International Colloquium and Workshop“Ganymede lander: science goals and experiments” Space Research Institute (IKI), Moscow, Russia 5-7 March 2013

Chemistry of the atmosphere-icy surfaceinterface at Ganymede

V.I. ShematovichInstitute of Astronomy RAS, 48 Pyatnitskaya str., Moscow 119017, Russia.

e-mail: shematov@inasan.ru

Outline:

-Plasma environment of Ganymede;-Surface composition and surface chemistry;-Surface-bounded atmosphere (exosphere);-Latitude-dependent models and results of calculations;-Atmosphere composition near the surface, adsorption fluxes, emission excitation rates and etc.

- Numerical model is based on the previous studies for Europa and Ganymede: Shematovich et al., Icarus, 2005 - DSMC model; Smyth & Marconi, Icarus, 2006 - MC model; Shematovich, SSR, 2008 - H2O ionization chemistry; Marconi, Icarus, 2007 - DSMC model; Cessateur et al., Icarus, 2012 – photo-absorption.

Ganymede in the Jovian System:

Images of Ganymede’s OI 135.6 nm emission for HST orbits on 1998 October 30 (Feldman et al., 2000).

Observations indicate that Ganymede has a significant O2 atmosphere, probably a subsurface ocean, and is the only satellite with its own magnetosphere.

Radiation environment of Ganymede:

The plasma interaction with the surface is a principal source of O2 and the plasma interaction with atmosphere is a principal loss process, therefore a large atmosphere does not accumulate ( Johnson et al. 1982).

High-energy plasma environmentat Ganymede (Cooper at al. 2001) – H+, O+, S+, O++,…

Electrons:-cold component with ne,c=70 cm-3 and Te,c=20 eV;-hot component with ne,h=? cm-3 and Te,h=??? eV.

Surface composition:•Ganymede’s surface composition determines the composition of its atmosphere. The surface is predominantly water ice with impact craters, ridges, possibly melted regions and trace species determining how its appearance varies;

•Ganymede’s surface is dominated by oxygen rich species – H2O and its radiolysis product O2, surface chemistry product H2O2, trace species CO2 , …;

•Trace surface species, which are possible atmospheric constituents, can be endogenic, formed by the irradiation, or have been implanted as magnetospheric plasma ions, as neutrals or grains from Io, or meteoroid and comet impacts.

Atmosphere-surface interface:Radiolysis can occur to depths of the order of tens of cm’s because of the penetration of the energetic incident radiation (Cooper et al., 2001). Mixing of these radiolytic products to greater depths occurs because of meteoroid bombardment (Cooper et al., 2001). This bombardment also produces a porous regolith (Buratti, 1995) composed of sintered grains (Grundy et al., 2001), which increases the effective radiation penetration depth.

The atmospheric O2 permeates pore space in the regolith. Macroscopic mass transport of trapped species by crustal subduction (Prockter and Pappalardo, 2000) is a macroscopic mass transport pump, which is needed to carry oxidants to Ganymede’s ocean.

Lower boundary – Radiation-induced ice chemistry (Johnson, 2010):

(i) Sputtering of icy surface by magnetospheric ions with energies of Е ~ 10 -1000 keV (Cooper at al. 2001) results in the ejection of parent molecules H2O and their radiolysis products O2 and H2 with energy spectra (Johnson et al. 1983) – non-thermal source

(ii) UV-photolysis of the icy satellite surface leads to the ejection of H2O and O2 with Maxwellian energy distribution with the mean surface temperature T ~ 70 -- 150 K, – thermal source;(iii) Returning H2 and O2 molecules are desorbed thermally – thermal source;(iv) Returning H2O, O, and OH stick with unit efficiency.

Atmosphere-surface interface:

Returning H2 and O2 molecules do not stick to the icy surface and are desorbed thermally, while returning H2O, O, and OH stick with unit efficiency.

Kn > 1 – atmosphere is effectively collisionless;

0.1 < Kn <1 – transitional region;

Kn < 0.1 near-surfacecollision-dominant layer.

Photolysis by (a) solar UV radiation, (b) impact by photo- and plasma electrons, and (c) atmospheric sputtering by high-energy magnetospheric ions:

•   Dissociation, direct and dissociative ionization :

• Momentum transfer, dissociation, ionization, and charge transfer in collisions with high-energy ions

Calculated models:

Model A: – subsolar region λ=15o - photolysisModel B: – polar region λ=90o - radiolysisModel C: – transitional region λ=45-75o -radiolysis+photolysis

Near-surface atmosphere of Ganymede: Model A (subsolar region)

Model A: - subsolar region λ=15o - photolysis

- surface temperature Ts(λ)=70o×cos(λ)0.75+80o in [K] Ts(λ=15)=148o

- upward flux of H2O due to the evaporation F(λ)=1.1×1031 ×Ts(λ)-0.5×exp(-5757/Ts(λ)) in [cm-2s-1] F(λ=15)=1.4×1013 cm-2s-1

- Maxwellian flux distribution by energy

Near-surface atmosphere of Ganymede: H2O kinetic energy distributions – Model A (subsolar region)

Spectrum of H2O upward flux Spectrum of H2O downward flux

Near-surface atmosphere of Ganymede: OH kinetic energy distributions – Model A (subsolar region)

Spectrum of OH upward flux Spectrum of OH downward flux

Energy spectra are non-thermal with the significant suprathermal tails – important for both escape from atmosphere and adsorption to surface!

Near-surface atmosphere of Ganymede: density

distributions – Model A (subsolar region)

Number densities – H2O-dominant atmosphere !

Column number densities

Near-surface atmosphere of Ganymede: Model B (pole region)

Model B: - polar region λ=90o – radiolysis and surface temperature Ts(λ=90)=80o in [K] - upward fluxes of H2O, OH, O, and H are due to the sputtering with energy spectra f(E)=2EU0/(E+U0)3, U0=0.055 eV FH2O(λ=90)=1.8×108 cm-2s-1 , FH,O,OH=1.0×107 cm- 2s-1

- upward fluxes of H2 and O2 are induced by sputtering but with Maxwellian flux distribution by energyFH2(λ=90)=2.8×109 cm-2s-1 , FO2(λ=90)=1.4×109 cm-2s-1

- H2 and O2 thermally desorb, why H2O, OH, O, and H stick to the ice with prob=1

Near-surface atmosphere of Ganymede : O2 kinetic energy distributions – Model B(pole)

Spectrum of O2 upward flux Spectrum of O2 downward flux

Spectrum of H2O upward flux Spectrum of H2O downward flux

Near-surface atmosphere of Ganymede : O2 kinetic energy distributions – Model B(pole)

Number densities O2-dominant atmosphere !

Column densities

Near-surface atmosphere of Ganymede: density

distributions – Model B(pole region)

The detailed behaviour of the species is complex because of the very different source characteristics and weak collisionality of the thin atmosphere.

Number densities H2O+O2-dominant atmosphere !

Near-surface atmosphere of Ganymede: density

distributions – Model C(transitional 45 – 75o region)

Near-surface atmosphere of Ganymede: Models BB and BBB (pole region)

- polar region λ=90o – radiolysis and surface temperature Ts(λ=90)=80o in [K] Model BB: - same as Model B but upward sputtering flux of H2O is 10 times higher;

Model BBB: - same as Model B but upward sputtering fluxes of H2O, OH, O, and H are 10 times higher;

BB –H2O –sputtering source x 10. O2-dominant atmosphere !

Near-surface atmosphere of Ganymede: density

distributions – Models BB and BBB(pole region)

BBB – H, O, OH, H2, O2, and H2O –sputtering source x 10. H2 + O2-dominant atmosphere !

Ionization chemistry in the H2O-dominant atmosphere

The parent H2O molecules are easily dissociated and ionized by the solar UV-radiation and the energetic magnetospheric electrons forming secondaries: chemically active radicals, O and OH, and ions, H+, H2+, O+, OH+, and H2O+ . Secondary ions in H2O-dominantatmospheres are efficiently transformed to H3O+ hydroxonium ions in the fast ion-molecular reactions;The H3O+ hydroxonium ion does not chemically interact with other neutrals, and is destroyed by dissociative recombination with thermal electrons producing H, H2, O, and OH (Shematovich, 2008).

Near-surface atmosphere of Europa: ionization chemistry in the H2O+O2-dominant atmosphere

In a mixed H2O + O2 atmosphere ionization chemistry results in the formation of a second major ion O2

+ - since O2 has a lower ionization potential than other species –H2, H2O, OH, CO2. When there is a significant admixture of H2 then O2

+ can be converted to the O2H+ through the fast reaction with H2 and then to the H3O+ through low speed ion-molecular reaction with H2O. Therefore, the minor O2H+ ion is an important indicator at what partition between O2 and H2O does ionization chemistry result in the major O2

+ or H3O+ ion (Johnson et al., 2006).

Near-surface atmosphere of Ganymede: ion distributions

Model A –subsolar region

Model B –polar region

Near-surface atmosphere of Ganymede is:

• of interest as an extension of its surface and indicator of surface composition and chemistry. Composition measurements are critical for our understanding of the matter transport near, onto surface, and in the subsurface layers;

• neutral and ion composition of the surface-bounded atmosphere is determined by the irradiation-induced ice chemistry through the surface sources of the parent molecules and of their dissociation products;

•There is a critical need for detailed modeling of the desorption of important trace surface constituents related to exo- and endogenic sources of the Ganymede’s surface composition.

Thank you for your attention!

Near-surface atmosphere of Ganymede: