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Synergism in magnetosphere-exosphere-ice interactions enhances gas trapping and radiation chemistry. Raúl A. Baragiola University of Virginia, Charlottesville, USA r [email protected]. Epistemology. Most of what we observe is the surface Models guide to interpret observations - PowerPoint PPT Presentation
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Synergism in magnetosphere-exosphere-ice interactions enhances gas trapping and radiation chemistry
Raúl A. BaragiolaUniversity of Virginia, Charlottesville,
USA
Epistemology
• Most of what we observe is the surface• Models guide to interpret observations• But models are underdetermined by data• Laboratory simulations constraint
possibilities– Historically, study each process in isolation,
then synthesize full picture
Laboratory Simulations
• Standard simulation goals– Exosphere: typ. 10-10 to 10-8 Torr– Temperature: <160 K– Ice: water, water + other gases, rocks– Ice from vapor deposition– Irradiation: particle type, energy, fluxes (?)– Time (not possible)– Gravity: usually ignored
Synergy
• Phenomena happen simultaneously• Evaporation, sputtering, photodesorption,
condensation, ion implantation, topographical alterations
• Previously, each phenomenon studied separately
• We started to study 2 at a time
5
Origin of condensed
O2, ozone at Ganymede
Telescope: Spencer et al., J. Geophys. Res. 1995
Noll et al., Nature 1996Lab: Bahr & Baragiola, J. Geophys. Res. 1998
Condensed O2
Ozone
Vidal, Bahr, Baragiola, Peters, Science 276, 1839 (1997)
Absorption by (O2)2
Sputtering and generation of atmospheres
Escape vs. redeposition
H2O
O2
H2
Ion
No model accounts quantitatively for condensed oxygen and ozone at
Ganymede and some other satellites
8
Radiation of ice in lab gives H2O2, O2, but no ozone
Experiments show– Sputtering of O2 (more for heavy
ions)– O2 trapped in ice (not enough to
explain Ganymede) (up to 30% close to surface)
– H2O2 <1 % – No ozone
0 50 100 150 2000.0
0.1
0.2
0.3
Depth (ML)
Frac
tiona
l O 2 Con
cent
ratio
n
After Irradiation at 130 KMeasured at 20 K
O2 from radiolysis with 100 keV Ar+Depth profile: Teolis et al, Phys Rev B (2005)
2900 2800 2700 2600
0.0
0.4
0.8
3.4 3.5 3.6 3.7 3.8
2900 2800 2700 26000.0
0.4
0.8
3.4 3.6 3.8
Nor
mal
ized
Opt
ical
Dep
th
Wavenumber (cm-1)
disp
irr
Europa80K
cr dh
cram
disp
110 K
Wavelength (µm)
ID of H2O2 in Europa, Loeffler & Baragiola Geophys. Res. Lett. (2005)
Water co-deposition enhances oxygen trapping and ozone synthesis
0 5 10 15 20 250
1
2
3
4
5
1016
O3 /
cm2
Fluence (1015 ions/cm2)
Start Simultaneous Condenation
240 260 280 300 320 3400.0
0.2
0.4
0.6
0.8
1.0
Ref
lect
ance
Rat
io
Wavelength (nm)
258 nm
O3
Teolis, Loeffler, Raut, Fama & Baragiola, Astrophys. J. Letters 644, L141 (2006)
Hartley band
Solves the Problem of Ozone on Ganymede (?)
Is exospheric Oxygen trapped in the surface ice?
O2 adsorption / desorption cycle in amorophous, porous ice
• Ice film grown at 70K, then cooled to 50K. • O2 pressure: of 5.5*10-7 Torr, 90 ML of O2 are adsorbed.• When removing the O2 ambient the trapped O2 diffuses out
0 1000 2000 3000 4000 50000
2
4
6
8
10
O2 C
olum
n D
ensi
ty (1
015O
2/cm
2 )
Time (s)
O2 pressure
5.5x10-7 Torr
valve to O2 closed
desorption andpump down
Ion-induced Compaction of Nanoporous Ice
Dangling bonds in internal surfaceSurface and volume decay differently with ion fluence
2.68 2.7 2.72 2.74
3740 3720 3700 3680 3660 36400.00
0.01
0.02
0.03
0.04
Opt
ical
Dep
th
Wavenumber (cm -1)
DB 1
DB 2
increasing fluence
40 K0.44m thick initially100 keV Ar+
Wavelength (m)
0.01 0.1 1 100.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
Nor
mal
ized
Por
osity
Nor
mal
ized
Ban
d A
rea
Irradiation Fluence (1013 ions/cm2 )
PDB
40 K, 0.44 m thick initially
Raut, Teolis, Loeffler, Vidal, Famá & Baragiola, J. Chem. Phys. 126 (2007) 244511
Raut, Famá, Loeffler & Baragiola, Astrophys. J. 687 (2008) 1070 ion fluence
OH vibrations in dangling bonds Fluences 10x smaller than for amorphization
Ice in space has been subject to prolonged irradiation, and therefore compacted.
Then how can it trap gases (e.g., in comets, icy satellites)?
Ion-enhanced adsorption and trapping
0 1000 2000 3000 4000 50000
2
4
6
8
10
O2 C
olum
n D
ensi
ty (1
015O
2/cm
2 )
Time (s)
O2 pressure
5.5x10-7 Torr
valve to O2 closed
desorption andpump down
When O2 is pumped out, the trapped O2 diffuses out
0 500 1000 1500 20000
5
10
15
20
25
Ar
O2
(1
015 /c
m2 )
Ar admitted
O2 admitted
pumped
Time (s)
pumped
Ion inducd adsorption
When O2 is pumped out, the trapped O2 does NOT diffuse out
Shi, Teolis & Baragiola, Phys Rev B 79 (2009) 235422
50 KeV H+ 4 x 1011 /cm2 s
2 µm ice film grown at 70K
Conclusions
• Without irradiation, adsorption above 70K is negligible. The amount of O2 adsorbed depends on film thickness and temperature. Adsorbed O2 cannot be trapped permanently above 50K.
• Ice compacted by irradiation in vacuum cannot adsorb gases.
• Irradiation enhances gas adsorption and retention at 50K. The enhancement depends on ion flux, ice thickness, ambient pressure as well as the continuity of the ion flux.
UV irradiation under gas exposure
Shi, J. et al. 2011, ApJ Lett. 738, L3
Enhanced O2 absorption with 193 nm light
But from radiation chemical products:hydrogen peroxide and ozone
Not from closing pores
Implications
• 193 nm photons can photolyse oxygen and penetrate ~2 meters in
the ice, much deeper than ionizing radiation. Thus, they can
produce radiation effects deeper in the surface than previously
considered.
• Deep photolysis does not require nanopores. It could happen in
loose grain structure of icy regoliths (macro porosity). Pores
significantly increase the residence time of adsorbed molecules,
enhancing photodissociation, and favoring molecular synthesis.
Shi, J. et al. 2011, ApJ Lett. 738, L3
Astrophysical ices have gas-filled pores stabilized
by radiation