Synergism in magnetosphere- exosphere-ice interactions enhances gas trapping and radiation chemistry Raúl A. Baragiola University 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 to 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 O 2, ozone at Ganymede Telescope: Spencer et al., J. Geophys. Res Noll et al., Nature 1996 Lab: Bahr & Baragiola, J. Geophys. Res Condensed O 2 Ozone Vidal, Bahr, Baragiola, Peters, Science 276, 1839 (1997) Absorption by (O 2 ) 2

Sputtering and generation of atmospheres Escape vs. redeposition Escape vs. redeposition H2OH2O O2 H2H2 Ion

No model accounts quantitatively for condensed oxygen and ozone at Ganymede and some other satellites

Radiation of ice in lab gives H 2 O 2, O 2, but no ozone Experiments show –Sputtering of O 2 (more for heavy ions) –O 2 trapped in ice (not enough to explain Ganymede) (up to 30% close to surface) <1 % –H 2 O 2 <1 % –No ozone O 2 from radiolysis with 100 keV Ar+ D epth profile: Teolis et al, Phys Rev B (2005) ID of H2O2 in Europa, Loeffler & Baragiola Geophys. Res. Lett. (2005) 8

Water co-deposition enhances oxygen trapping and ozone synthesis 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?

O 2 adsorption / desorption cycle in amorophous, porous ice Ice film grown at 70K, then cooled to 50K. O 2 pressure: of 5.5*10 -7 Torr, 90 ML of O 2 are adsorbed. When removing the O 2 ambient the trapped O 2 diffuses out

Ion-induced Compaction of Nanoporous Ice Dangling bonds in internal surface Surface and volume decay differently with ion fluence Raut, Teolis, Loeffler, Vidal, Famá & Baragiola, J. Chem. Phys. 126 (2007) 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 When O 2 is pumped out, the trapped O 2 diffuses out When O 2 is pumped out, the trapped O 2 does NOT diffuse out Shi, Teolis & Baragiola, Phys Rev B 79 (2009) KeV H + 4 x /cm 2 s 2 µm ice film grown at 70K

Conclusions Without irradiation, adsorption above 70K is negligible. The amount of O 2 adsorbed depends on film thickness and temperature. Adsorbed O 2 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 O 2 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