiation belts. A phase-angle approach is used to find initial guesses for a Lambert targeter to find patched-conic Callisto–Ganymede transfers. A B-plane targeter using grid search methodology is used to backward target Earth to find launch conditions. Twenty-nine distinct patched-conic trajectories were found from Earth to Callisto–Ganymede–JOI capture throughout the search space from 2020–2060. Five promising trajectories were found that launch from Earth between July 11, 2023 and July 20, 2023, and arrive at Jupiter between February and September 2026. These trajectories were numerically integrated using GMAT and, in the author’s opinion, are excellent candidates for use on NASA’s planned Europa Clipper mission.

The first European mission dedicated to the exploration of Jupiter and its icy moons (JUpiter ICy moons Explorer—JUICE) will be launched in 2022 and will reach its final destination in 2030. The main goals of this mission are to understand the internal structure of the icy crusts of three Galilean satellites (Europa, Ganymede, and Callisto) and, ultimately, to detect Europa's subsurface ocean, which is believed to be the closest to the surface among those hypothesized to exist on these moons. JUICE will be equipped with the 9 MHz subsurface-penetrating radar RIME (Radar for Icy Moon Exploration), which is designed to image the ice down to a depth of 9 km. Moreover, a parallel mission to Europa, which will host onboard REASON (Radar for Europa Assessment and Sounding: Ocean to Near-surface) equipped with 9MHz and 60MHz antennas, has been recently approved by NASA. The success of these experiments strongly relies on the accurate prediction of the radar performance and on the optimal processing and interpretation of radar echoes that, in turn, depend on the dielectric properties of the materials composing the icy satellite crusts. In the present review we report a complete range of potential ice types that may occur on these icy satellites to understand how they may affect the results of the proposed missions. First, we discuss the experimental results on pure and doped water ice in the framework of the Jaccard theory, highlighting the critical aspects in terms of a lack of standard laboratory procedures and inconsistency in data interpretation. We then describe the dielectric behavior of extraterrestrial ice analogs like hydrates and icy mixtures, carbon dioxide ice and ammonia ice. Building on this review, we have selected the most suitable data to compute dielectric attenuation, velocity, vertical resolution, and reflection coefficients for such icy moon environments, with the final goal being to estimate the potential capabilities of the radar missions as a function of the frequency and temperature ranges of interest for the subsurface sounders. We present the different subsurface scenarios and associated radar signal attenuation models that have been proposed so far to simulate the structure of the crust of Europa and discuss the physical and geological nature of various dielectric targets potentially detectable with RIME. Finally, we briefly highlight several unresolved issues that should be addressed, in near future, to improve our capability to produce realistic electromagnetic models of icy moon crusts. The present review is of interest for the geophysical exploration of all solar system bodies, including the Earth, where ice can be present at the surface or at relatively shallow depths.

Triple-satellite-aided capture sequences use gravity-assist flybys of three of Jupiter’s four massive Galilean moons to help capture a spacecraft into orbit about Jupiter. A novel triple-satellite-aided capture uses sequential flybys of Callisto, Io, and Ganymede to reduce the ΔV required to capture into orbit about Jupiter. An optimal broken-plane maneuver is added between Earth and Jupiter to form a complete chemical/impulsive interplanetary trajectory from Earth to Jupiter. Such a trajectory can yield significant fuel savings over single and double-flyby capture schemes while maintaining a brief and simple interplanetary transfer phase. The developed methods maintain flexibility for adaptation to similar launch, cruise, and capture conditions.

Jupiter is one of the major targets for planetary exploration, and dust in the Jovian system is of great interest to researchers in the field of planetary science. In this paper, we review the five dust populations outside the ring system: grains in the region of the Galilean moons, potential dust from plumes on Europa, Jovian stream particles, particles in the outer region of the Jovian system ejected from the irregular satellites, and dust in the region of the Trojan asteroids. The physical environment for the dust dynamics is described, including the gravity, the magnetic field and the plasma environment. For each population, the dust sources are described, and the relevant perturbation forces are discussed. Observations and results from modeling are reviewed, and the distributions of the individual dust populations are shown. The understanding of the Jovian dust environment allows to assess the dust hazard to spacecraft, and to characterize the material exchange between the Jovian moons, their surface properties and distribution of non-icy constituents., Comment: 17 pages, 8 figures. Accepted by Astrodynamics

