Ganymede’s and Europa’s Neutral Imaging Experiment

(GENIE) at the Jupiter’s icy moons

A. Milillo, and the GENIE Team

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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.

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Ion impact onto an icy surface

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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.

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Comparative observationsSimilar 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 regionsOn 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

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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.

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Surface released velocity spectra @ Europa

(Plainaki et al., 2011, Icarus).

Major components of released particles are H2O (produced by direct ion sputtering),O2 and H2 (produced by radiolysis and sputtering).H (via ion back scattering) could be relevant at velocities above 100 km/s.

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Escape velocity: 2 km/s

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Neutral particles release @ Europa

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(Plainaki et al., Icarus, 2012).

The ENP differential flux above 10 eV from the surface can be estimated about 5·108 p.cle/(cm2 s sr eV) at Europa

O2 released by H+, O+ and S+ impact and radiolysis

H2O released by H+, O+ and S+ sputtering. Note that IS is a stochiometric process, so it releases all the species trapped into the surface.

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Leading

Trailing 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

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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

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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 release @ Ganymede

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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+ FLU

X (cm

-2 s-1 keV

-1)

Leading

Trailing

Jupiter

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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

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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.

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Synergic science goals with other JUICE payload experiments are:• To characterize the complete exospheric energy distributionwith INMS and UV spectrometer;• To characterize the surface release processwith 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

auroraewith Particle Package, magnetometer and UV imaging.

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GENIE pointingTo 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.

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•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

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High-angular resolution low-energy neutral atom detectionby means of micro-shuttering techniques

Heritage at IAPS

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SERENA/ELENA will be delivered to ESA in mid 2013

BepiColombo-MPO/SERENA-ELENA

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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.

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Example: ELENA shutter at 10 kHz

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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.

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Conclusions

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•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, 10-12 Sep. 2012