A. Milillo, and the GENIE Team

Golden Age of of Solar System Exploration Ganymede’s and Europa’s Neutral Imaging Experiment (GENIE) GENIE is a high-angular-resolution detector of Energetic Neutral Particles (energy range 10s eV – few keV) (ENP) based on the ToF technique, optimized to flown in the Jupiter’s environment. Its objective is to map the origin sites of the ENP of the icy moons’ exospheres, in order to investigate the interaction between the surface and the environment. Rome, Sep. 2012

Ion impact onto an icy surface Golden Age of of Solar System Exploration Intense ion fluxes impacting onto the icy moons’ surfaces produce neutral particle release that originates the exospheres. The observations at proper angular resolution of the higher- energy neutrals produced by the plasma-surface interaction will provide an instantaneous 2D imaging. Rome, Sep. 2012

Golden Age of of Solar System Exploration Comparative observations Similar energetic ion fluxes are expected at Europa and Ganymede, composed by a similar iced surface but with the difference of an internal magnetic field that could shield the plasma or define preferential entries and plasma precipitation regions On the contrary, the ion fluxes at Callisto are considerably lower. Similar observations in different environment like the three moons would offer the chance to investigate the satellites evolution in the Jupiter system.

Europa vs Ganymede Golden Age of of Solar System Exploration Europa is the simplest case since it has not an internal magnetic field. The plasma precipitation is just due to the interaction with the obstacle. The release variations at low spatial scale are mainly driven by different surface properties. Ganymede with its internal magnetic field has a complex interaction with the Jupiter’s magnetospheric plasma. Rome, Sep. 2012

Surface released velocity Europa (Plainaki et al., 2011, Icarus). Major components of released particles are H 2 O (produced by direct ion sputtering), O 2 and H 2 (produced by radiolysis and sputtering). H (via ion back scattering) could be relevant at velocities above 100 km/s. Golden Age of of Solar System Exploration Escape velocity: 2 km/s Rome, Sep. 2012

Neutral particles Europa Golden Age of of Solar System Exploration (Plainaki et al., Icarus, 2012). The ENP differential flux above 10 eV from the surface can be estimated about 5·10 8 p.cle/(cm 2 s sr eV) at Europa O 2 released by H +, O + and S + impact and radiolysis H 2 O released by H +, O + and S + sputtering. Note that IS is a stochiometric process, so it releases all the species trapped into the surface. Rome, Sep Leading Trailing Leading Detection of ENP at high spatial resolution from the surface will permit to investigate the global asymmetries and to relate the local surface release efficiency to surface features and purity of ice. This will be an important piece of the evolution puzzle.

Ganymede’s surface Rome, Sep. 2012Golden Age of of Solar System Exploration Ganymede is the unique discovered moon in the Solar system with a dipolar magnetic field. Very close correspondence has been demonstrated between the observed higher-albedo polar cap boundary and open/closed field lines boundary (Khurana et al., 2007).

Ganymede’s magnetosphere Rome, Sep. 2012Golden Age of of Solar System Exploration The Jupiter’s plasma overcomes the moon from the trailing side. At Ganymede, as in the Mercury’s case, the fluxes precipitate in the open-field lines areas. (Massetti courtesy, 2011) 1 keV10 keV100 keV (Jia et al. 2008)

ENP Ganymede Golden Age of of Solar System Exploration 1-keV O + simulations by Massetti in the MHD magnetic field model by Jia et al., 2008 The magnetic field permits the exosphere generation only in specific regions. Leading O + FLUX (cm -2 s -1 keV -1 ) LeadingTrailing Jupiter Rome, Sep Detection of ENP at high spatial resolution from the surface will permit to dynamically map the precipitating regions (auroral mapping) and to relate them to surface features (like different albedo). This is a kind of second vantage point observation for precipitating plasma. This will be an outstanding new way to investigate the coupling between the Jupiter’s plasma and the Ganymede’s magnetic field. H2O released by O+ sputtering simulation by Mura

Primary and synergic GENIE science goals Golden Age of of Solar System Exploration For each Galiean moon: Ganymede, Europa and Callisto, GENIE is aimed: To characterize in space and in energy the radiating component of the exospheres; To study the interactions of the moons with the Jovian magnetosphere; To discriminate and depict the exospheres generation mechanisms. Moreover, the possibility to operate GENIE at the three moons permits To compare the different environments. For each Galiean moon: Ganymede, Europa and Callisto, GENIE is aimed: To characterize in space and in energy the radiating component of the exospheres; To study the interactions of the moons with the Jovian magnetosphere; To discriminate and depict the exospheres generation mechanisms. Moreover, the possibility to operate GENIE at the three moons permits To compare the different environments. Rome, Sep Synergic science goals with other JUICE payload experiments are: To characterize the complete exospheric energy distribution with INMS and UV spectrometer; To characterize the surface release process with Particle Package and magnetometer; To investigate weathering and erosion of surface features; with Surface multi-wavelength spectroscopy; To determine how much the Ganymede escape influence the Jupiter’s aurorae with Particle Package, magnetometer and UV imaging. Synergic science goals with other JUICE payload experiments are: To characterize the complete exospheric energy distribution with INMS and UV spectrometer; To characterize the surface release process with Particle Package and magnetometer; To investigate weathering and erosion of surface features; with Surface multi-wavelength spectroscopy; To determine how much the Ganymede escape influence the Jupiter’s aurorae with Particle Package, magnetometer and UV imaging.

Golden Age of of Solar System Exploration GENIE pointing To see the surface emission, the sensor must point toward the moon surface (nadir). For instance, in order to resolve surface features of the order of 30 km, an angular resolution of about 5 degrees is required from an altitude of 350 km above the moon surface. Rome, Sep. 2012

Golden Age of of Solar System Exploration Entrance 5°x60° with ion deflector. START section: Shutter System (BC/SERENA-ELENA heritage) ToF chamber STOP section: signal detection system Anticoincidence section: background detection system. GENIE basic concept Rome, Sep. 2012

High-angular resolution low-energy neutral atom detection by means of micro-shuttering techniques Heritage at IAPS Golden Age of of Solar System Exploration SERENA/ELENA will be delivered to ESA in mid 2013

BepiColombo-MPO/SERENA-ELENA Golden Age of of Solar System Exploration The basic sections of ELENA sensor are: a charge particle deflector for ions suppression; an entrance with a grating system which may work as: a shuttering system, based of coupled moving slits of nanometric dimension, which permits the neutrals to enter in the sensor only when the slits are aligned, defining the detection START time. UV suppressor; a ToF chamber; a STOP system based on the technique of MCP technologies. Rome, Sep. 2012

Example: ELENA shutter at 10 kHz Golden Age of of Solar System Exploration At MEFISTO facility in Bern University, the functionality of the ELENA shutter has been tested to verify the open/closed timing. 1-keV-H beam through the shuttering system detected by the MCP. Rome, Sep. 2012

Conclusions Golden Age of of Solar System Exploration Without ENP observations there is no way to univocally relate the exosphere to surface features and to monitor instantaneously the effect of plasma precipitation onto the surface. ENP investigation is the link between magnetospheric science and surface science. ENP detection could be a support for interpreting other instrument observations. This kind of measurement is new, especially for icy surfaces where sputtering is the dominant process; hence, any observation will produce a big science return. ENP observations are feasible, even if the noise issue must be carefully addressed, especially during the Europa flybys. GENIE has no strong constraints (distances or pointing requirements) for the spacecraft. The required GENIE resources are not demanding for the spacecraft. Rome, Sep. 2012