In 1995, the Galileo spacecraft arrived at Jupiter to conduct follow-up experiments on pathfinder Pioneer and key Voyager discoveries especially at Io, Europa, Ganymede and Callisto. These new observations helped expand our scientific knowledge of the prominent Galilean satellites; studies revealed diversity with respect to their geology, internal structure, evolution and degree of past and present activity. Jupiter's diverse Galilean satellites, of which three are believed to harbor internal oceans, are central to understanding the habitability of icy worlds. Galileo provided for the first time compelling evidence of a near-surface global ocean on Europa. Furthermore, by understanding the Jupiter system and unraveling the history of its evolution from initial formation to the emergence of possible habitats and life, gives insight into how giant planets and their satellite systems form and evolve. Most important, new light is shed on the potential for the emergence and existence of life in icy satellite oceans. In 2009, NASA released a detailed Jupiter Europa Mission Study (EJSM) that proposed an ambitious Flagship Mission to understand more fully the satellites Europa and Ganymede within the context of the Jovian system. Key to EJSM is the NASA led Jupiter Europa Orbiter (JEO) and the ESA led Jupiter Ganymede Orbiter (JGO). JEO and JGO would execute a choreographed exploration of the Jovian system before settling into orbit around Europa and Ganymede, respectively. The National Academies Planetary Decadal Survey, 2011 has listed the NASA-led JEO as the second highest priority mission for the decade 2013-2022, and if chosen it would be launched in 2020 with arrival at Jupiter in 2025. If the JEO mission is not chosen it is anticipated that there will be opportunities in future decadal cycles. Jupiter Orbit Insertion (JOI) begins a 30-month Jovian system tour followed by nine months of science mapping after Europa Orbit Insertion (EOI) in July 2028. The orbiter will ultimately impact the surface of Europa after the mission is completed. The current JEO mission concept includes a range of instruments on the payload, to monitor dynamic phenomena (such as Io's volcanoes and Jupiters atmosphere), map the Jovian magnetosphere and its interactions with the Galilean satellites, and characterize water oceans beneath the ice shells of Europa and Ganymede. The payload includes a low mass (3.7 Kg) and low power (< 5 W) Thermal Instrument (TI) concept for measuring possible warm thermal anomalies on Europa s cold surface caused by recent (< 10,000 years) eruptive activity. Regions of anomalously high heat flow will be identified by thermal mapping using a nadir pointing, push-broom filter radiometer that provides far-IR imagery in two broad band spectral wavelength regions, 8-20 m and 20-100 m, for surface temperature measurements with better than a 2 K accuracy and a spatial resolution of 250 m/pixel obtained from a 100 Km orbit. The temperature accuracy permits a search for elevated temperatures when combined with albedo information. The spatial resolution is sufficient to resolve Europa's larger cracks and ridge axial valleys. In order to accomplish the thermal mapping, the TI uses sensitive thermopile arrays that are readout by a custom designed low-noise Multi-Channel Digitizer (MCD) ASIC that resides very close to the thermopile linear array outputs. Both the thermopile array and the MCD ASIC will need to show full functionality within the harsh Jovian radiation environment, operating at cryogenic temperatures, typically 150 K to 170 K. In the following, a radiation mitigation strategy together with a low risk Radiation-Hardened-By-Design (RHBD) methodology using commercial foundry processes is given for the design and manufacture of a MCD ASIC that will meet this challenge.

