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We study the dynamics of the thermal O+ and H+ ions at Ganymede's magnetopause when Ganymede is inside and outside of the Jovian plasma sheet using a three-dimensional hybrid model of plasma (kinetic ions, fluid electrons). We present the global structure of the electric fields and power density (E · J) in the magnetosphere of Ganymede and show that the power density at the magnetopause is mainly positive and on average is +0.95 and +0.75 nW/m3 when Ganymede is inside and outside the Jovian plasma sheet, respectively, but locally it reaches over +20 nW/m3. Our kinetic simulations show that ion velocity distributions at the vicinity of the upstream magnetopause of Ganymede are highly non-Maxwellian. We investigate the energization of the ions interacting with the magnetopause and find that the energy of those particles on average increases by a factor of 8 and 30 for the O+ and H+ ions, respectively. The energy of these ions is mostly within 1-100 keV for both species after interaction with the magnetopause, but a few percentages reach to 0.1-1 MeV. Our kinetic simulations show that a small fraction (25%) of the corotating Jovian plasma reach the magnetopause, but among those >50% cross the high-power density regions at the magnetopause and gain energy. Finally, we compare our simulation results with Galileo observations of Ganymede's magnetopause crossings (i.e., G8 and G28 flybys). There is an excellent agreement between our simulations and observations, particularly our simulations fully capture the size and structure of the magnetosphere. [ABSTRACT FROM AUTHOR]

Jovian magnetospheric plasma irradiates the surface of Ganymede and is postulated to be the primary agent that changes the surface brightness of Ganymede, leading to asymmetries between polar and equatorial regions as well as between the trailing and leading hemispheres. As impinging ions sputter surface constituents as neutrals, ion precipitation patterns can be remotely imaged using the Energetic Neutral Atoms (ENA) measurement technique. Here we calculate the expected sputtered ENA flux from the surface of Ganymede to help interpret future observations by ENA instruments, particularly the Jovian Neutrals Analyzer (JNA) onboard the JUpiter ICy moon Explorer (JUICE) spacecraft. We use sputtering models developed based on laboratory experiments to calculate sputtered fluxes of H2O, O2, and H2. The input ion population used in this study is the result of test particle simulations using electric and magnetic fields from a hybrid simulation of Ganymede's environment. This population includes a thermal component (H+ and O+ from 10 eV to 10 keV) and an energetic component (H+, O++, and S+++ from 10 keV to 10 MeV). We find a global ENA sputtering rate from Ganymede of 1.42 × 1027 s-1, with contributions from H2, O2, and H2O of 34%, 17%, and 49% respectively. We also calculate the energy distribution of sputtered Energetic Neutral Atoms (ENAs), give an estimate of a typical JNA count rate at Ganymede, and investigate latitudinal variations of sputtered fluxes along a simulated orbit track of the JUICE spacecraft. Our results demonstrate the capability of the JNA sensor to remotely map ion precipitation at Ganymede. Plain Language Summary Particles trapped by Jupiter's magnetic field interact with Jupiter's moons. Ganymede, the largest of those moons, lacks a dense atmosphere to protect its surface from these energetic Jovian particles, but Ganymede's magnetic field is strong enough to influence their trajectory: charged particles are deflected away from equatorial regions to polar regions, resulting in uneven particle precipitation patterns at the surface of Ganymede. When ions hit the surface of Ganymede, they eject particles from the surface, in a process referred to as sputtering. Those particles are mostly neutral and therefore unaffected by Ganymede's magnetic fields, so we can image where ions hit the surface of Ganymede by measuring ejected neutral particles. The Jovian Neutrals Analyzer (JNA) will fly onboard the JUpiter ICy moon Explorer (JUICE) spacecraft and will measure sputtered neutrals in the vicinity of Ganymede. To help interpret the data to be collected by JNA, we used models derived from laboratory experiments to simulate what JNA will observe at Ganymede. Our results show that JNA will be able to show us where ions hit the surface of Ganymede, which is important as uneven ion precipitation is thought to explain why Ganymede's poles are brighter than its equatorial regions. [ABSTRACT FROM AUTHOR]

Ganymede is the only Solar System moon that generates a permanent magnetic field. Dynamics within the Ganymedean magnetosphere is thought to be driven by energy‐transfer interactions on its upstream magnetopause. Previously in Kaweeyanun et al. (2020), https://doi.org/10.1029/2019GL086228 we created a steady‐state analytical model of Ganymede's magnetopause and predicted global‐scale magnetic reconnection to occur frequently throughout the surface. This paper subsequently provides the first assessment of Kelvin‐Helmholtz (K‐H) instability growth on the magnetopause. Using the same analytical model, we find that linear K‐H waves are expected on both Ganymedean magnetopause flanks. Once formed, the waves propagate downstream at roughly half the speed of the external Jovian plasma flow. The Ganymedean K‐H instability growth is asymmetric between magnetopause flanks due to the finite Larmor radius effect arising from large gyroradii of Jovian plasma ions. A small but notable enhancement is expected on the sub‐Jovian flank according to the physical understanding of bulk plasma and local ion flows alongside comparisons to the well‐observed magnetopause of Mercury. Further evaluation shows that nonlinear K‐H vortices should be strongly suppressed by concurring global‐scale magnetic reconnection at Ganymede. Reconnection is therefore the dominant cross‐magnetopause energy‐transfer mechanism and driver of global‐scale plasma convection within Ganymede's magnetosphere. Plain Language Summary: Ganymede is the largest moon of Jupiter, and the only moon in the Solar System that can maintain a permanent magnetic field. Current research suggests Ganymede contains two internal magnetic field sources—a molten iron core and a subsurface ocean. The study of Ganymede's magnetic environment will be a primary objective for the JUpiter ICy moon Explorer (JUICE), the first moon‐orbiting satellite mission set to launch in 2022. Ganymede is surrounded by flows of plasma (energized gas) which are normally deflected away by the magnetic field along a boundary called the upstream magnetopause. However, the magnetic shield can be broken through interactions on the magnetopause such as Kelvin‐Helmholtz (K‐H) instability. Using a mathematical model established in Kaweeyanun et al. (2020), https://doi.org/10.1029/2019GL086228 we first determine that K‐H instability can grow as waves along Ganymede's magnetopause flank regions, and that the growth is enhanced on the magnetopause flank that is closest to Jupiter due to motions of local plasma. Finally, using Mercury as a comparison case, we argue that K‐H waves are unlikely to grow into turbulent vortices that can inject plasma across the magnetopause, as they will be torn apart by another magnetopause process known as magnetic reconnection. Key Points: We present the first assessment of Kelvin‐Helmholtz (K‐H) instability on Ganymede's upstream magnetopause using an analytical modelLinear K‐H waves can grow on both magnetopause flanks with small enhancement on the sub‐Jovian flank due to the finite Larmor radius effectNonlinear K‐H vortices should be suppressed by magnetic reconnection, so the latter likely dominates cross‐magnetopause plasma transport [ABSTRACT FROM AUTHOR]

