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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]

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]

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 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]

Abstract: The large icy moons of Jupiter contain vast quantities of liquid water, a key ingredient for life. Ganymede and Callisto are weaker candidates for habitability than Europa, in part because of the model-based assumption that high-pressure ice layers cover their seafloors and prevent significant water–rock interaction. Water–rock interactions may occur, however, if heating at the rock–ice interface melts the high pressure ice. Highly saline fluids would be gravitationally stable, and might accumulate under the ice due to upward migration, refreezing, and fractionation of salt from less concentrated liquids. To assess the influence of salinity on Ganymede׳s internal structure, we use available phase-equilibrium data to calculate activity coefficients and predict the freezing of water ice in the presence of aqueous magnesium sulfate. We couple this new equation of state with thermal profiles in Ganymede׳s interior—employing recently published thermodynamic data for the aqueous phase—to estimate the thicknesses of layers of ice I, III, V, and VI. We compute core and silicate mantle radii consistent with available constraints on Ganymede׳s mass and gravitational moment of inertia. Mantle radii range from 800 to 900km for the values of salt and heat flux considered here (4–44mWm−2 and 0 to 10wt% MgSO4). Ocean concentrations with salinity higher than 10wt% have little high pressure ice. Even in a Ganymede ocean that is mostly liquid, achieving such high ocean salinity is permissible for the range of likely S/Si ratios. However, elevated salinity requires a smaller silicate mantle radius to satisfy mass and moment-of-inertia constraints, so ice VI is always present in Ganymede׳s ocean. For lower values of heat flux, oceans with salinity as low as 3wt% can co-exist with ice III. Available experimental data indicate that ice phases III and VI become buoyant for salinity higher than 5wt% and 10wt%, respectively. Similar behavior probably occurs for ice V at salinities higher than 10wt%. Flotation can occur over tens of kilometers of depth, indicating the possibility for upward ‘snow’ or other exotic modes of heat and material transport. [Copyright &y& Elsevier]

Abstract: The present study is aimed at resolving the issue of pitch-angle diffusion by whistler mode waves near Ganymede and to understand the interaction of Ganymede with magnetosphere of Jupiter. A new electron distribution function, which is a combination of Maxwellian and kappa loss-cone distribution, is used to calculate whistler mode wave growth rates. The distribution function is fitted to observed distribution near Ganymede. Wave growth rates have been used to obtain bounce-averaged pitch-angle diffusion coefficients along the field line connecting Ganymede and Jupiter. Variations of electron density and magnetic field along the particle bounce trajectory and a non dipolar field have been taken into account. Pitch-angle variation of diffusion coefficients is parameterized in a simple form . D o is diffusion coefficient (independent of pitch angle). The calculated energy dependence of diffusion coefficients (D o ) is in good agreement with observed energy dependence of coefficient D o . Wave magnetic field ~16pT is required to match observed and calculated diffusion coefficients. The exponent p takes values from ~1 to 4 for electron energies 15keV to 1MeV, respectively. It is found that contribution of Ganymede interaction region to pitch-angle scattering is quite small (~4% to 20%). Further analysis of Galileo energetic particle detector (EPD) and plasma wave data or new particle and wave data near Ganymede from JUNO mission to Jupiter are necessary to confirm the findings of present work. [Copyright &y& Elsevier]

Abstract: Exploration of subsurface oceans on Jupiter׳s icy moons is a key issue of the icy moons׳ geology. Radar is in fact the only sounding technique which is able to penetrate their icy mantles, which can be many kilometers thick. Surface clutter, i.e. scattering of the radio waves on the rough surface, is known to be one of the most important problems of subsurface radar probing. Adequate numerical modeling of this scattering is required on all stages of subsurface radar experiment, including design of an instrument, operational strategy planning and data interpretation. In the present paper, a computer simulation technique for numerical simulations of radar sounding of rough surfaces is formulated in general form. Subsurface radar location of the ocean beneath Ganymedian ice with chirp radar signals has been simulated. [Copyright &y& Elsevier]

Abstract: The detection of Energetic Neutral Particles (ENP) above 10eV can unequivocally relate a surface-bound exosphere to surface features and can monitor instantaneously the effect of plasma precipitation onto the surface. In the framework of a mission to Jupiter's moons, 2D imaging of plasma precipitation will provide important information on the plasma circulation at the orbits of the moons. Furthermore, a joint measurement of precipitating ions will permit an estimation of the efficiency of the release process. Coupled measurements of ENP and gas composition will improve our knowledge of surface release mechanisms. Ganymede's and Europa's Neutral Imaging Experiment (GENIE) is a high-angular-resolution detector, based on the ToF (Time of Flight) technique, that can detect ENP (energy range >10eV–few keV) in the Jupiter environment thanks to an innovative design and technology. Its objective is to map the sites of origin of the ENP of the icy moons’ exospheres to investigate the interaction between the surface and the environment. Finally, coupling GENIE with an ion sensor and a mass spectrometer will be an outstanding opportunity to better understand the magnetosphere–moon coupling within the Jupiter system and compare the surface interaction with plasma in the diverse moons. In this paper, the scientific objectives and requirements of ENP detection are summarized and the description of the innovative design concept of GENIE is given, together with the signal and background noise simulation. [Copyright &y& Elsevier]