We base our search for the right instrumentation for detecting biosignatures on Europa on the analogy suggested by the recent work on polar ecosystems in the Canadian Arctic at Ellesmere Island. In that location sulphur patches (analogous to the Europan patches) are accumulating on glacial ice lying over saline springs rich in sulphate and sulphide. Their work reinforces earlier analogies in Antarctic ecosystems that are appropriate models for possible habitats that will be explored by the European Space Agency JUpiter ICy Moons Explorer (JUICE) mission to the Jovian System. Its Jupiter Ganymede Orbiter (JGO) will include orbits around Europa and Ganymede. The Galileo orbital mission discovered surficial patches of non-ice elements on Europa that were widespread and, in some cases possibly endogenous. This suggests the possibility that the observed chemical elements in the exoatmosphere may be from the subsurface ocean. Spatial resolution calculations of Cassidy and co-workers are available, suggesting that the atmospheric S content can be mapped by a neutral mass spectrometer, now included among the selected JUICE instruments. In some cases, large S-fractionations are due to microbial reduction and disproportionation (although sometimes providing a test for ecosystem fingerprints, even though with Sim – Bosak – Ono we maintain that microbial sulphate reduction large sulphur isotope fractionation does not require disproportionation. We address the question of the possible role of oxygen in the Europan ocean. Instrument issues are discussed for measuring stable S-isotope fractionations up to the known limits in natural populations of δ34 ≈ −70‰. We state the hypothesis of a Europa anaerobic oceanic population of sulphate reducers and disproportionators that would have the effect of fractionating the sulphate that reaches the low-albedo surficial regions. This hypothesis is compatible with the time-honoured expectation of Kaplan and co-workers (going back to the 1960s) that the distribution range of 32S/34S in analysed extra-terrestrial material appears to be narrower than the isotopic ratio of H, C or N and may be the most reliable for estimating biological effects. In addition, we discuss the necessary instruments that can test our biogenic hypothesis. First of all we hasten to clarify that the last-generation miniaturized mass spectrometer we discuss in the present paper are capable of reaching the required accuracy of ‰ for the all-important measurements with JGO of the thin atmospheres of the icy satellites. To implement the measurements, we single out miniature laser ablation time-of-flight mass spectrometers that are ideal for the forthcoming JUICE probing of the exoatmospheres, ionospheres and, indirectly, surficial low-albedo regions. Ganymede's surface, besides having ancient dark terrains covering about one-third of the total surface, has bright terrains of more recent origin, possibly due to some internal processes, not excluding biological ones. The geochemical test could identify bioindicators on Europa and exclude them on its large neighbour by probing relatively recent bright terrains on Ganymede's Polar Regions.

This paper has the goal of searching for natural trajectories that can be used for a particle or a spacecraft coming from a region of the space far from Jupiter system to be captured into this system by making close approaches with the Galilean satellites of the planet. The opposite situation is also possible and escape trajectories can also be found. This type of maneuver is called “Swing-By” and it is usual in astrodynamics. It was used in many space missions to reduce the fuel consumption by gaining or loosing energy from the gravity of a celestial body. Several famous examples are the Voyager, Cassini, Galileo and other missions. The idea of the present research is to study this type of maneuver using the Galilean satellites of Jupiter, to search for trajectories that change the two-body energy (particle or spacecraft)–(Jupiter) from positive to negative (a capture trajectory) or from negative to positive (an escape trajectory). Those trajectories can be used for a spacecraft going or leaving the planet Jupiter or to explain how particles can be captured or escape from Jupiter system by close approaches with the Galilean satellites. Initial conditions are varied to cover the whole possible alternatives and then small regions of captures and escapes are identified. After that, a study is made to see the accuracy of the Tisserand’s method when applied to those close approach trajectories.

The implementation of the deep space expeditions to the outer Solar system is the one of main priorities of modern astrodynamics. One of the irritating factors for this is the probability of the life determination in the underground ocean at the Jovian icy moon Europa. In that regard the realization of the cosmic projects using the spacecraft — orbiters around the Jovian Galilean Moons and especially the descent modules implementation on their surface is one of the basic focuses of the current international cosmic space researches collaboration. The mission design of such cosmic projects must be executed using new advanced methods for their planning: Low Energy Tours exploring, Favourable Total Radiation Doze of the spacecraft (SC) provision, and tolerable maintenance of the SC’s Time of Flight during the Jupiter system phase. Furthermore, the saving character of the trajectory design for the artificial satellite of the Galilean moon necessary required the smallness of the SC’s velocity relative this moon, i.e. the smallness of the SC’s hyperbolic velocity excess vector. In this work multibody bunch adaptive synthesis algorithms to overcome ballistic determinism expressed by existing of the integral of Jacobi in the basic CR3BP (circular restricted three body problem) are developed. It is shown that the diversity of the gravity of the ensemble in the Jovian system, not only complicates the accurate ballistic mission analysis, but, when its multibody effective use, allowing you to tailor flowed algorithms for the constructing scenarios of gravity assist maneuvers for interplanetary missions design, including circumvention of dangerous radiation areas, as well as landing at selected Galilean moon at a reasonable cost, resources and time.

NASA’s Juno mission launched in 2011 and will explore Jupiter and its near environment starting in 2016. Planetary protection requirements for avoiding the contamination of Europa have been taken into account in the Juno mission design. In particular Juno’s polar orbit, which enables scientific investigations of parts of Jupiter’s environment never before visited, also greatly assist avoiding close flybys of Europa and the other Galilean satellites. The science mission is designed to conclude with a deorbit burn that disposes of the spacecraft in Jupiter’s atmosphere. Compliance with planetary protection requirements is verified through a set of analyses including analysis of initial bioburden, analysis of the effect of bioburden reduction due to the space and Jovian radiation environments, probabilistic risk assessment of successful deorbit, Monte-Carlo orbit propagation, and bioburden reduction in the event of impact with an icy body.

Irregular satellites are dormant comet-like bodies that reside on distant prograde and retrograde orbits around the giant planets. They are likely to be captured objects. Dynamical modeling work indicates they may have been caught during a violent reshuffling of the giant planets ∼4 Gy ago (Ga) as described by the so-called Nice model. According to this scenario, giant planet migration scattered tens of Earth masses of comet-like bodies throughout the Solar System, with some comets finding themselves near giant planets experiencing mutual encounters. In these cases, gravitational perturbations between the giant planets were often sufficient to capture the comet-like bodies onto irregular satellite-like orbits via three-body reactions. Modeling work suggests these events led to the capture of on the order of ∼0.001 lunar masses of comet-like objects on isotropic orbits around the giant planets. Roughly half of the population was readily lost by interactions with the Kozai resonance. The remaining half found themselves on orbits consistent with the known irregular satellites. From there, the bodies experienced substantial collisional evolution, enough to grind themselves down to their current low-mass states. Here we explore the fate of the putative irregular satellite debris in the Jupiter system. Pulverized by collisions, we hypothesize that the carbonaceous chondrite-like material was beaten into small enough particles that it could be driven toward Jupiter by Poynting–Robertson (P–R) drag forces. Assuming its mass distribution was dominated by D > 50 μm particles, we find that >40% ended up striking the Galilean satellites. The majority were swept up by Callisto, with a factor of 3–4 and 20–30 fewer particles reaching Ganymede and Europa/Io, respectively. Collision evolution models indicate most of this material arrived about 4 Ga, but some is still arriving today. We predict that Callisto, Ganymede, Europa, and Io were buried about 4 Ga by ∼120–140 m, 25–30 m, 7–15 m, and 7–8 m of dark debris, respectively. The first two values are consistent with observations of the deepest dark lag deposits found on the most ancient terrains of Callisto and Ganymede. The rest of the debris was likely worked into the crusts of these worlds by geologic and impact processes. This suggests the debris is a plausible source of the dark lag material found in Europa’s low-lying crevices. More speculatively, it is conceivable that the accreted dark particles were a significant source of organic material to Europa’s subsurface ocean.

The New Horizons encounter with the Pluto system was a historic achievement in planetary exploration. Launched on January 19, 2006, the spacecraft executed its close encounter with Pluto on July 14, 2015, acquiring the first-ever close up data of Pluto, its five known satellites, and the surrounding plasma and particle environment. During its 9½ year cruise, the spacecraft also conducted a flyby of an asteroid in 2006 and a Jupiter gravity assist in 2007 during which over 700 observations of Jupiter, the Galilean satellites, and the plasma and particle environment near Jupiter were acquired. Led by Principal Investigator Alan Stern, New Horizons was the first launch of NASA’s New Frontiers Program and the first mission to Pluto and the Kuiper Belt.

The JUpiter ICy moons Explorer (JUICE) is an ESA L-class mission concept designed to explore the Galilean satellites of the Jovian system. Although the current mission science goals do not include any astrophysical observations, we find that the planned period of the JUICE mission is optimal for in situ measurements of Interstellar Dust (ISD), due to highly increased flux levels at that time. In case that JUICE carries a dust detector, this could lead to in situ high-resolution mass spectra of ISD grains. Such compositional information on the ISD grains is important for understanding the origins of solar/planetary systems, and therefore could represent a valuable addition to the core JUICE mission science.

We use numerical models, supported by our laboratory data, to predict the dust densities of ejecta outflux at any altitude within the Hill spheres of Europa and Ganymede. The ejecta are created by micrometeoroid bombardment and five different dust populations are investigated as sources of dust around the moons. The impacting dust flux (influx) causes the ejection of a certain amount of surface material (outflux). The outflux populates the space around the moons, where a part of the ejecta escapes and the rest falls back to the surface. These models were validated against existing Galileo DDS (Dust Detector System) data collected during Europa and Ganymede flybys. Uncertainties of the input parameters and their effects on the model outcome are also included. The results of this model are important for future missions to Europa and Ganymede, such as JUICE (JUpiter ICy moon Explorer), recently selected as ESA's next large space mission to be launched in 2022.

The morphology of impact craters on the icy Galilean satellites differs from craters on rocky bodies. The differences are thought due to the relative weakness of ice and the possible presence of sub-surface water layers. Digital elevation models constructed from Galileo images were used to measure a range of dimensions of craters on the dark and bright terrains of Ganymede. Measurements were made from multiple profiles across each crater, so that natural variation in crater dimensions could be assessed and averaged scaling trends constructed. The additional depth, slope and volume information reported in this work has enabled study of central peak formation and development, and allowed a quantitative assessment of the various theories for central pit formation. We note a possible difference in the size-morphology progression between small craters on icy and silicate bodies, where central peaks occur in small craters before there is any slumping of the crater rim, which is the opposite to the observed sequence on the Moon. Conversely, our crater dimension analyses suggest that the size-morphology progression of large lunar craters from central peak to peak-ring is mirrored on Ganymede, but that the peak-ring is subsequently modified to a central pit morphology. Pit formation may occur via the collapse of surface material into a void left by the gradual release of impact-induced volatiles or the drainage of impact melt into sub-crater fractures.

Classical methods to analyze the surface composition of atmosphereless planetary objects from an orbiter are IR and gamma ray spectroscopy and neutron backscatter measurements. The idea to analyze surface properties with an in-situ instrument has been proposed by Johnson et al. (1998) . There, it was suggested to analyze Europa's thin atmosphere with an ion and neutral gas spectrometer. Since the atmospheric components are released by sputtering of the moon's surface, they provide a link to surface composition. Here we present an improved, complementary method to analyze rocky or icy dust particles as samples of planetary objects from which they were ejected. Such particles, generated by the ambient meteoroid bombardment that erodes the surface, are naturally present on all atmosphereless moons and planets. The planetary bodies are enshrouded in clouds of ballistic dust particles, which are characteristic samples of their surfaces. In situ mass spectroscopic analysis of these dust particles impacting onto a detector of an orbiting spacecraft reveals their composition. Recent instrumental developments and tests allow the chemical characterization of ice and dust particles encountered at speeds as low as 1 km/s and an accurate reconstruction of their trajectories. Depending on the sampling altitude, a dust trajectory sensor can trace back the origin of each analyzed grain with about 10 km accuracy at the surface. Since the detection rates are of the order of thousand per orbit, a spatially resolved mapping of the surface composition can be achieved. Certain bodies (e.g., Europa) with particularly dense dust clouds, could provide impact statistics that allow for compositional mapping even on single flybys. Dust impact velocities are in general sufficiently high at orbiters about planetary objects with a radius >1000 km and with only a thin or no atmosphere. In this work we focus on the scientific benefit of a dust spectrometer on a spacecraft orbiting Earth's Moon as well as Jupiter's Galilean satellites. This ‘dust spectrometer' approach provides key chemical and isotopic constraints for varying provinces or geological formations on the surfaces, leading to better understanding of the body's geological evolution.