We present reflectance spectra of Ganymede's leading and trailing hemispheres in the wavelength range 138–215 nm, obtained by the Hubble Space Telescope Cosmic Origins Spectrograph (HST/COS) in 2014. The most notable feature of both spectra is the absence of a sharp water absorption edge at ~165 nm, seen in laboratory measurements of ice reflectivity and in previous observations of Saturn's icy moons and rings. Rather than displaying a sharp change in the reflectivity at the wavelength of the water ice absorption edge, Ganymede's reflectance gradually increases with wavelength at λ > 165 nm. We show that the observed shape of Ganymede's UV reflectance is inconsistent with intimate mixture models of pure ice with UV‐dark materials including tholins, amorphous carbon, graphite, and silicates. However, we find that intraparticle models, in which a small proportion of a UV‐absorbing contaminant is trapped as inclusions within the ice matrix, are able to suppress the 165 nm feature at contaminant concentrations of <1%. We show that models of ice with inclusions of silicates, Triton‐type tholin, or H2O2 are able to produce the observed gradual increase in reflectivity at λ > 165 nm, but additional absorbing surface materials are required to produce a good fit to Ganymede's previously observed near‐UV and visible reflectance. Plain Language Summary: We use the Hubble Space Telescope to study how Jupiter's moon Ganymede reflects sunlight at ultraviolet (UV) wavelengths. Ganymede's surface is known to contain significant amounts of water ice, which is very reflective at UV wavelengths longer than 165 nm, but reflects very little light at shorter wavelengths. We find that Ganymede does not show the same abrupt change in reflectivity at 165 nm as pure water ice. We use computer models that account for how light interacts with different materials to try and explain why we cannot see evidence of water ice in UV observations of Ganymede, even though we know ice is present there. We find that models that include pure water ice cannot explain our observations, but if we model ice containing a small fraction of impurities, we get a good match with Ganymede's UV reflectance. Models like the ones we produced for this study will be useful for interpreting observations of Ganymede and Jupiter's other icy moons, Europa and Callisto, by the ultraviolet instrument on the upcoming JUpiter ICy moons Explorer (JUICE) mission. Determining which impurities exist in icy moon surfaces helps us to understand the habitability of these worlds—JUICE's main scientific goal. Key Points: Ganymede's UV reflectance spectrum lacks the sharp H2O feature at 165 nm observed at Saturn's icy moonsAdding small fractions (<1%) of contaminants within intraparticle models of H2O suppresses the 165 nm featureThe contaminant materials found to produce the best fit to Ganymede's UV spectrum were silicates, a Triton‐type tholin, and H2O2 [ABSTRACT FROM AUTHOR]

The largest moon in the solar system, Ganymede, is the only moon known to possess a strong intrinsic magnetic field and a corresponding magnetosphere. Using the latest version of Space Weather Modeling Framework (SWMF), we study the upstream plasma interactions and dynamics in this sub‐Alfvénic system. Results from the Hall magnetohydrodynamics (MHD) and the coupled MHD with embedded particle‐in‐cell (MHD‐EPIC) models are compared. We find that under steady upstream conditions, magnetopause reconnection occurs in a nonsteady manner, and the energy partition between electrons and ions is different in the two models. Flux ropes of Ganymede's radius in length form on the magnetopause at a rate about 3 min and create spatiotemporal variations in plasma and field properties. Upon reaching proper grid resolutions, the MHD‐EPIC model can resolve both electron and ion kinetics at the magnetopause and show localized nongyrotropic behavior inside the diffusion region. The estimated global reconnection rate from the models is about 80 kV with 60% efficiency, and there is weak evidence of ∼1 min periodicity in the temporal variations due to the dynamic reconnection process. Key Points: Simulated flux rope of the length of Ganymede's radius forms at a rate of ∼2 min at the upstream magnetopauseIon and electron scale kinetic physics near the reconnection site are revealed with coupled kinetic modelA general and robust method to measure the global reconnection rate is proposed [ABSTRACT FROM AUTHOR]

Laplace‐like resonances among Ganymede, Europa, and Io may have once led Ganymede to have an eccentricity (presently e = 0.0013) as high as ~0.07 (Showman & Malhotra, 1997, https://doi.org/10.1006/icar.1996.5669). While diurnal stresses at Ganymede today are small (less than 10 kPa), a previous period of higher eccentricity may have allowed for an order of magnitude increase in the diurnal tidal stresses that could drive fault initiation and result in a past period of active tectonism. To investigate the role of tidal stresses on faulting, we use the numerical model SatStress (Wahr et al., 2009, https://doi.org/10.1016/j.icarus.2008.11.002) to calculate diurnal tidal stresses on Ganymede's surface assuming e = 0.05, representative of a more eccentric orbit in Ganymede's past. We resolve normal and shear stresses onto discrete mapped fault segments and assess Coulomb failure criteria along three inferred shear zones on Ganymede's surface: Dardanus Sulcus, Tiamat Sulcus, and Nun Sulci. While Coulomb failure is not expected from diurnal stressing at any of the three shear zones for a diurnal model of present‐day (low) eccentricity, we do predict Coulomb failure for a past, high eccentricity case, in isolated diurnal slip windows and limited to very shallow depths (~100 to 250 m). In these cases, this model may provide an alternative to invoking additional stresses such as nonsynchronous rotation or true polar wander, as in previous studies (Cameron et al., 2019, https://doi.org/10.1016/j.icarus.2018.09.002). Additionally, this model is in general agreement with the sense of inferred shear from imagery and structural mapping. Plain Language Summary: As the Galilean satellites (Io, Europa, Ganymede, and Callisto) orbit Jupiter, the gravity of each body influences each other, causing tidal stresses, much like how the Moon causes ocean tides on Earth. The orbits of Jupiter's moons are elliptical rather than circular, meaning the tidal stresses vary as the moons change distance from Jupiter. The orbits of Io, Europa, and Ganymede are currently in a configuration, or orbital resonance, where for every one orbit around Jupiter that Ganymede completes, Europa completes two, and Io completes four. This orbital resonance also influences tidal stresses. High enough tidal stresses may have driven past geologic activity, such as the strike‐slip faults we observe on Ganymede's surface. Strike‐slip faulting occurs when fault walls move past one another laterally, like the San Andreas fault in California. This study examines images of Ganymede and models the amount of tidal stress necessary to have moved these strike‐slip faults based on diurnal stresses alone, without the need to invoke more complicated additional stress configurations. We find that a past orbital resonance may have increased Ganymede's tidal stresses enough to allow for shallow movement along a strike‐slip fault. We also find that the direction of the movement of the fault predicted in our models generally agrees with the direction of motion inferred from the image. Key Points: Shallow strike slip may be possible on Ganymede during a past, high eccentricity orbitDiurnal tidal stresses may have driven shallow (<250 m) strike‐slip failureModels match the sense of inferred shear from images and structural mapping efforts [ABSTRACT FROM AUTHOR]

The largest moon in the solar system, Ganymede, is also the only moon known to possess a strong intrinsic magnetic field and a corresponding magnetosphere. Using the new version of Hall magnetohydrodynamic with embedded particle‐in‐cell model with a self‐consistently coupled resistive body representing the electrical properties of the moon's interior, improved inner boundary conditions, and the flexibility of coupling different grid geometries, we achieve better match of magnetic field with measurements for all six Galileo flybys. The G2 flyby comparisons of plasma bulk flow velocities with the Galileo Plasma Subsystem data support the oxygen ion assumption inside Ganymede's magnetosphere. Crescent shape, nongyrotropic, and nonisotropic ion distributions are identified from the coupled model. Furthermore, we have derived the energy fluxes associated with the upstream magnetopause reconnection of ∼10−7W/cm2 based on our model results and found a maximum of 40% contribution to the total peak auroral emissions. Key Points: Hall MHD‐EPIC model of Ganymede's magnetosphere uses realistic inner boundary conditions and energy‐conserving PIC schemeIon‐scale kinetics at upstream magnetopause are fully resolved, shown by the nongyrotropic/anisotropic distributionsElectron precipitation of ∼10−7 W/cm2 shows up to half of the peak emission brightness contributed by upstream reconnection [ABSTRACT FROM AUTHOR]

