Synergism in magnetosphere-exosphere-ice interactions enhances gas trapping and radiation chemistry

Raúl A. BaragiolaUniversity of Virginia, Charlottesville,

USA

raul@Virginia.edu

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)

Fra

ctio

na

l O 2 C

on

ce

ntr

atio

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

Europa

80K

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

101

6 O3 / c

m2

Fluence (1015 ions/cm2)

Start Simultaneous Condenation

240 260 280 300 320 3400.0

0.2

0.4

0.6

0.8

1.0

Re

flect

an

ce R

atio

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

olu

mn

De

ns

ity

(1

015

O2/c

m2 )

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 initially

100 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

olu

mn

De

ns

ity

(1

015

O2/c

m2 )

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 /

cm

2 )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