Missions to explore Europa have been imagined ever since the Voyager mission first suggested that Europa was geologically very young. Subsequently, the Galileo spacecraft supplied fascinating new insights into this satellite of Jupiter. Now, an international team is proposing a return to the Jupiter system and Europa with the Europa Jupiter System Mission (EJSM). Currently, NASA and ESA are designing two orbiters that would explore the Jovian system and then each would settle into orbit around one of Jupiter’s icy satellites, Europa and Ganymede. In addition, the Japanese Aerospace eXploration Agency (JAXA) is considering a Jupiter magnetospheric orbiter and the Russian Space Agency is investigating a Europa lander. The Jupiter Europa Orbiter (JEO) would be the NASA-led portion of the EJSM; JEO would address a very important subset of the complete EJSM science objectives and is designed to function alone or in conjunction with ESA’s Jupiter Ganymede Orbiter (JGO). The JEO mission concept uses a single orbiter flight system that would travel to Jupiter by means of a multiple-gravity-assist trajectory and then perform a multi-year study of Europa and the Jupiter system, including 30 months of Jupiter system science and a comprehensive Europa orbit phase of 9 months. The JEO mission would investigate various options for future surface landings. The JEO mission science objectives, as defined by the international EJSM Science Definition Team, include: A. Europa’s ocean: Characterize the extent of the ocean and its relation to the deeper interior. B. Europa’s ice shell: Characterize the ice shell and any subsurface water, including their heterogeneity, and the nature of surface–ice–ocean exchange. C. Europa’s chemistry: Determine global surface compositions and chemistry, especially as related to habitability. D. Europa’s geology: Understand the formation of surface features, including sites of recent or current activity, and identify and characterize candidate sites for future in situ exploration. E. Jupiter system: Understand Europa in the context of the Jupiter system. The JEO orbital mission would provide critical measurements to support the scientific and technical selection of future landed options. The primary challenge of a Europa mission is to perform in Jupiter’s radiation environment, radiation damage being the life-limiting parameter for the flight system. Instilling a system-level radiation-hardened-by-design approach very early in the mission concept would mitigate the pervasive mission and system-level impacts (including trajectory, configuration, fault protection, operational scenarios, and circuit design) that can otherwise result in runaway growth of cost and mass. This paper addresses the JEO mission concept developed by a joint team from JPL and the Applied Physics Laboratory to address the science objectives defined by an international science definition team formed in 2008, while designing for the Jupiter environment.

Impact craters on icy satellites display a wide range of morphologies, some of which have no counterpart on rocky bodies. Numerical simulation studies have struggled to reproduce the diversity of features, such as central pits and transitions in crater depth with increasing diameter, observed on the icy Galilean satellites. The transitions in crater depth (at diameters of about 26 and 150 km on Ganymede and Callisto) have been interpreted as reflecting subsurface structure. Using the CTH shock physics code, we model the formation of craters with diameters between 400 m and about 200 km on Ganymede using different subsurface temperature profiles. Our calculations include recent improvements in the model equation of state for H 2 O and quasi-static strength parameters for ice. We find that the shock-induced formation of dense high-pressure polymorphs (ices VI and VII) creates a gap in the crater excavation flow, which we call discontinuous excavation. For craters larger than about 20 km, discontinuous excavation concentrates a hot plug of material (>270 K and mostly on the melting curve) in the center of the crater floor. The size and occurrence of the hot plug are in good agreement with the observed characteristics of central pit craters, and we propose that a genetic link exists between them. We also derive depth versus diameter curves for different internal temperature profiles. In a 120 K isothermal crust, calculated craters larger than about 30 km diameter are deeper than observed and do not reproduce the transition at about 26 km diameter. Calculated crater depths are shallower and in good agreement with observations between about 30 and 150 km diameter using a warm thermal gradient representing a convective interior. Hence, the depth-to-diameter transition at about 26 km reflects thermal weakening of ice. Finally, simulation results generally support the hypothesis that the anomalous interior morphologies for craters larger than 100 km are related to the presence of a subsurface ocean.

The ice-covered world Europa—one of the four large Galilean satellites of Jupiter—may be the best place in the solar system to look for currently existing life beyond Earth.

Satellite-aided capture is a mission design concept used to reduce the delta-v required to capture into a planetary orbit. The technique employs close flybys of a massive moon to reduce the energy of the planet-centered orbit. A sequence of close flybys of two or more of the Galilean moons of Jupiter may further decrease the delta-v cost of Jupiter orbit insertion. A Ganymede-Io sequence can save 207 m/s of delta-v over a single Io flyby. A phase angle analysis based on the Laplace resonance is used to find triple-satellite-aided capture sequences involving Io, Europa, and Ganymede. Additionally, the near-resonance of Callisto and Ganymede is used to find triple-satellite-aided capture sequences involving Callisto, Ganymede, and another moon. A combination of these techniques is used to find quadruple-satellite-aided capture sequences that involve gravity-assists of all four Galilean moons. These sequences can save a significant amount of delta-v and have the potential to benefit both NASA’s Jupiter Europa orbiter mission and ESA’s Jupiter Ganymede orbiter mission.