Abstract The outer H 2 O ice shell of Europa, a Galilean satellite of Jupiter thought to exhibit a global interior water ocean, shows evidence for plate-like surface motions along tectonic boundaries. In this study, we investigate whether forces similar to those that drive plate tectonics on Earth may drive plate motions on icy satellites. We focus on whether the forces of slab pull and ridge push, driven by thermal and compositional buoyancy, can overcome the mechanical forces resisting plate motions to sustain interaction between surface ice and the interior ocean. We find that Earth-like plate tectonics is implausible within the outer ice shell of icy satellites because the forces resisting self-sustaining plate cycling are of significantly greater order than the driving forces. In the case of Ganymede and Triton, if it has a thick shell, plate tectonics would require warm, low-viscosity slabs to remain intact for timescales that are orders of magnitude longer than those of interior convection. Highlights • Earth-like plate tectonics unlikely in the outer ice shells of icy satellites • Mechanical resisting forces are much greater than driving buoyancy forces. • Buoyancy forces most important on large ocean worlds w/ thick ice shells • Secular stresses on the order of 10 MPa required to produce Europa features • Forcing most likely to be from a decoupled ice shell or global freezing/thawing [ABSTRACT FROM AUTHOR]

Highlights • Nonsynchronous rotation may have assisted strike-slip faulting on Ganymede. • Comparisons of inferred and modeled sense of slip are compatible at many faults. • Strike-slip failure may be possible down to ∼2.5 km depth. Abstract High-resolution Galileo data of Ganymede's complex surface provide strong and ubiquitous evidence of strike-slip motion: en echelon structures, strike-slip duplexes, laterally offset pre-existing features, and strained craters. In a previous study, we performed a detailed mapping of these strike-slip morphologies within nine regions of Ganymede: Dardanus Sulcus, Tiamat Sulcus, Nun Sulci, Byblus Sulcus, Nippur and Philus Sulci, the Transitional Terrain of Northern Marius Regio, Anshar Sulcus, Arbela Sulcus, and Uruk Sulcus. Strike-slip indicators were inferred in various combinations at each site, in addition to extensional processes. However, the prominence of strike-slip indicators suggests shearing has been an important process to the structural development of Ganymede's surface. Moreover, tidal stresses on Ganymede, under particular circumstances, may have been sufficient to induce Coulomb failure and generate strike-slip faulting. Here we investigate the role of both diurnal and non-synchronous rotation (NSR) tidal stresses on Ganymede using the numerical code SatStress. We resolve normal and shear tractions onto discrete fault segments of specified orientation and assess Coulomb failure potential for the nine inferred fault zones. Testing a range of subsurface fault depths (z = 0–5 km) and ice friction (μ f = 0.2–0.6), we find that tidal stress models of combined diurnal and NSR stress readily promote Coulomb failure within each studied fault zone. High friction (μ f = 0.6) limits failure depths to ∼1 km, while low friction (μ f = 0.2) extends failure depths to ∼2 km, consistent with elastic thickness estimates. We also compare each fault zone's predicted shear sense to the inferred shear sense from our structural mapping efforts and find compatible senses of shear among six of the nine regions that exhibit notable fault offset and/or prevalent inferences of en echelon , duplexes, and strained craters. In addition, principal stress orientations and failure feasibility computed on a global scale suggest that combined diurnal and NSR tidal stresses promote Coulomb failure within the shallow (< 2 km) icy lithosphere across much of Ganymede. Coulomb failure is limited near the equator by large compressive NSR stresses, but stresses at mid- to high-latitudes readily promote Coulomb failure along a wide range of fault orientations. Based on these results, we infer that nonsynchronous rotation may have assisted the formation and evolution of strike-slip structures on Ganymede. [ABSTRACT FROM AUTHOR]

The technique of mutual approximations accurately gives the central instant of the maximum apparent approximation of two moving natural satellites in the plane of the sky. This can be used in ephemeris fitting to infer the relative positions of satellites with high precision. Only mutual phenomena – occultations and eclipses – can achieve better results. However, mutual phenomena only occur every six years in the case of Jupiter. Mutual approximations do not have this restriction and can be observed at any time in the year as long as the satellites are visible. In this work, we present 104 central instants determined from the observations of 66 mutual approximations between the Galilean moons carried out at different sites in Brazil and France during the period 2016–2018. For 28 events, we have at least two independent observations. All telescopes were equipped with a narrow-band filter centred at 889 nm with a width of 15 nm to eliminate the scattered light from Jupiter. The telescope apertures ranged between 25 and 120 cm. For comparison, the precision of the positions obtained with classical CCD astrometry is about 100 mas, for mutual phenomena it can be 10 mas or less, and the average internal precision obtained with mutual approximations is 11.3 mas. This new type of simple, yet accurate, observations can significantly improve the orbits and ephemeris of Galilean satellites and thus it can be very useful for the planning of future space missions to the Jovian system. [ABSTRACT FROM AUTHOR]

The volume-averaged or mean arithmetical (MA) magnetic field vector in the whole dynamo region is already known once the observable magnetic dipole and region size of a planet/moon are known. This MA field is 1.4 of the root-mean-square (RMS) dipole field at the outer dynamo boundary. The Earth, Jupiter and Saturn have MA fields of 1mT and small inclinations of their dipoles to the axis of rotation. This common feature for similar dynamos could be in these rather different planets. Uranus and Neptune have MA fields of ~0.1mT and their dipoles are strongly inclined requiring not so clear modifications in dynamo modelling. A similar MA field value is for Ganymede (a moon of Jupiter), whereas an almost symmetric to the rotation axis field and rather asymmetric to the equator is observed. Almost the same field spatial configuration is observed in Mercury, where the MA field is ~0.01mT. There is no active dynamo in Venus, Mars, Moon and in some other planets/moons, whereas an existence of ancient/future self magnetic fields is possible there. The geodynamo RMS field typical value is about a few mT, as it is estimated from the well-known Christensen-Aubert (2006) scaling law. That gives a geodynamo RMS/MA field ratio of the order 10. The same ratio could be expected in Jupiter and Saturn, whereas one of Starchenko (2018) scaling law gives a ratio ~100 for such geodynamo-type system that is consistent with some observational estimations and dynamo models. Uranus, Neptune and Ganymede, perhaps, have the RMS field just slightly (up to a few times) exceeding the MA field. The intermittency of the internal magnetic field also could be roughly measured by this RMS/MA field ratio for the objects with a sufficiently different from zero MA field. [ABSTRACT FROM AUTHOR]

Abstract The surface of Ganymede is characterized by intense tectonic deformation. The two terrains, light and dark, show morphotectonic features, namely, grooves and furrows. These represent kilometric systems at the regional scale, interpreted as extensional structures (e.g., horst and graben systems). Currently, evidence of significant compression has not yet been recognized, leaving open the opportunity to clarify the tectonic balance of Ganymede's surface. Debate still exists on the geodynamics responsible for surface deformation, and the investigation of these features contributes to understanding the internal processes. To understand the deformation that shapes the surface of the satellite, we explore the tectonic setting of the light terrain region of Uruk Sulcus. Using structural geology methodologies by remote sensing and systematic analysis of grooves, we recognize three groove systems within the sulcus and one furrow system in the neighboring dark terrain. Uruk Sulcus is also characterized by the presence of two domains of lineaments, subtle linear texture anisotropies related to the influence of tectonic processes on morphology. Comparison of the combined results from the analyses of the recognized groove systems with the lineament domains allowed us to infer the stress field responsible for their development in the studied area. According to these results, an evolutionary tectonic scenario for Uruk Sulcus is proposed: a tectonic corridor characterized by dextral transpression, with 58% of the internal compressional component approximately N-S oriented and responsible for localized, E-W extensional features among crustal blocks of the shear zone. Highlights • A new high-resolution Voyager and Galileo image mosaic of Uruk Sulcus is produced. • Automatic lineament analysis allowed inferring the stress field at Uruk Sulcus. • Structural analysis allows identifying groove and furrow systems. • Variation of lithospheric thickness/brittle deformation is recognized. • Uruk Sulcus is a transpressive right-lateral shear corridor. [ABSTRACT FROM AUTHOR]

