Irradiation-enhanced adsorption and trapping ofO2on nanoporous water ice

Abstract

We have found that irradiation with 50-150 keV protons enhances gas adsorption in nanoporous amorphous ice by creating high-energy binding sites. If irradiation is done in vacuum, the ice is compacted and does not adsorb significantly in subsequent exposure to gas. If irradiation occurs while the ice is exposed to an ambient O 2 pressure, adsorption is enhanced by a factor as high as 5.5 compared to unirradiated ice. The maximum concentration of adsorbed O 2 increased with increasing pressure and decreasing ion flux, achieving ϳ6% throughout the ion-penetration depth in the low-flux limit ͑at 7.2ϫ 10 −5 mbar and 50 K͒. After simultaneous irradiation and oxygen exposure, the adsorbed O 2 could be retained in the ice when the ambient oxygen was removed, indicating that ion impacts close nanopores containing O 2 . Similar results, but with somewhat lower gas/ice ratios where obtained with Ar, which has a lower binding energy than O 2 . The experiments suggest a mechanism for gas trapping in comets and icy satellites in the outer Solar system.

Cometary ices in forming protoplanetary disc midplanes

“…Similar trapping mechanisms have also been seen experimentally for other volatiles, such as N2, CH4, CO2, O2, NH3; and noble gases, such as Ar, Kr, Xe (Ayotte et al 2001;Notesco, Bar-Nun & Owen 2003;Bar-Nun, Notesco & Owen 2007;Gálvez et al 2008;Shi, Teolis & Baragiola 2009;Fayolle et al 2011;Yokochi et al 2012;Bar-Nun & Laufer 2013;Fresneau et al 2014). It has also been suggested that irradiation of H2O ice may collapse pores in the amorphous structure, thereby further enhancing trapping of volatiles (e.g., Shi, Teolis & Baragiola 2009).…”

“…It has also been suggested that irradiation of H2O ice may collapse pores in the amorphous structure, thereby further enhancing trapping of volatiles (e.g., Shi, Teolis & Baragiola 2009). Such experimental findings imply that weakly bound volatiles can be preserved in the modelled discs at temperatures that are warmer than their respective thermal desorption regimes.…”

Trace constituents of Europa's atmosphere

“…For residence times above the horizontal line labeled "lifetime against sputtering," a molecule is more likely to be sputtered or photodesorbed along with H 2 O before being thermally desorbed. All data come from Sandford and Allamandola (1993), except for the O 2 , which comes from Shi et al (2007), and the "lifetime against sputtering," which comes from Paranicas et al (2002). tion, and, as discussed later, a laboratory measurement on pure H 2 O might not be applicable at Europa's surface.…”

“…2 come from Sandford and Allamandola (1993), except for the O 2 curve, which came from Shi et al (2007). There was no data available for SO 2 on H 2 O ice, so we used the SO 2 on SO 2 curve.…”

“…tion, and, as discussed later, a laboratory measurement on pure H 2 O might not be applicable at Europa's surface. The O 2 curve (Shi et al, 2007) may be considered an upper limit to O 2 's residence time since the data was acquired using "microporous" water ice; ice that is porous at the scale of nanometers (as distinguished from regolith porosity). Using this data with our model requires that atmospheric molecules are fully thermally accommodated in the brief period when they are adsorbed on the surface.…”