The four Galilean satellites are thought to harbor one or even two global internal liquid layers beneath their surface layer. The iron core of lo and Ganymede is most likely (partially) liquid and also the core of Europa may be liquid. Furthermore, there are strong indications for the existence of a subsurface ocean in Europa, Ganymede, and Callisto. Here, we investigate whether libration observations can be used to prove the existence of these liquid layers and to constrain the thickness of the overlying solid layers. For lo, the presence of a small liquid core increases the libration of the mantle by a few percent with respect to an entirely solid lo and mantle libration observations could be used to determine the mantle thickness with a precision of several tens of kilometers given that the libration amplitude can be measured with a precision of 1 m. For Europa, Ganymede, and Callisto, the presence of a water ocean close to the surface increases by at least an order of magnitude the ice shell libration amplitude with respect to an entirely solid satellite. The shell libration depends essentially on the shell thickness and to a minor extent on the density difference between the ocean and the ice shell. The possible presence of a liquid core inside Europa and Ganymede has no noticeable influence on their shell libration. For a precision of several meters on the libration measurements, in agreement with the expected accuracy with the NASA/ESA EJSM orbiter mission to Europa and Ganymede, an error on the shell thickness of a few tens kilometers is expected. Therefore, libration measurements can be used to detect liquid layers such as lo's core or water subsurface oceans in Europa, Ganymede, and Callisto and to constrain the thickness of the overlying solid surface layers. (C) 2010 Elsevier Inc. All rights reserved.

The surface composition of Europa is of great importance for understanding both the internal evolution of Europa and its putative ocean. The Near Infrared Mapping Spectrometer (NIMS) investigation on Galileo observed Europa and the other Galilean satellites from 0.7 to 5.2 μm with spatial resolution down to a few kilometers during flybys by the spacecraft as it orbited Jupiter. These data have been analyzed and results published over the life of the Galileo mission and afterward. One result was the discovery of hydrated minerals at some locations on Europa and Ganymede. The data are noisy, especially for Europa, due to radiation affecting the NIMS electronics and detectors, and other artifacts are also present. The NIMS data are now being reprocessed using the accumulated knowledge gained over the entire missions to remove noise spikes and compensate for some other defects in the data. We are analyzing these reprocessed data in an attempt to defined better the nature of the hydrate spectral features and improve their interpretation. We report here on analyses of two NIMS reprocessed observations for the 0.7–3-μm region. A revised hydrate spectrum is calculated and mapped in detail across two lineaments. The spectrum shows the expected distorted water features but little or no spectral structure in these features. A narrow, weak spectral feature appears at 1.344 μm, which is weakly correlated with lower albedo. Several other weak features may be present but are difficult to confirm in these limited data sets. The hydrate signature shows the greatest strength within and toward the center of the lineaments, confirming and strengthening the association of the hydrate with these endogenic features. This trend may indicate that the material in the lineaments is youngest toward the center and has more water frost coverage toward the edge. A small, visually dark, circular feature has a spectrum that shows both hydrate and crystalline water ice features and perhaps contains a hydrate different in spectral characteristics and perhaps composition than found in the lineament.

The four massive Galilean satellites are believed to have formed within a circumplanetary disk during the last stages of Jupiter's formation. While the existence of a circum-jovian disk is supported by hydrodynamic simulations, no consensus exists regarding the origin and delivery mechanisms of the building blocks of the forming satellites. The opening of a gap in the circumsolar disk would have efficiently isolated Jupiter from the main sources of solid material. However, a reservoir of planetesimals should have existed at the outer edge of Jupiter's gap, where solids were trapped and accumulated over time. Here we show that the formation of Saturn's core within this reservoir, or its prompt inward migration, allows planetesimals to be redistributed from this reservoir towards Jupiter and the inner Solar System, thereby providing enough material to form the Galilean satellites and to populate the Main Belt with primitive asteroids. We find that the orbit of planetesimals captured within the circumjovian disk are circularized through friction with gas in a compact system comparable to the current radial extent of the Galilean satellites. The decisive role of Saturn in the delivery mechanism has strong implications for the occurrence of massive moons around extrasolar giant planets as they would preferentially form around planets within multiple planet systems., Comment: 17 pages (incl. Appendix); 10 Figures; Accepted for publication in AJ

In order to understand the evolution of planetary satellite surfaces and atmospheres it is important to understand their external environments. In this paper we look at the interactions between plasma in planetary magnetospheres and satellite atmospheres responsible for the production and loss of atmospheric mass. We focus on the processes which take place in the tenuous atmospheres of the Galilean satellites and in the Enceladus water plume. We also review the impact histories of satellites in the outer solar system, and compare the record of impacts in the inner and outer solar system.