Highlights • We extensively map nine sites of varying terrain types in both sub- and anti-Jovian hemispheres using Galileo imagery. • Numerous examples of strike-slip morphological indicators at every site suggest strike-slip tectonism is important to Ganymede's evolutionary history. • Mapped sites share similarities with each other, both in the occurrence of strike-slip indicators and in the stages of deformation when comparing neighboring regions. Abstract The heavily fractured surface of Ganymede displays many morphologically distinctive regions of inferred distributed shear and strike-slip faulting that may be important to the structural development of its surface. To better understand the role of strike-slip tectonism in shaping this complex icy surface, we perform detailed mapping at nine sites using Galileo and Voyager imagery, noting key examples of strike-slip morphologies where present. These four morphological indicators are: en echelon structures, strike-slip duplexes, laterally offset pre-existing features, and possible strained craters. We map sites of both light, grooved terrain (Nun Sulci, Dardanus Sulcus, Tiamat Sulcus, Uruk Sulcus, and Arbela Sulcus), and terrains that are transitional from dark to light terrains (Nippur and Philus Sulci, Byblus Sulcus, Anshar Sulcus, and the Transitional Terrain of Northern Marius Regio). At least one, if not more, of the four strike-slip morphological indicators are observed at every site, suggesting strike-slip tectonism is indeed important to Ganymede's evolutionary history. Byblus Sulcus is the only mapped site where the presence of strike-slip indicators is limited to only a few en echelon structures; every other mapped site displays examples of at least two types, with Arbela Sulcus containing candidate examples of all four. In addition, quantification of morphological characteristics suggests related rotation between sites, as evidenced by the predominant NW/SE trend of mapped features within the light terrain present in five different sites (Nun, Tiamat, Uruk, Nippur/Philus, Byblus, and Anshar Sulcus). Moreover, incorporation of strike-slip tectonism with pre-existing observations of extensional behavior provides an improved, synoptic representation of Ganymede's tectonic history. [ABSTRACT FROM AUTHOR]

Context. We consider the three-body mean motion resonance defined by the Jovian moons Io, Europa, and Ganymede, which is commonly known as the Laplace resonance. In terms of the moons' mean longitudes λ1 (Io), λ2 (Europa), and λ3 (Ganymede), this resonance is described by the librating argument φL ≡ λ1 − 3λ2 + 2λ3 ≈ 180°, which is the sum of φ12 ≡ λ1 − 2λ2 + ϖ2 ≈ 180° and φ23 ≡ λ2 − 2λ3 + ϖ2 ≈ 0°, where ϖ2 denotes Europa's longitude of perijove. Aims. In particular, we construct approximate models for the evolution of the librating argument φL over the period of 100 yr, focusing on its principal amplitude and frequency, and on the observed mean motion combinations n1 − 2n2 and n2 − 2n3 associated with the quasi-resonant interactions above. Methods. First, we numerically propagated the Cartesian equations of motion of the Jovian system for the period under examination, and by comparing the results with a suitable set of ephemerides, we derived the main dynamical effects on the target quantities. Using these effects, we built an alternative Hamiltonian formulation and used the normal forms theory to precisely locate the resonance and to semi-analytically compute its main amplitude and frequency. Results. From the Cartesian model we observe that on the timescale considered and with ephemerides as initial conditions, both φL and the diagnostics n1 − 2n2 and n2 − 2n3 are well approximated by considering the mutual gravitational interactions of Jupiter and the Galilean moons (including Callisto), and the effect of Jupiter's J2 harmonic. Under the same initial conditions, the Hamiltonian formulation in which Callisto and J2 are reduced to their secular contributions achieves larger errors for the quantities above, particularly for φL. By introducing appropriate resonant variables, we show that these errors can be reduced by moving in a certain action-angle phase plane, which in turn implies the necessity of a tradeoff in the selection of the initial conditions. Conclusions. In addition to being a good starting point for a deeper understanding of the Laplace resonance, the models and methods described are easily generalizable to different types of multi-body mean motion resonances. Thus, they are also prime tools for studying the dynamics of extrasolar systems. [ABSTRACT FROM AUTHOR]

Jupiter’s aurorae are produced in its upper atmosphere when incoming high-energy electrons precipitate along the planet’s magnetic field lines. A northern and a southern main auroral oval are visible, surrounded by small emission features associated with the Galilean moons. We present infrared observations, obtained with the Juno spacecraft, showing that in the case of Io, this emission exhibits a swirling pattern that is similar in appearance to a von Kármán vortex street. Well downstream of the main auroral spots, the extended tail is split in two. Both of Ganymede’s footprints also appear as a pair of emission features, which may provide a remote measure of Ganymede’s magnetosphere. These features suggest that the magnetohydrodynamic interaction between Jupiter and its moon is more complex than previously anticipated. [ABSTRACT FROM AUTHOR]

Ganymede’s core most likely possesses an active dynamo today, which produces a magnetic field at the surface of  ∼ 719 nT. Thermochemical convection triggered by cooling of the core is a feasible power source for the dynamo. Experiments of different research groups indicate low pressure gradients of the melting temperatures for Fe–FeS core alloys at pressures prevailing in Ganymede’s core ( < 10 GPa). This may entail that the core crystallizes from the top instead of from the bottom as is expected for Earth’s core. Depending on the core sulfur concentration being more iron- or more sulfur-rich than the eutectic concentration either snowing iron crystals or a solid FeS layer can form at the top of the core. We investigate whether these two core crystallization scenarios are capable of explaining Ganymede’s present magnetic activity. To do so, we set up a parametrized one-dimensional thermal evolution model. We explore a wide range of parameters by running a large set of Monte Carlo simulations. Both freezing scenarios can explain Ganymede’s present-day magnetic field. Dynamos of iron snow models are rather young ( < 1 Gyr), whereas dynamos below the FeS layer can be both young and much older ( ∼ 3.8 Gyr). Successful models preferably contain less radiogenic heat sources in the mantle than the chondritic abundance and show a correlation between the reference viscosity in the mantle and the initial core sulfur concentration. [ABSTRACT FROM AUTHOR]

