Europa’s exosphere generation C. Plainaki, A. Milillo, A. Mura, S. Orsini Isituto di Fisica dello...

Europa’s exosphere generationC. Plainaki, A. Milillo, A. Mura, S.

Orsini

Isituto di Fisica dello

Spazio Interplanetario, Rome, ITALY

2nd SERENA-HEWG Meeting, Mykonos-Greece, June-2009

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



Europa’s surface characteristics• very young surface: between 106 – 109 years (Moore et al., 1998) covered mainly (>99%) by H2O ice (Clark, 2004).• average density ~2.989 g/cm3 (Anderson et al., 1998)• traces of non-icy material: H2O2 (0.13 % by number of molecules), SO2 and CO2, with hemispherical distributions (Tiscareno and Geissler, 2003; McCord et al, 1998).

Io Europa Ganymede Callisto

Galilean Satellites in comparison with Earth and Moon

Mean Radius : 1.569d3 km (0.245 RE)Mean Mass: 4.8d22 kgEquarorial surface gravity: 1.314 m/s2 Surface temperature: 102 KEscape velocity: 2.025km/sOrbital period: 3.551days (average orbital speed: 13.740km/s)Inclination: 0.470 (to Jupiter’s equator)

Europa


H+ O+ 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 ∙ 1010 cm-2s-1.

Ion fluxes Photon fluxes

Courtesy of NASA

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).



The Europa’s Generated Exosphere (EUGE) Modelis 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.


Ion Sputtering at Europa

Parameter name

Symbol (unit)

Suggested Value

Europa Radius

RE (km) 1569

Europa Mass

ME (kg) 4.8 ∙1022

Energy of the incident

particle

Ei (keV) 100

Incident flux

Fi

(part/cm2s)

H+ C+ O+ S+

1.5 ∙107

Paranicas et al., 2003

1.8 ∙106

Strazzulla et al., 2003

1.5 ∙106


9∙106


Mass of the incident particle

mi

(amu) 1 12 16 32

Sputtering Yield

Y(part/ion)

6Shi et al.,

1995

10Rocard, et al. (1986)

50Ip et al.,

1997; 1998; Johnson,

1990; Shi et al., 1995

30Johnson,

1990

Mass of the ejected particle

me

(amu)18

Binding energy

Eb

(eV)0.45 (Johnson, 1998)

0.05 (Boring et al., 1984; Haring et al., 1984)

Sputtering Input Parameters used in this analysis

Assumptions• Only H2O 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 H2O) (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).


Energy distribution of the sputtered H2O particles (for impinging 10 keV and 100 keV S+ flux)

2

31

( , ) ( )

0

e e be i b

s i i e b m

e i b

E E EA E E E

f E E E E T

E E E

where Ee is the energy of the ejected particle, Ei is the energy of the incident particle and Eb is the binding energy

Ion Sputtering at Europa

Energy distribution function

Escape energy: 0.38 eV


Ion back scattering and neutralizationWhen an ion is scattered from a target atom at an angle θ, the ratio of the scattered-ion energy E to the incident energy, Ei, can be calculated using the laws of conservation of energy and momentum.

22 2 22 1 1

1 2

sin cos

i

M M MEK

E M M

Kinematic factor

where M1, M2 are the masses of the incident ion and target atom, respectively, θ is the scattering angle, Ei is the incident energy of the ion and E is the energy of

the back-scattered ion.

Back-scattering exists for cases where θ>900.

Kinematic factor as a function of various target masses scattered by O-atoms (θ~1500).


1 exp ARBS

N LP

M

Ion back scattering and neutralizationThe 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 cm2 and σ=~1.04·10-22 cm2 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 PRBS = 1.4 %. The free path of the 100keV H+ is~ 1.2 μm. In this case PRBS = 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.

221 2

24 sin ( / 2)R i

Z Z ed

d E


Thermal desorption

Europa’s temperature: 86 K - 132 K (Spencer et al., 1999).

The thermal energy of an ice H2O 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 H2O, i.e. 0.45 eV per molecule (Johnson, 1998).

Consequently the particle release via the TD mechanism can be considered negligible.


Photon stimulated desorption

1

4PSD photon PSDf f Fd

where fPSD is the photon flux per unit area per time, integrated over the photon energy range > 7eV and equal to 5.8 ∙ 1010 cm-2s-1, F is the fraction of H2O ice on the surface, equal to 0.99, d is the surface density, equal to 1.1∙1015 molecules/cm2 (Dulieu et al., 2005) and σPSD is the PSD cross-section, calculated equal to 10-18 cm2 .

Energy distribution function

2exp

( )( )ee

PSDB eB e

EEf

k Tk T

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):


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

Results of the MC simulations - IS

Total sputtered particle flux Total sputtered particle density

x particles m-2 -1s


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 (2.7·1013 H2O/m2/s and 7.9·109 H2O/m3, respectively). These results are in general in good agreement with estimates obtained by other researchers. The total sputtering rate was calculated to be 1.7·1027 H2O/s. The actual value of the total sputtering rate at some point on the planet’s surface may exhibit variations.

Sputtered H2O flux (upper panels) and density (lower panels) for different types of impinging ions of energy ~100 keV

Plainaki et al., EGU2009

H+ C+ O+ S+


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 1.3·1027 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 H2O 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)


S+


Results of the MC simulations - PSD

H2O particles emitted via PSD from the iced surface of Europa

• The H2O PSD flux is of the same order of magnitude of the sputtered one. • The H2O density released via PSD (1.38 ·109 H2O/m3 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 ·1022 s-1.

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 (2.7·1013 H2O /m2 /s and 7.9·109 H2O /m3, 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 1.7·1027 H2O/s. The actual value of the total sputtering rate at some point on the planet’s surface may exhibit variations.

The H2O density released via PSD (1.38 ·109 H2O/m3 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


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 1.3·1027 s-1, while the fraction of escaping particles via PSD is 0.53% thus meaning a total rate 8.34 ·1022 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 Eb=0.05eV) and 48.7% (for Eb=0.45eV) of the total sputtered flux.


Conclusions

Europa’s exosphere generation C. Plainaki, A. Milillo, A. Mura, S. Orsini Isituto di Fisica dello...

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