International audience; Of the terrestrial planets, Earth and Mercury have self-sustained fields while Mars and Venus do not. Magnetic field data recorded at Ganymede have been interpreted as evidence of a self-generated magnetic field. The other icy Galilean satellites have magnetic fields induced in their subsurface oceans while Io and the Saturnian satellite Titan apparently are lacking magnetic fields of internal origin altogether. Parts of the lunar crust are remanently magnetized as are parts of the crust of Mars. While it is widely accepted that the magnetization of the Martian crust has been caused by an early magnetic field, for the Moon alternative explanations link the magnetization to plasma generated by large impacts. The necessary conditions for a dynamo in the terrestrial planets and satellites are the existence of an iron-rich core that is undergoing intense fluid motion. It is widely accepted that the fluid motion is caused by convection driven either by thermal buoyancy or by chemical buoyancy or by both. The chemical buoyancy is released upon the growth of an inner core. The latter requires a light alloying element in the core that is enriched in the outer core as the solid inner core grows. In most models, the light alloying element is assumed to be sulfur, but other elements such as, e.g., oxygen, silicon, and hydrogen are possible. The existence of cores in the terrestrial planets is either proven beyond reasonable doubt (Earth, Mars, and Mercury) or the case for a core is compelling as for Venus and the Moon. The Galilean satellites Io and Ganymede are likely to have cores judging from Galileo radio tracking data of the gravity fields of these satellites. The case is less clear cut for Europa. Callisto is widely taken as undifferentiated or only partially differentiated, thereby lacking an iron-rich core. Whether or not Titan has a core is not known at the present time. The terrestrial planets that do have magnetic fields either have a well-established inner core with known radius and density such as Earth or are widely agreed to have an inner core such as Mercury. The absence of an inner core in Venus, Mars, and the Moon (terrestrial bodies that lack fields) is not as well established although considered likely. The composition of the Martian core may be close to the Fe-FeS eutectic which would prevent an inner core to grow as long as the core has not cooled to temperatures around 1500 Kelvin. Venus may be on the verge of growing an inner core in which case a chemical dynamo may begin to operate in the geologically near future. The remanent magnetization of the Martian and the lunar crust is evidence for a dynamo in Mars' and possibly the Moon's early evolution and suggests that powerful thermally driven dynamos are possible. Both the thermally and the chemically driven dynamo require that the core is cooled at a sufficient rate by the mantle. For the thermally driven dynamo, the heat flow from the core into the mantle must by larger than the heat conducted along the core adiabat to allow a convecting core. This threshold is a few mW m(-2) for small planets such as Mercury, Ganymede, and the Moon but can be as large as a few tens mW m(-2) for Earth and Venus. The buoyancy for both dynamos must be sufficiently strong to overcome Ohmic dissipation. On Earth, plate tectonics and mantle convection cool the core efficiently. Stagnant lid convection on Mars and Venus are less efficient to cool the core but it is possible and has been suggested that Mars had plate tectonics in its early evolution and that Venus has experiened episodic resurfacing and mantle turnover. Both may have had profound implications for the evolution of the cores of these planets. It is even possible that inner cores started to grow in Mars and Venus but that the growth was frustrated as the mantles heated following the cessation of plate tectonics and resurfacing. The generation of Ganymede's magnetic field is widely debated. Models range from magneto-hydrodynamic convection in which case the field will not be self-sustained to chemical and thermally-driven dynamos. The wide range of possible compositions for Ganymede's core allows models with a completely liquid near eutectic Fe-FeS composition as well as models with Fe inner cores or cores in with iron snowfall.

In the spirit of the symposium and the theme of the session of this presentation, “Our solar system after Galileo, the grand vision,” I review briefly a relatively recently discovered phenomenon in the solar system – existence of atmospheres on certain moons, including Io, one of the four moons Galileo discovered four centuries ago. The origin of such atmospheres is discussed, and comparisons are made between various gassy moons.