1. Europa’s exosphere generation C. Plainaki, A. Milillo, A. Mura, S. Orsini Isituto di Fisica dello Spazio Interplanetario, Rome, ITALY 2nd SERENA-HEWG Meeting, Mykonos-Greece, June-2009

2. O u t l i n e Europa’s characteristics and radiation environment Neutral particle release processes at Europa’s surface The EUropa’s Generated Exosphere (EUGE) Model - Assumptions - Simulations results Conclusions 2nd SERENA-HEWG Meeting, Mykonos-Greece, June-2009

3. Europa’s surface characteristics very young surface: between 10 6 – 10 9 years (Moore et al., 1998) covered mainly (>99%) by H 2 O ice (Clark, 2004). average density ~2.989 g/cm 3 (Anderson et al., 1998) traces of non-icy material: H 2 O 2 (0.13 % by number of molecules), SO 2 and CO 2, with hemispherical distributions (Tiscareno and Geissler, 2003; McCord et al, 1998). Io Europa GanymedeCallisto Galilean Satellites in comparison with Earth and Moon Mean Radius : 1.569d3 km (0.245 R E ) Mean Mass: 4.8d22 kg Equarorial surface gravity: 1.314 m/s 2 Surface temperature: 102 K Escape velocity: 2.025km/s Orbital period: 3.551days (average orbital speed: 13.740km/s) Inclination: 0.47 0 (to Jupiter’s equator) Europa

4. 2nd SERENA-HEWG Meeting, Mykonos-Greece, June-2009 H+H+ O+O+ S+S+ Energetic ion fluxes observed by Galileo spacecraft impacting on the surface of Europa. (Paranicas et al., GRL, 2002) Radiation environment @ Europa The photon flux per unit area per time, at the vicinity of Europa, integrated over the photon energy range > 7eV, is equal to 5.8 ∙ 10 10 cm -2 s -1. Ion fluxesPhoton fluxes Courtesy of NASA

5. What processes happen on the surface of Europa Ion Sputtering (IS) : removal of a part of atoms or molecules from a solid surface, due to the interaction of a projectile ion with target electrons and nuclei, as well as secondary cascades of collisions between target atoms (Sigmund, 1981)  refractories (e.g. Si, Al, Mg) and volatiles, from 1 to > 100 eV) Ion Back-scattering and Neutralization (IBSN): scattering of the impinging ions by the molecules of the surface (determined by the Coulomb potential) and neutralization on their way out. Photon Stimulated Desorption (PSD) : desorption of neutrals or ions as a result of direct excitation of surface atoms by photons (Hurych, 1988).  volatiles, < 1 eV (e.g. H, Na, K, C etc.) Thermal Desorption (TD) : exists when the thermal energy of an atom exceeds the surface binding energy  volatiles, < 1 eV (e.g. H, Na, K, C etc.) Micrometeoroid Impact Vaporization (MIV) : caused by micrometeorites hitting the planetary surface. Schematic view of the main mechanisms acting on a surface exposed to the solar system environment (Leblanc et al., 2007). 2nd SERENA-HEWG Meeting, Mykonos-Greece, June-2009

6. The Europa’s Generated Exosphere (EUGE) Model is intended to study the generation of Europa’s exosphere as a result of its surface interaction with the Jupiter’s magnetospheric plasma. MODEL ASSUMPTIONS Uniform distribution of impinging ions (magnetic field not taken into account) Release Processes : IS, IBSN, PSD, TD, MIV. PSD takes place only in the illuminated side of the moon, whereas IS is considered uniform. Surface composition : Water ice. (1) incoming ion, (2) scattering, (3) neutralization and scattering, (4) sputtering or recoiling (5) electron emission (6) photon emission (7) adsorption (8) displacement Overview of various ion-surface interactions.

7. 2nd SERENA-HEWG Meeting, Mykonos-Greece, June-2009 Ion Sputtering at Europa Parameter name Symbol (unit) Suggested Value Europa Radius R E (km)1569 Europa Mass M E (kg)4.8 ∙10 22 Energy of the incident particle E i (keV)100 Incident flux F i (part/cm 2s) H+H+ C+C+ O+O+ S+S+ 1.5 ∙10 7 Paranicas et al., 2003 1.8 ∙10 6 Strazzulla et al., 2003 1.5 ∙10 6 Paranicas et al., 2003 9∙10 6 Paranicas et al., 2003 Mass of the incident particle m i (amu)1121632 Sputtering Yield Y (part/ion ) 6 Shi et al., 1995 10 Rocard, et al. (1986) 50 Ip et al., 1997; 1998; Johnson, 1990; Shi et al., 1995 30 Johnson, 1990 Mass of the ejected particle m e (amu) 18 Binding energy E b (eV) 0.45 (Johnson, 1998) 0.05 (Boring et al., 1984; Haring et al., 1984) Sputtering Input Parameters used in this analysis Assumptions Only H 2 O sputtered molecules are being released Sputtering Yields for for “H-like” and “O-like” ions were compiled byShi et al. (1995). Binding energy is assumed 0.45 eV (sublimation energy of H 2 O) (Johnson, 1998). For the case of S +, we perform two separate runs of our model to account also for the a much lower “effective binding energy” equal to 0.05 eV per molecule (Boring et al., 1984; Haring et al., 1984).

8. 2nd SERENA-HEWG Meeting, Mykonos-Greece, June-2009 Energy distribution of the sputtered H 2 O particles (for impinging 10 keV and 100 keV S + flux) where E e is the energy of the ejected particle, E i is the energy of the incident particle and E b is the binding energy Ion Sputtering at Europa Energy distribution function Escape energy: 0.38 eV

9. 2nd SERENA-HEWG Meeting, Mykonos-Greece, June-2009 Ion back scattering and neutralization When an ion is scattered from a target atom at an angle θ, the ratio of the scattered- ion energy E to the incident energy, E i, can be calculated using the laws of conservation of energy and momentum. Kinematic factor where M 1, M 2 are the masses of the incident ion and target atom, respectively, θ is the scattering angle, E i is the incident energy of the ion and E is the energy of the back-scattered ion. Back-scattering exists for cases where θ>90 0. Kinematic factor as a function of various target masses scattered by O-atoms (θ~150 0 ).

10. 2nd SERENA-HEWG Meeting, Mykonos-Greece, June-2009 Ion back scattering and neutralization The probability that an ion is back-scattered from a target atom can be given as a function of the cross-section of the interaction: Estimation of the IBSN probability For H + impinging ions of energy 10 keV or 100 keV, σ=~1.04·10 -20 cm 2 and σ=~1.04·10 -22 cm 2 respectively. The free path of the 10 keV H + inside the ice is ~ 400 nm, calculated as in Ziegler and Manoyan (1988). In this case P RBS = 1.4 %. The free path of the 100keV H + is~ 1.2 μm. In this case P RBS = 0.043 %. The neutral fraction of low energy back-scattered protons decreases with increasing energy. For backscattered H + energies larger than 4.5 keV per impinging H +, the neutral fraction becomes smaller than 30% (Almulhem, 2005). The total probability that neutral particles will come out after both back-scattering and neutralization processes is very low (< 0.013% for 100 keV ions and <0.42% for 10 keV ions) and consequently IBSN can be considered negligible.

11. 2nd SERENA-HEWG Meeting, Mykonos-Greece, June-2009 Thermal desorption Europa’s temperature: 86 K - 132 K (Spencer et al., 1999). The thermal energy of an ice H 2 O molecule ranges between 0.011 eV and 0.017eV. The binding energy holding the ice molecules on the surface of Europa, can be considered to be characterized by the sublimation energy of H 2 O, i.e. 0.45 eV per molecule (Johnson, 1998). Consequently the particle release via the TD mechanism can be considered negligible.

12. 2nd SERENA-HEWG Meeting, Mykonos-Greece, June-2009 Photon stimulated desorption where f PSD is the photon flux per unit area per time, integrated over the photon energy range > 7eV and equal to 5.8 ∙ 10 10 cm -2 s -1, F is the fraction of H 2 O ice on the surface, equal to 0.99, d is the surface density, equal to 1.1∙10 15 molecules/cm 2 (Dulieu et al., 2005) and σ PSD is the PSD cross- section, calculated equal to 10 -18 cm 2. Energy distribution function The neutral flux released via PSD from the icy surface of Europa can be calculated on the basis of the following equation (Wurz and Lammer, 2003):

13. 2nd SERENA-HEWG Meeting, Mykonos-Greece, June-2009 The MIV refers to the impact vaporization caused by micrometeorites hitting the surface of a planet. According to Cooper et al. (2001), the energy input from meteoroids is of several orders of magnitude less than the particle energy fluxes and as a result the meteoroid energy effects, although they may appear locally, they are not significant for the global energy influx onto any of the Galilean satellites. Micrometeoroid Impact Vaporization