Ganymede is the solar system's only known moon with an intrinsic, global magnetic field. This field is strong enough to stand off the incident Jovian magnetospheric flow to form a small, complex magnetosphere around the satellite. Ganymede's magnetosphere is thought to be responsible for variable surface weathering patterns, the production of a neutral exosphere, and the generation of UV aurorae near Ganymede's open‐closed field line boundaries; however, the exact details and underlying mechanisms are poorly understood. We use results from three‐dimensional hybrid models of Ganymede's magnetosphere and a three‐dimensional particle tracing model to quantify the dynamics of thermal and energetic Jovian ions as they interact with Ganymede's magnetosphere and precipitate to the surface. We identify the formation of quasi‐trapped ionic radiation belts in the model and variable surface weathering. Most of the particle precipitation occurs in Ganymede's polar caps, yet ions also precipitate onto Ganymede's equatorial region in lesser amounts due to particle shadowing of quasi‐trapped ions in Ganymede's ionic radiation belts. Model results predict that within Jupiter's central plasma sheet, total ion fluxes to Ganymede's polar, leading, and trailing hemispheres are 50 × 106, 10 × 106, and 0.06 × 106 cm−2 · s−1, respectively. Finally, convolution of incident ions fluxes with neutral sputtering yields for icy bodies predicts neutral sputtered fluxes in Ganymede's polar, leading, and trailing hemispheres of 1.3 × 109, 4.8 × 108, and 1.2 × 108 neutrals cm−2 · s−1, respectively. Together, we estimate that Ganymede loses 7.5 × 1026 neutral particles per second, or assuming a mean mass of 18 amu, approximately 23 kg/s, half that estimated for Europa. Key Points: Thermal and energetic ion tracing is performed using hybrid simulations of Ganymede's magnetosphereSurface ion fluxes are concentrated in Ganymede's poles, with additional energetic ion precipitation in equatorial regionWe discuss implications for surface weathering and exospheric generation at Ganymede by ion irradiation [ABSTRACT FROM AUTHOR]

Viscously relaxed craters provide a window into the thermal history of Ganymede, a satellite with copious geologic signs of past high heat flows. Here we present measurements of relaxed craters in four regions for which suitable imaging exists: near Anshar Sulcus, Tiamat Sulcus, northern Marius Regio, and Ganymede's south pole. We describe a technique to measure apparent depth, or depth of the crater with respect to the surrounding terrain elevation. Measured relaxation states are compared with results from finite element modeling to constrain heat flow scenarios [see companion paper: Bland et al. (2017)]. The presence of numerous, substantially relaxed craters indicates high heat flows—in excess of 30–40 mW m −2 over 2 Gyr, with many small (<10 km in diameter) relaxed craters indicating even higher heat flows. Crater relaxation states are bimodal for some equatorial regions but not in the region studied near the south pole, which suggests regional variations in Ganymede's thermal history. [ABSTRACT FROM AUTHOR]

Water and hydroxyl, once thought to be found only in the primitive airless bodies that formed beyond roughly 2.5–3 AU, have recently been detected on the Moon and Vesta, which both have surfaces dominated by evolved, non-primitive compositions. In both these cases, the water/OH is thought to be exogenic, either brought in via impacts with comets or hydrated asteroids or created via solar wind interactions with silicates in the regolith or both. Such exogenic processes should also be occurring on other airless body surfaces. To test this hypothesis, we used the NASA Infrared Telescope Facility (IRTF) to measure reflectance spectra (2.0–4.1 µm) of two large near-Earth asteroids (NEAs) with compositions generally interpreted as anhydrous: 433 Eros and 1036 Ganymed. OH is detected on both of these bodies in the form of absorption features near 3 µm. The spectra contain a component of thermal emission at longer wavelengths, from which we estimate thermal inertias of 167 ± 98 J m −2 s −1/2 K −1 for Eros (consistent with previous estimates) and 214 ± 80 J m −2 s −1/2 K −1 for Ganymed, the first reported measurement of thermal inertia for this object. These observations demonstrate that processes responsible for water/OH creation on large airless bodies also act on much smaller bodies. [ABSTRACT FROM AUTHOR]

Ganymede, the largest moon in the solar system, has a fully differentiated interior with a layer of high-pressure (HP) ice between its deep ocean and silicate mantle. In this paper, we study the dynamics of this layer using a numerical model of two-phase ice–water mixture in two-dimensional Cartesian geometry. While focusing on the generation of water at the silicate/HP ice interface and its upward migration towards the ocean, we investigate the effect of bottom heat flux, the layer thickness, and the HP ice viscosity and permeability. Our results suggest that melt can be generated at the silicate/HP ice interface for small layer thickness ( ≲ 200 km) and high values of heat flux ( ≳ 20 mW m − 2 ) and viscosity ( ≳ 10 15  Pa s). Once generated, the water is transported through the layer by the upwelling plumes. Depending on the vigor of convection, it stays liquid or it may freeze before melting again as the plume reaches the temperate (partially molten) layer at the boundary with the ocean. The thickness of this layer as well as the amount of melt that is extracted from it is controlled by the permeability of the HP ice. This process constitutes a means of transporting volatiles and salts that might have dissolved into the melt present at the silicate/HP ice interface. As the moon cools down, the HP ice layer becomes less permeable because the heat flux from the silicates decreases and the HP ice layer thickens. [ABSTRACT FROM AUTHOR]

The advanced Russian project Laplace-P is aimed at developing and launching two scientific spacecraft (SC)--Laplace-P1 (LP1 SC) and Laplace-P2 (LP2 SC)--designed for remote and in-situ studies of the system of Jupiter and its moon Ganymede. The LP1 and LP2 spacecraft carry an orbiter and a lander onboard, respectively. One of the orbiter's objectives is to map the surface of Ganymede from the artificial satellite's orbit and to acquire the data for the landing site selection. The main objective of the lander is to carry out in-situ investigations of Ganymede's surface. The paper describes the scientific goals and objectives of the mission, its special features, and the LP1 and LP2 mission profiles during all of the phases--from the launch to the landing on the surface of Ganymede. [ABSTRACT FROM AUTHOR]

Far-ultraviolet observations of Ganymede's atmospheric emissions were obtained with the Space Telescope Imaging Spectrograph (STIS) onboard of the Hubble Space Telescope (HST) on several occasions between 1998 and 2014. We analyze the Lyman- α emission from four HST campaigns in order to constrain the abundance and variation of atomic hydrogen in Ganymede's atmosphere. We apply a forward model that estimates surface reflection and resonant scattering in an escaping corona of the solar Lyman- α flux, taking into account the effects of the hydrogen in the interplanetary medium. The atmospheric emissions around Ganymede's disk derived for the observations taken between 1998 and 2011 are consistent with a hydrogen corona in the density range of (5–8) × 10 3 cm −3 at the surface. The hydrogen density appears to be generally stable in that period. In 2014, Ganymede's corona brightness is approximately 3 times lower during two observations of Ganymede's trailing hemisphere and hardly detectable at all during two observations of the leading hemisphere. We also investigate extinction of Ganymede's coronal emissions in the Earth's upper atmosphere or geocorona. For small Doppler shifts, resonant scattering in the geocorona of the moon corona emissions can effectively reduce the brightness observed by HST. In the case of the 2014 leading hemisphere observations, an estimated extinction of 80% might explain the non-detection of Ganymede's hydrogen corona. Geocoronal extinction might also explain a previously detected hemispheric difference from Callisto's hydrogen corona. [ABSTRACT FROM AUTHOR]

