Adsorption-driven Gas Trapping in Cometary Ice Analogs

Icy planetary bodies carry volatiles in abundances that should reflect the conditions of their formation. However, the interpretation of volatile abundance data from comets is complicated by the fact that the governing parameters and underlying mechanism of volatile trapping are poorly constrained. To unravel the mechanism of gas trapping in water ice and shed light on the origin of comets, gas-trapping experiments were conducted under isothermal conditions with a focus on the relationship between adsorption properties and the quantities of trapped gas. The adsorption data suggest that the adsorption energies of ice surfaces are heterogeneous, and surface properties depend on initial ice-deposition temperatures as well as on the effect of thermal annealing. Sites with different adsorption energies result in temperature dependence of contributing adsorption sites. Moreover, adsorption sites characterized by higher energy contribute predominantly at low pressures, relevant to the protosolar nebula, and at higher temperatures. When new ice was added to the well-characterized ice surface under controlled Ar pressure, the amount of trapped gas was proportional to the amount of gas adsorbed on the newly generated external surface, suggesting the burial of adsorbed gas as the primary mechanism of gas trapping. The ratio of trapped to newly adsorbed gas reflecting the internal to external surface areas ranged between 5 and 20. Assuming the formation of comparable water ice under the pressure-temperature profile of the protosolar nebula, Ar/H₂O~10^-5 reported from 67P/Churyumov-Gerasimenko could have formed at about 40 K.