14. Results of the MC simulations - IS Total sputtered particle flux Total sputtered particle density x particles m -2 -1 s 2nd SERENA-HEWG Meeting, Mykonos-Greece, June-2009 Flux (particles m -2 s -1 ) Density (particles m -3 )  The most significant sputtered-particle flux and density come from the S + impinging ions and they are equal to 67% with respect to the total ones ( 2.7·10 13 H 2 O/m 2 /s and 7.9·10 9 H 2 O/m 3, respectively). These results are in general in good agreement with estimates obtained by other researchers.  The total sputtering rate was calculated to be 1.7·10 27 H 2 O/s. The actual value of the total sputtering rate at some point on the planet’s surface may exhibit variations. Sputtered H 2 O flux (upper panels) and density (lower panels) for different types of impinging ions of energy ~100 keV Plainaki et al., EGU2009 H+H+ C+C+ O+O+ S+S+

15. 2nd SERENA-HEWG Meeting, Mykonos-Greece, June-2009 Results of the MC simulations - IS The fraction of escaping particles (E>0.38 eV) via IS is 78%, thus meaning a total rate of 1.3·10 27 s-1. The net erosion rate of Europa surface is calculated to be 2.7 m/100Myear. Given the Europa’s surface age (50 Myears, according to Zahnle (2001)) we calculate that a H 2 O frost layer as thick as 1.37 m can have been formed over the whole surface of the planet (assuming that the erosion rate has remained constant over the past million of years and that the orbit of Europa has not been changed) ESCAPE EMISSION FROM EUROPA Flux (particles m -2 s -1 ) Plainaki et al., EGU2009 S+S+

16. 2nd SERENA-HEWG Meeting, Mykonos-Greece, June-2009 Results of the MC simulations - PSD H 2 O particles emitted via PSD from the iced surface of Europa The H 2 O PSD flux is of the same order of magnitude of the sputtered one. The H 2 O density released via PSD (1.38 ·10 9 H 2 O/m 3 on the surface of the illuminated side) is lower than that due to sputtering by a factor of 6.  The fraction of escaping particles via PSD is 0.53% thus meaning a total rate 8.34 ·10 22 s -1. Flux (particles m -2 s -1 ) Density (particles m -3 ) Plainaki et al., EGU2009

17.  The most significant sputtered-particle flux and density come from the S + impinging ions and they are equal to 67% with respect to the total ones (2.7·10 13 H 2 O /m 2 /s and 7.9·10 9 H 2 O /m 3, respectively). These results are in general in good agreement with estimates obtained by other researchers. Furthermore, due to the variation of the Jupiter plasma, the IS can contribute locally in different weights.  The total sputtering rate for Europa was calculated to be 1.7·10 27 H 2 O/s. The actual value of the total sputtering rate at some point on the planet’s surface may exhibit variations.  The H 2 O density released via PSD (1.38 ·10 9 H 2 O/m 3 on the surface of the illuminated side) is lower than that due to sputtering by a factor of 6.  On the dark side, the PSD density, due to ballistic trajectories, is 2 orders of magnitude lower than that on the illuminated side. Considering that the particles emitted in the dayside travelling to the night side have to cross a longer path inside the atmosphere, the night-side density extracted from our model is probably overestimated. Depending on the mean free path assumed (ranging from 13 km to 78 km), one can estimate that only 0.3% to 3% of the simulated particles are able to reach the night-side. Conclusions 2nd SERENA-HEWG Meeting, Mykonos-Greece, June-2009

18.  The exospheric neutral density, retrieved by the Galileo electron density measurements, seems to be higher than that calculated by the EUGE model. Therefore, some other neutral generation mechanism should be considered (sublimation?).  The contribution of IBSN to the total neutral flux emitted from the surface of Europa can be considered negligible.  The fraction of escaping particles via IS is 78%, thus meaning a total rate of 1.3·10 27 s -1, while the fraction of escaping particles via PSD is 0.53% thus meaning a total rate 8.34 ·10 22 s -1.  A suggestion for defining the active release process is to discriminate the particle energy spectra and detect the Sputtered High Energy (> 10 eV) Atoms (SHEA) and /or to have a good mass spectrometer able to detect the low component of refractories. The SHEA ratio constitutes 24.4% (for E b =0.05eV) and 48.7% (for E b =0.45eV) of the total sputtered flux. 2nd SERENA-HEWG Meeting, Mykonos-Greece, June-2009 Conclusions