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

Abstract: Radar sounding of Jovian icy satellites has great potential to address specific science questions such as the presence of subsurface liquid water. Radargrams acquired over Mars polar caps allow observing clear echoes up to kilometers depth. However, Jovian icy satellites display dramatically different surface topographies. In order to assess possible issues arising from such surface topographies on radar sounding, we performed a study on different DEMs (Digital Elevation Models) obtained on Ganymede. Topographic data are derived using stereo and photoclinometric analysis of Galileo and Voyager images at resolutions of 16–629m. Main results are presented in this paper. Overall we found that Ganymede''s surface is quite rough, with mean slopes at 630m scale varying from 3.5° to 8°, smoothest terrains being found within sulcii. This will be a major challenge for the design of radar sounders and parameters should be chosen accordingly in order to correctly sound this planetary body. Previous studies have shown similar concern for Europa. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: Neutron spectroscopy has the potential to address many of the scientific objectives of the Jupiter Ganymede Orbiter mission. The most significant challenge is understanding the response of a neutron spectrometer to the intense radiation environment around Ganymede. This study uses data from the Lunar Prospector Neutron Spectrometer to benchmark simulations of the performance of a similar instrument in Ganymede orbit. A solar particle event observed by the Lunar Prospector Neutron Spectrometer serves as a surrogate for the charged particle environment around Ganymede, facilitating a study of the instrument response using data acquired in an environment very similar to the radiation environment found at Ganymede. Based on the conclusions of this study, modifications to a Lunar Prospector-style neutron spectrometer are suggested to allow for the operation of such an instrument in Ganymede orbit. Simulations of the expected signal, based on current models of the surface composition, show this instrument would be capable of compositionally characterizing the nature and extent of surface ice deposits. [Copyright &y& Elsevier]

Abstract: Crater''s density distribution versus satellitographical longitude was searched for seven icy satellites: two of Jupiter (Ganymede and Callisto) and five of Saturn (Mimas, Tethys, Dione, Rhea and Iapetus). Craters were classified according to their size. Four bins of the craters'' diameter were used. Density distributions were found in the longitudinal sectors of the near-equatorial stripes that circumscribe the satellites. The size distributions (R-plots) were done independently for each of the eight longitudinal sectors of the satellites. Searching for the leading/trailing (apex/antapex) and the near-side/far-side asymmetry was done. It was found that the crater density is longitudinally asymmetric for all seven satellites being studied. However, the apex–antapex asymmetry is much less pronounced than predicted by theory of , for impacts on the satellites by ecliptic comets. We conclude that the impact craters observed on the considered satellites are mostly originated from planetocentric swarm of debris. In that case longitudinal asymmetry is not expected, as stated by . However, cratering longitudinal asymmetry that we observe for Mimas perfectly agrees with calculations of . It is very likely that important part of craters on Mimas were formed due to impacts of ejecta originated from crater Herschel. [Copyright &y& Elsevier]

Abstract: We present modeled images of Io at a variety of distances from the surface as a function of imager aperture size and wavelength. We consider the science objectives that could be achieved from missions engaged in long range remote-sensing of Io during the approach to the Jovian system and subsequently from orbit around Europa or Ganymede, in both the visible and near infrared wavelength ranges. We find that basic global mapping objectives in the visible can be met with a traditional 0.5m telescope design. A more ambitious 1.5m telescope could accomplish much more detailed objectives such as topographical measurements, and determination of flow patterns and thermal sources for individual active regions on Io. [ABSTRACT FROM AUTHOR]

We describe an evolutionary turbulent one-dimensional model of the Jovian subnebula, based on the previous models of the solar nebula of Dubrulle (Icarus 106 (1993) 59), and of Drouart et al. (Icarus 140 (1999) 129), as well as on the evolutionary turbulent model of the subnebula of Saturn of Mousis et al. (Icarus 156 (2002a) 162). We show that the conversion of N2 to NH3 and that of CO to CH4 were inhibited in the Jovian subnebula, in conflict with the conclusions of Prinn and Fegley (Astrophys. J. 249 (1981) 308). We argue that grains from which ultimately formed Galilean satellites were initially produced in the cooling feeding zone of Jupiter prior to the formation of the subdisk surrounding the giant planet. It is assumed that hydrates of NH3 and clathrate hydrates of CO, CH4, and N2 formed in the feeding zone (Astrophys. J. Lett. 550 (2001a) L227; Astrophys. J. Lett. 559 (2001b) L183) were incorporated in planetesimals embedded in the cold outer part of the Jovian subnebula. Under the assumption that planetesimals which formed Ganymede and Callisto migrated from the outer region and did not outgas during this migration, the per mass abundances of NH3, N2, CO, and CH4 with respect to H2O in the interiors of these satellites are estimated. Calculated values depend upon the poorly known relative abundances of these species in the solar nebula. However, they provide an interpretation of the presence of NH3 suspected in subsurface oceans of Ganymede and Callisto, and which is consistent with the measurement of the internal magnetic field of these satellites measured by the Galileo mission (Geophys. Res. Lett. 24 (1997) 2155; J. Geophys. Res. 104 (1999) 4609). [Copyright &y& Elsevier]