Laboratory Investigations into the Physisorption of Volatiles in Nonice Materials Relevant to the Outer Solar System

Details Laboratory Investigations into the Physisorption of Volatiles in Nonice Materials Relevant to the Outer Solar System Laboratory Investigations into the Physisorption of Volatiles in Nonice Materials Relevant to the Outer Solar System

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.p13g..04h&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #P13G-04

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6008 Composition (1060), 6218 Jovian Satellites, 6280 Saturnian Satellites

Scientific paper

We propose that physisorption may be responsible for the prsence of CO2, and potentially the other volatiles, detected in the surfaces of the icy Galilean and Saturnian satellites. Physisorption is caused by weak inter- molecular van der Waals forces between the adsorbate molecules and areas of asymmetric charge distribution within the adsorbing host material. The van der Waals force can exist between CO2 and an adsorbant when a charge asymmetry in the adsorbent induces a weak dipole within the otherwise symmetric CO2 molecule. This partial electronic polarization of the CO2 is thus a function of the structure of the host molecule and the charge distribution within the host molecule. Complex silicates and potentially other materials offer excellent structures for the adsorption of CO2 by this mechanism because of their significant asymmetric distribution of charges within a unit cell as well as by their large microporosity. However, at room temperature, the strength of the van der Waals bond is insufficient to keep the CO2 adsorbed when the host material is exposed to vacuum. We have found that CO2 can remain physisorb onto some clays when the CO2 partial pressure is effectively zero under ultra-high vacuum (UHV) if the adsorbant is cooled to the surface temperatures of the icy satellites of Jupiter and Saturn. CO2 remains adsorbed onto the clay mineral montmorillonite for 10s of minutes when exposed to a vacuum of approx. 1E-8 Torr at approx. 125K. However, CO2 does not adsorb onto serpentine, goethite, or palagonite under these conditions. A small amount may adsorb onto kaolinite. These are materials materials that may have similar microporosities, but whose unit cells possess much less charge asymmetry than smectites. When heated above 150K under vacuum, the CO2 desorbs from the montmorillonite within a few minutes. We infer that the presence of a strong charge asymmetry which can induce a strong dipole in the CO2 molecule is the most important requirement for physisorption at cryogenic temperatures.

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