Chemistry of the atmosphere-icy surface interface at Ganymede

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Chemistry of the atmosphere-icy surfaceinterface
at Ganymede
International Colloquium and Workshop Ganymede
lander science goals and experiments Space
Research Institute (IKI), Moscow, Russia 5-7
March 2013

• V.I. Shematovich
• Institute of Astronomy RAS, 48 Pyatnitskaya str.,
  Moscow 119017, Russia. e-mail
  shematov_at_inasan.ru

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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.

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Ganymede in the Jovian System
Observations indicate that Ganymede has a
significant O2 atmosphere, probably a subsurface
ocean, and is the only satellite with its own
magnetosphere.
Images of Ganymedes OI 135.6 nm emission for
HST orbits on 1998 October 30 (Feldman et al.,
2000).
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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 environment
• at Ganymede (Cooper at al. 2001) H, O, S,
  O,
• Electrons
• cold component with ne,c70
• cm-3 and Te,c20 eV
• hot component with ne,h? cm-3 and Te,h??? eV.

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Surface composition

• Ganymedes 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
• Ganymedes 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.

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Atmosphere-surface interface
Radiolysis can occur to depths of the order of
tens of cms 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
Ganymedes ocean.
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Lower boundary Radiation-induced ice chemistry
(Johnson, 2010)
(i) Sputtering of icy surface by magnetospheric
ions with energies of ? 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.
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Atmosphere-surface interface
Kn gt 1 atmosphere is effectively
collisionless 0.1 lt Kn lt1 transitional
region Kn lt 0.1 near-surface collision-dominant
layer.
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.
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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

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Calculated models
Model A subsolar region ?15o -
photolysis Model B polar region ?90o -
radiolysis Model C transitional region
?45-75o - radiolysisphotolysis
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Near-surface atmosphere of Ganymede Model A
(subsolar region)

• Model A
• subsolar region ?15o - photolysis
• surface temperature Ts(?)70ocos(?)0.7580o in
  K
• Ts(?15)148o
• upward flux of H2O due to the evaporation
• F(?)1.11031 Ts(?)-0.5exp(-5757/Ts(?)) in
  cm-2s-1
• F(?15)1.41013 cm-2s-1
• Maxwellian flux distribution by energy

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Near-surface atmosphere of Ganymede H2O kinetic
energy distributions Model A (subsolar region)
Spectrum of H2O upward flux
Spectrum of H2O downward flux
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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!
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Near-surface atmosphere of Ganymede density
distributions Model A (subsolar region)
Column number densities
Number densities H2O-dominant atmosphere !
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Near-surface atmosphere of Ganymede Model B
(pole region)

• Model B
• polar region ?90o radiolysis and surface
  temperature Ts(?90)80o in K
• upward fluxes of H2O, OH, O, and H are due to
  the sputtering with energy spectra
  f(E)2EU0/(EU0)3, U00.055 eV
• FH2O(?90)1.8108 cm-2s-1 , FH,O,OH1.0107
  cm- 2s-1
• upward fluxes of H2 and O2 are induced by
  sputtering but with Maxwellian flux distribution
  by energy
• FH2(?90)2.8109 cm-2s-1 , FO2(?90)1.4109
  cm-2s-1
• - H2 and O2 thermally desorb, why H2O, OH, O, and
  H stick to the ice with prob1

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Near-surface atmosphere of Ganymede O2 kinetic
energy distributions Model B(pole)
Spectrum of O2 upward flux
Spectrum of O2 downward flux
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Near-surface atmosphere of Ganymede O2 kinetic
energy distributions Model B(pole)
Spectrum of H2O upward flux
Spectrum of H2O downward flux
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Near-surface atmosphere of Ganymede density
distributions Model B(pole region)
Number densities O2-dominant atmosphere !
Column densities
The detailed behaviour of the species is complex
because of the very different source
characteristics and weak collisionality of the
thin atmosphere.
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Near-surface atmosphere of Ganymede density
distributions Model C(transitional 45 75o
region)
Number densities H2OO2-dominant atmosphere !
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Near-surface atmosphere of Ganymede Models BB
and BBB (pole region)
- polar region ?90o radiolysis and surface
temperature Ts(?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
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Near-surface atmosphere of Ganymede density
distributions Models BB and BBB(pole region)
BB H2O sputtering source x 10. O2-dominant
atmosphere !
BBB H, O, OH, H2, O2, and H2O sputtering
source x 10. H2 O2-dominant atmosphere !
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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-dominant atmospheres 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).
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Near-surface atmosphere of Europa ionization
chemistry in the H2OO2-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).
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Near-surface atmosphere of Ganymede ion
distributions
Model B polar region
Model A subsolar region
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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 Ganymedes
  surface composition.
• Thank you for your attention!

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Near-surface atmosphere of Ganymede