The Jupiter Icy Moons Explorer (JUICE) mission will perform detailed measurements of the properties of the Galilean moons, with a nominal in-system science-mission duration of about 3.5 years. Using both the radio tracking data, and (Earth- and JUICE-based) optical astrometry, the dynamics of the Galilean moons will be measured to unprecedented accuracy. This will provide crucial input to the determination of the ephemerides and physical properties of the system, most notably the dissipation in Io and Jupiter. The data from Planetary Radio Interferometry and Doppler Experiment (PRIDE) will provide the lateral position of the spacecraft in the International Celestial Reference Frame (ICRF). In this article, we analyze the relative quantitative influence of the JUICE-PRIDE observables to the determination of the ephemerides of the Jovian system and the associated physical parameters. We perform a covariance analysis for a broad range of mission and system characteristics. We analyze the influence of VLBI data quality, observation planning, as well as the influence of JUICE orbit determination quality. This provides key input for the further development of the PRIDE observational planning and ground segment development. Our analysis indicates that the VLBI data are especially important for constraining the dynamics of Ganymede and Callisto perpendicular to their orbital planes. Also, the use of the VLBI data makes the uncertainty in the ephemerides less dependent on the error in the orbit determination of the JUICE spacecraft itself. Furthermore, we find that optical astrometry data of especially Io using the JANUS instrument will be crucial for stabilizing the solution of the normal equations. Knowledge of the dissipation in the Jupiter system cannot be improved using satellite dynamics obtained from JUICE data alone, the uncertainty in Io's dissipation obtained from our simulations is similar to the present level of uncertainty. [ABSTRACT FROM AUTHOR]

We used 126 Voyager-1 and -2 as well as 87 Galileo images of Ganymede and searched for stereo images suitable for digital 3D stereo analysis. Specifically, we consider image resolutions, stereo angles, as well as matching illumination conditions of respective stereo pairs. Lists of regions and local areas with stereo coverage are compiled. We present anaglyphs and we selected areas, not previously discussed, for which we constructed Digital Elevation Models and associated visualizations. The terrain characteristics in the models are in agreement with our previous notion of Ganymede morphology, represented by families of lineaments and craters of various sizes and degradation stages. The identified areas of stereo coverage may serve as important reference targets for the Ganymede Laser Altimeter (GALA) experiment on the future JUICE (Jupiter Icy Moons Explorer) mission. [ABSTRACT FROM AUTHOR]

As the youngest features on Ganymede, ray craters are useful in revealing the sources of recent impactors and surface modification processes on the satellite. We examine craters with D > 10 km on Ganymede from images obtained by the Voyager and Galileo spacecraft to identify ray craters and study their spatial distributions. Furthermore, we carefully select images of appropriate solar and emission angles to obtain unbiased ray crater densities. As a result, we find that the density of large ray craters (D > 25 km) on the bright terrain exhibits an apex-antapex asymmetry, and its degree of asymmetry is much lower than the theoretical estimation for ecliptic comets. For large craters (D > 25 km), ecliptic comets ought to be less important than previously assumed, and a possible explanation is that nearly isotropic comets may play a more important role on Ganymede than previously thought. We also find that small ray craters (10 km < D < 25 km) on the bright terrain and ray craters (D > 10 km) on the dark terrain show no apex-antapex asymmetry. We interpret that the distribution difference between the terrain types comes from preferential thermal sublimation on the dark terrain, while the distribution difference between large and small ray craters suggests that rays of small craters are more readily erased by some surface modification processes, such as micrometeorite gardening. [ABSTRACT FROM AUTHOR]

Ganymede's atmosphere is produced by radiative interactions with its surface, sourced by the Sun and the Jovian plasma. The sputtered and thermally desorbed molecules are tracked in our Exospheric Global Model (EGM), a 3-D parallelized collisional model. This program was developed to reconstruct the formation of the upper atmosphere/exosphere of planetary bodies interacting with solar photon flux and magnetospheric and/or the solar wind plasmas. Here, we describe the spatial distribution of the H 2 O and O 2 components of Ganymede's atmosphere, and their temporal variability along Ganymede's rotation around Jupiter. In particular, we show that Ganymede's O 2 atmosphere is characterized by time scales of the order of Ganymede's rotational period with Jupiter's gravity being a significant driver of the spatial distribution of the heaviest exospheric components. Both the sourcing and the Jovian gravity are needed to explain some of the characteristics of the observed aurora emissions. As an example, the O 2 exosphere should peak at the equator with systematic maximum at the dusk equator terminator. The sputtering rate of the H 2 O exosphere should be maximum on the leading hemisphere because of the shape of the open/close field lines boundary and displays some significant variability with longitude. [ABSTRACT FROM AUTHOR]

The formation of Ganymede's sulci is likely related to extensional tectonics that affected this largest icy satellite of Jupiter. Through geometric and structural analyses we reconstructed the pre-deformed terrains and we recognized two different modes of extension associated with sulci. In the first mode, smooth sulci constitute spreading centers between two dark terrain plates, similar to the fast oceanic spreading centers on Earth. Here extension is primarily accommodated by crustal accretion of newly formed icy crust. In the second mode, dark terrain extension is mainly accommodated by swaths of normal fault systems analogous to Earth's continental crustal rifts. A comparison with terrestrial extensional analogues, based on the fault displacement/length (D max /L) ratio, spacing and morphology, showed that magmato-tectonic spreading centers and continental crustal rifts on Earth follow the same relative patterns observed on Ganymede. Our results suggest that the amount of extensional strain may have previously been underestimated since the occurrence of spreading centers may have played a major role in the tectonic evolution of the globally expanded Ganymede. We also discuss a possible model for the origin of the different modes of extension in the context of the global expansion of the satellite. [ABSTRACT FROM AUTHOR]

In this contribution, an efficient technique to design direct (i.e., without intermediate flybys) low-energy trajectories in multi-moon systems is presented. The method relies on analytical two-body approximations of trajectories originating from the stable and unstable invariant manifolds of two coupled circular restricted three-body problems. We provide a means to perform very fast and accurate computations of the minimum-cost trajectories between two moons. Eventually, we validate the methodology by comparison with numerical integrations in the three-body problem. Motivated by the growing interest in the robotic exploration of the Jovian system, which has given rise to numerous studies and mission proposals, we apply the method to the design of minimum-cost low-energy direct trajectories between Galilean moons, and the case study is that of Ganymede and Europa. [ABSTRACT FROM AUTHOR]

Recent satellite inferences of hydrous sulfates as recurrent minerals on the surface of icy planetary bodies link with the potential mineral composition of their interior. Blödite, a mixed Mg-Na sulfate, is here taken as representative mineral of icy satellites surface to investigate its crystal structure and stability at conditions of the interior of icy bodies. To this aim we performed in situ synchrotron angle-dispersive X-ray powder diffraction experiments on natural blödite at pressures up to ∼10.4 GPa and temperatures from ∼118.8 K to ∼490.0 K using diamond anvil cell technique to investigate the compression behavior and establish a low-to-high temperature equation of state that can be used as reference when modeling the interior of sulfate-rich icy satellites such as Ganymede. The experimentally determined volume expansivity, α, varies from 7.6 (7) 10 −5 K −1 at 0.0001 GPa (from 118.8 to 413.15 K) to 2.6 (3) 10 −5 K −1 at 10 GPa (from 313.0 to 453.0 K) with a δα/δ P coefficient = −5.6(9)10 −6 GPa −1 K −1 . The bulk modulus calculated from the least squares fitting of P-V data on the isotherm at 413 K using a second-order Birch - Murnaghan equation of state is 38(5) GPa, which gives the value of δK/δ T equal to 0.01(5) GPa K −1 . The thermo-baric behavior of blödite appears strongly anisotropic with c lattice parameter being more deformed with respect to a and b . Thermogravimetric analyses performed at ambient pressure showed three endotherms at 413 K, 533 K and 973 K with weight losses of approximately 11%, 11% and 43% caused by partial dehydration, full dehydration and sulfate decomposition respectively. Interestingly, no clear evidence of dehydration was observed up to ∼453 K and ∼10.4 GPa, suggesting that pressure acts to stabilize the crystalline structure of blödite. The data collected allow to write the following equation of state, V ( P, T ) = V 0 [1 + 7.6(7)10 − 5 Δ T − 0.026(3) P − 5.6(9)10 − 6 P Δ T −6.6(9)10 − 6 P Δ T )] from which the density of blödite can be determined at conditions of the mantle of the large icy satellites of Jupiter. Blödite has higher density, bulk modulus and thermal stability than similar hydrous sulfates (e.g. mirabilite and epsomite) implying, therefore, a different contribution of these minerals to the extent of deep oceans in icy planets and their distribution over the local geotherms. [ABSTRACT FROM AUTHOR]

Diffuse auroral intensities of neutral atomic oxygen OIλ1356Å emission on Ganymede due to whistler mode waves are estimated. Pitch angle diffusion of magnetospheric electrons into the loss cone due to resonant wave-particle interaction of whistler mode waves is considered, and the resulting electron precipitation flux is calculated. The analytical yield spectrum approach is used for determining the energy deposition of electrons precipitating into the atmosphere of Ganymede. It is found that the intensities (4-30R) calculated from the precipitation of magnetospheric electrons observed near Ganymede are inadequate to account for the observational intensities (≤100R). This is in agreement with the conclusions reached in previous works. Some acceleration mechanism is required to energize the magnetospheric electrons. In the present work we consider the heating and acceleration of magnetospheric electrons by electrostatic waves. Two particle distribution functions (Maxwellian and kappa distribution) are used to simulate heating and acceleration of electrons. Precipitation of a Maxwellian distribution of electrons can produce about 70R intensities of OIλ1356Å emission for electron temperature of 150 eV. A kappa distribution can also yield a diffuse auroral intensity of similarmagnitude for a characteristic energy of about 100 eV. The maximum contribution to the estimated intensity results from the dissociative excitation of O2. Contributions from the direct excitation of atomic oxygen and cascading in atomic oxygen are estimated to be only about 1 and 2% of the total calculated intensity, respectively. The findings of this work are relevant for the present JUNO and future JUICE missions to Jupiter. These missions will provide new data on electron densities, electron temperature and whistler mode wave amplitudes in the magnetosphere of Jupiter near Ganymede. [ABSTRACT FROM AUTHOR]

Recent work has raised the potential for Jupiter's decametric radiation to be used as a source for passive radio sounding of its icy moons. Two radar sounding instruments, the Radar for Icy Moon Exploration (RIME) and the Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) have been selected for ESA and NASA missions to Ganymede and Europa. Here, we revisit the projected performance of the passive sounding concept and assess the potential for its implementation as an additional mode for RIME and REASON. We find that the Signal to Noise Ratio (SNR) of passive sounding can approach or exceed that of active sounding in a noisy sub-Jovian environment, but that active sounding achieves a greater SNR in the presence of quiescent noise and outperforms passive sounding in terms of clutter. We also compare the performance of passive sounding at the 9 MHz HF center frequency of RIME and REASON to other frequencies within the Jovian decametric band. We conclude that the addition of a passive sounding mode on RIME or REASON stands to enhance their science return by enabling sub-Jovian HF sounding in the presence of decametric noise, but that there is not a compelling case for implementation at a different frequency. [ABSTRACT FROM AUTHOR]

Jupiter's satellites are subject to strong tidal forces which result in variations of the gravitational potential and deformations of the satellites' surfaces on the diurnal tidal cycle. Such variations are described by the Love numbers $$k_2$$ and $$h_2$$ for the tide-induced potential variation due to internal mass redistribution and the radial surface displacement, respectively. The phase-lags $$ \phi _{k_2}$$ and $$ \phi _{h_2}$$ of these complex numbers contain information about the rheological and dissipative states of the satellites. Starting from interior structure models and assuming a Maxwell rheology to compute the tidal deformation, we calculate the phase-lags in application to Ganymede and Europa. For both satellites we assume a decoupling of the outer ice-shell from the deep interior by a liquid subsurface water ocean. We show that, in this case, the phase-lag difference $$\varDelta \phi = \phi _{k_2}- \phi _{h_2}$$ can provide information on the rheological and thermal state of the deep interiors if the viscosities of the deeper layers are small. In case of Ganymede, phase-lag differences can reach values of a few degrees for high-pressure ice viscosities $${<}10^{14}$$ Pa s and would indicate a highly dissipative state of the deep interior. In this case $$\varDelta \phi $$ is dominated by dissipation in the high-pressure ice layer rather than dissipation within the ice-I shell. These phase lags would be detectable from spacecraft in orbit around the satellite. For Europa $$\varDelta \phi $$ could reach values exceeding $$20^\circ $$ and phase-lag measurements could help distinguish between (1) a hot dissipative silicate mantle which would in thermal equilibrium correspond to a very thin outer ice-I shell and (2) a cold deep interior implying that dissipation would mainly occur in a thick (several tens of km) outer ice-I shell. These measurements are highly relevant for ESA's Jupiter Icy Moons Explorer (JUICE) and NASA's Europa Multiple Flyby Mission, both targeted for the Jupiter system. [ABSTRACT FROM AUTHOR]

Following an approach based on the Milankovitch elements, equations able to describe the long-term evolution of the trajectory of a probe orbiting a celestial body (primary body) have been analytically determined, taking into account the perturbative effects deriving from the planetary oblateness and from one or more perturbing bodies lying on the equatorial plane of the primary body. Using these equations, three families of orbits, characterized by constant orbit elements on average, have been obtained. Whereas the first family, composed of elliptical polar orbits, ensures frozen conditions on ail the orbit elements, the other two lead to null variations of semimajor axis, eccentricity, inclination, and argument of pericenter. For the latter two families, the concept of critical inclination, which, in the case of only planetary oblateness, assumes the well-known values 63.43 and 116.57 deg, has been generalized adding the effect of several attracting bodies. Interesting solutions have been found for the observation of Jupiter's moons Ganymede and Callisto and for Saturn's moon Titan. [ABSTRACT FROM AUTHOR]

The new technique for generation of coordinate control point networks based on photogrammetric processing of heterogeneous planetary images (obtained at different time, scale, with different illumination or oblique view) is developed. The technique is verified with the example for processing the heterogeneous information obtained by remote sensing of Ganymede by the spacecraft Voyager-1, -2 and Galileo. Using this technique the first 3D control point network for Ganymede is formed: the error of the altitude coordinates obtained as a result of adjustment is less than 5 km. The new control point network makes it possible to obtain basic geodesic parameters of the body (axes size) and to estimate forced librations. On the basis of the control point network, digital terrain models (DTMs) with different resolutions are generated and used for mapping the surface of Ganymede with different levels of detail (Zubarev et al., 2015b). [ABSTRACT FROM AUTHOR]

The paper discusses the formation and dynamics of the rarefied gas envelope near the icy surface of Jupiter's moon Ganymede. Being the most massive icy moon, Ganymede can form a rarefied exosphere with a relatively dense near-surface layer. The main parent component of the gas shell is water vapor, which enters the atmosphere due to thermal degassing, nonthermal radiolysis, and other active processes and phenomena on the moon's icy surface. A numerical kinetic simulation is performed to investigate, at the molecular level, the formation, chemical evolution, and dynamics of the mainly HO- and O-dominant rarefied gas envelopes. The ionization processes in these rarefied gas envelopes are due to exposure to ultraviolet radiation from the Sun and the magnetospheric plasma. The chemical diversity of the icy moon's gas envelope is attributed to the primary action of ultraviolet solar photons and plasma electrons on the rarefied gas in the HO- or O-dominant atmosphere. The model is used to calculate the formation and development of the chemical diversity in the relatively dense near-surface envelope of Ganymede, where an important contribution comes from collisions between parent molecules and the products of their photolysis and radiolysis. [ABSTRACT FROM AUTHOR]

The co-rotating plasma around Jupiter precipitates on the surfaces of the jovian moons, where it is not hindered by a local magnetic field. Precipitating ions lead to the emission of energetic neutral atoms, which are produced via backscattering and sputtering processes, from the surface. The European Space Agency’s JUICE mission to Jupiter carries as part of the Particle Environment Package experiment an imaging energetic neutral atom spectrometer called the jovian Neutrals Analyzer (JNA). When it is in orbit around Ganymede, JNA will measure the energetic neutral atom flux emitted from the surface of Ganymede in the energy range from 10 eV to 3300 eV. The surface of Ganymede consists of a large fraction of water ice. To characterize the expected energetic neutral atom fluxes from water ice due to precipitating jovian plasma, we impacted protons and singly charged oxygen ions with energies up to 33 keV on a salty water ice target kept at Ganymede surface conditions. Emitted energetic atoms were measured energy- and mass-resolved using the JNA prototype instrument. The data show high yields for energetic neutral atoms per incident ion in the JNA energy range. For incident protons, energetic neutral atom yields between 0.28 at 1 keV and ∼40 at 33 keV were observed. For incident singly charged oxygen ions, the observed energetic neutral atom yield ranged from 0.8 for at 3 keV to ∼170 at 23 keV. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The paper presents the results of observations of Jupiter and its Galilean moons, performed with the Normal Astrograph ( D/ F=0.33 m/3.5 m, CCD S2C, FOV 18' × 16') at Pulkovo observatory during 2009-2011. We obtained 140 positions of the Galilean satellites and 42 calculated positions of Jupiter in the system of UCAC4 (ICRS, J2000.0) catalogue. The equatorial coordinates of satellites have been compared to eight most current theories of planetary and satellite motion. The average residuals between the observed and calculated coordinates (O-C) using the selected theories of motion do not exceed 0.08″. The behavior and magnitudes of the (O-C) residuals in the positions of Ganymede and Callisto satellites show that the theory of their motion was elaborated worse than in the case of Io and Europa satellites. Distribution of the (O-C) residuals for Callisto differs from a normal distribution. Comparison of the calculated equatorial coordinates of Jupiter, based on the observed positions of satellites and their theoretical Jovi-centric coordinates calculated using the INPOP10 theory of planetary motion revealed satisfactory results. The average residuals are, respectively, (O-C) = 0.040″ and (O-C) =-0.053″ assuming the normal distribution law. [ABSTRACT FROM AUTHOR]

The aurora on Ganymede results from the interaction of Jovian plasma disk electrons with Ganymede's tenuous atmosphere. At the same time, the electrons surrounding Ganymede have energy insufficient for the excitation of emission in the observed lines. They can acquire the necessary acceleration through various processes, such as reconnection of Ganymede's intrinsic magnetic field with the Jovian magnetic field in the upstream and downstream regions. The generation mechanism of a field-aligned potential drop on auroral field lines is considered. The lack of charge carriers required for the generation of the necessary current closing the Alfvén-wing field-aligned currents is shown to be responsible for the formation of an accelerating potential and the acceleration of thermal magnetospheric electrons. The total current in the circuit is determined by the calculated cross-field potential drop applied to Ganymede and by the Pedersen conductivity of Ganymede's ionosphere. [ABSTRACT FROM AUTHOR]

We computed a 3D control point network for Ganymede using combinations of 126 Voyager-1 and -2 and 87 Galileo images, benefiting from reconstructed trajectory data for the three spacecraft and a more complete Galileo image data base than was available for earlier studies. Using more than 3000 control point coordinates, we determine global shape parameters, including mean radius, spheroid- and ellipsoidal axes, and make tests for various equilibrium shape models, constrained by the most recent estimates for gravity field parameters. We confirm that Ganymede has a pronounced ellipsoidal shape, approximately aligned with the Jupiter-direction, in agreement with Ganymede being in tidal equilibrium. The point heights, suffering from large individual errors, do not reveal any large-scale topography below our typical error levels (97% <5 km). By analysis of data residuals we search for, but cannot detect Ganymede longitudinal forced librations. We conclude that libration amplitudes cannot be larger than 0.1° (corresponding to a lateral displacement of 4.6 km at the equator). [ABSTRACT FROM AUTHOR]

Invaluable information about the interior of icy satellites orbiting close to the giant planets can be gained by monitoring the response of the satellite's surfaces to external tidal forces. Due to its geodetic accuracy, laser altimetry is the method of choice to measure time-dependent radial surface displacements from orbit. We present an instrument performance model with special focus on the capabilities to determine the corresponding tidal Love number h 2 and apply the model to the Ganymede Laser Altimeter (GALA) on board of the Jupiter Icy Moons Explorer (JUICE). Based on the instrument and spacecraft performance, we derive the range error and the measurement capabilities of the GALA instrument to determine the amplitude of the tide induced radial displacement of Ganymede's surface using the cross-over technique. We find that h 2 of Ganymede can be determined with an accuracy of better than 2% by using data acquired during the nominal mission. Furthermore, we show that this accuracy is sufficient to confirm the presence of a putative subsurface water ocean and, additionally, to constrain the thickness of the overlaying ice shell to ± 20km. [ABSTRACT FROM AUTHOR]

The JUpiter ICy moons Explorer mission of the European Space Agency, that will be launched in the year 2022, will study the geodesy and physical characteristics of Jupiter and its icy moons Europa, Callisto and Ganymede. The final phase of this mission, planned for the year 2032, is still under investigation and foresees phases in circular orbit around Ganymede. In this paper two types of science orbits around Ganymede, that optimize the observational performances and the probe life-time, are proposed. While the first type allows, at “zero cost”, the obtaining of repeated observations of a given point of the surface, the second type, with a limited cost, adds to the periodicity of observation, the condition of synchronism with the Sun. This synchronism permits both the achievement of a condition of permanent sunlight for the probe, thus leading to an extension of the mission duration, and a minimization of the variations of spectral signature due to the Sun's position. [ABSTRACT FROM AUTHOR]

The Galileo Dust Detection System (DDS) detected a population of micron-sized grains in and amongst the orbits of Io, Europa, Ganymede and Callisto. Previous studies, using roughly 50% of the data now available, concluded that the dominant sources for the impacts were magnetospherically captured interplanetary particles largely on retrograde orbits ( Colwell et al., 1998b; Thiessenhusen et al., 2000 ) and impact-generated ejecta from the Galilean satellites ( Krüger et al., 1999b; Krivov et al., 2002a ). Here we revisit the problem with the full data set and broaden our consideration to include four additional source populations: debris from the outer satellites, interplanetary and interstellar grains and particles accelerated outwards from Io and the jovian rings. We develop a model of detectable orbits at each Galileo position and we find that about 10% of the impact data require non-circular orbits with eccentricities greater than 0.1. In addition, ~3% of impacts require orbital solutions with eccentricities in excess of 0.7. Using the spatial distribution of particles, we are able to exclude, as dominant sources, all the additional source populations except for outer satellite particles. A study of DDS directional information demonstrates that none of the six standard sources fit the data well and thus a combination of sources is necessary. There are insufficient data to uniquely identify the relative strengths of the various contributions. However, we find an excess of large particles that is consistent with retrograde trajectories. [ABSTRACT FROM AUTHOR]