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 surface interface at Ganymede V.I. Shematovich Institute of Astronomy RAS, 48 Pyatnitskaya str., Moscow , Russia.

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, DSMC model; Smyth & Marconi, Icarus, MC model; Shematovich, SSR, H 2 O ionization chemistry; Marconi, Icarus, DSMC model; Cessateur et al., Icarus, 2012 – photo-absorption.

Ganymede in the Jovian System: Images of Ganymede’s OI nm emission for HST orbits on 1998 October 30 (Feldman et al., 2000). Observations indicate that Ganymede has a significant O 2 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 O 2 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 environment at Ganymede (Cooper at al. 2001) – H +, O +, S +, O ++,… Electrons: -cold component with n e,c =70 cm -3 and T e,c =20 eV; -hot component with n e,h =? cm -3 and T e,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 – H 2 O and its radiolysis product O 2, surface chemistry product H 2 O 2, trace species CO 2, …; 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 O 2 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 ~ keV (Cooper at al. 2001) results in the ejection of parent molecules H 2 O and their radiolysis products O 2 and H 2 with energy spectra (Johnson et al. 1983) – non-thermal source (ii) UV-photolysis of the icy satellite surface leads to the ejection of H 2 O and O 2 with Maxwellian energy distribution with the mean surface temperature T ~ K, – thermal source; (iii) Returning H 2 and O 2 molecules are desorbed thermally – thermal source; (iv) Returning H 2 O, O, and OH stick with unit efficiency.

Atmosphere-surface interface: Returning H 2 and O 2 molecules do not stick to the icy surface and are desorbed thermally, while returning H 2 O, O, and OH stick with unit efficiency. Kn > 1 – atmosphere is effectively collisionless; 0.1 < Kn <1 – transitional region; Kn < 0.1 near-surface collision-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=15 o - photolysis Model B: – polar region λ=90 o - radiolysis Model C: – transitional region λ=45-75 o - radiolysis+photolysis

Near-surface atmosphere of Ganymede: Model A (subsolar region) Model A: - subsolar region λ=15 o - photolysis - surface temperature T s (λ)=70 o ×cos(λ) o in [K] T s (λ=15)=148 o - upward flux of H 2 O due to the evaporation F(λ)=1.1×10 31 ×T s (λ )-0.5 ×exp(-5757/T s (λ)) in [cm -2 s -1 ] F(λ=15)=1.4×10 13 cm -2 s -1 - Maxwellian flux distribution by energy

Near-surface atmosphere of Ganymede: H 2 O kinetic energy distributions – Model A (subsolar region) Spectrum of H 2 O upward fluxSpectrum of H 2 O 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 – H 2 O-dominant atmosphere ! Column number densities

Near-surface atmosphere of Ganymede: Model B (pole region) Model B: - polar region λ=90 o – radiolysis and surface temperature T s (λ=90)=80 o in [K] - upward fluxes of H 2 O, OH, O, and H are due to the sputtering with energy spectra f(E)=2EU 0 /(E+U 0 ) 3, U 0 =0.055 eV F H2O (λ=90)=1.8×10 8 cm -2 s -1, F H,O,OH =1.0×10 7 cm - 2 s -1 - upward fluxes of H 2 and O 2 are induced by sputtering but with Maxwellian flux distribution by energy F H2 (λ=90)=2.8×10 9 cm -2 s -1, F O2 (λ=90)=1.4×10 9 cm -2 s -1 - H 2 and O 2 thermally desorb, why H 2 O, OH, O, and H stick to the ice with prob=1

Near-surface atmosphere of Ganymede : O 2 kinetic energy distributions – Model B(pole) Spectrum of O 2 upward fluxSpectrum of O 2 downward flux

Spectrum of H 2 O upward fluxSpectrum of H 2 O downward flux Near-surface atmosphere of Ganymede : O 2 kinetic energy distributions – Model B(pole)

Number densities O 2 -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 H 2 O+O 2 -dominant atmosphere ! Near-surface atmosphere of Ganymede: density distributions – Model C(transitional 45 – 75 o region)

Near-surface atmosphere of Ganymede: Models BB and BBB (pole region) - polar region λ=90 o – radiolysis and surface temperature T s (λ=90)=80 o in [K] Model BB: - same as Model B but upward sputtering flux of H 2 O is 10 times higher; Model BBB: - same as Model B but upward sputtering fluxes of H 2 O, OH, O, and H are 10 times higher;

BB –H 2 O –sputtering source x 10. O 2 -dominant atmosphere ! Near-surface atmosphere of Ganymede: density distributions – Models BB and BBB(pole region) BBB – H, O, OH, H 2, O 2, and H 2 O –sputtering source x 10. H 2 + O 2 -dominant atmosphere !

Ionization chemistry in the H 2 O-dominant atmosphere The parent H 2 O 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+, H 2 +, O+, OH+, and H 2 O+. Secondary ions in H 2 O-dominant atmospheres are efficiently transformed to H 3 O+ hydroxonium ions in the fast ion-molecular reactions; The H 3 O+ hydroxonium ion does not chemically interact with other neutrals, and is destroyed by dissociative recombination with thermal electrons producing H, H 2, O, and OH (Shematovich, 2008).

Near-surface atmosphere of Europa: ionization chemistry in the H 2 O+O 2 -dominant atmosphere In a mixed H 2 O + O 2 atmosphere ionization chemistry results in the formation of a second major ion O since O 2 has a lower ionization potential than other species –H 2, H 2 O, OH, CO 2. When there is a significant admixture of H 2 then O 2 + can be converted to the O 2 H + through the fast reaction with H 2 and then to the H 3 O + through low speed ion- molecular reaction with H 2 O. Therefore, the minor O 2 H + ion is an important indicator at what partition between O 2 and H 2 O does ionization chemistry result in the major O 2 + or H 3 O + 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: