Physisorption of CO 2 on non-ice materials relevant to icy satellites

Abstract

CO 2 is known to adsorb onto clay and other minerals when a significant atmospheric pressure is present. We have found that CO 2 can also adsorb onto some clays when the CO 2 partial pressure is effectively zero under ultra-high vacuum (UHV) if cooled to the surface temperatures of the icy satellites of Jupiter and Saturn. The strength of adsorption and the spectral characteristics of the adsorbed CO 2 infrared (IR) ν 3 absorption band near 4.25 μm depend on the composition and temperature of the adsorbent. CO 2 remains adsorbed onto the clay mineral montmorillonite for > 10 s of min when exposed to a vacuum of ∼ 1 × 10 −8 Torr at ∼ 125 K . CO 2 does not adsorb onto serpentine, goethite, or palagonite under these conditions. A small amount may adsorb onto kaolinite. When heated above 150 K under vacuum, the CO 2 desorbs from the montmorillonite within a few minutes. The ν 3 absorption band of CO 2 adsorbed onto montmorillonite at 125 K is similar to that of the CO 2 detected on the saturnian and Galilean satellites and is markedly different from CO 2 adsorbed onto montmorillonite at room temperature. We infer the adsorption process is physisorption and postulate that this mechanism may explain the presence and spectral characteristics of the CO 2 detected in the surfaces of these outer satellites.