CO2 in the Surfaces of the Galilean and Saturnian satellites

Details CO2 in the Surfaces of the Galilean and Saturnian satellites CO2 in the Surfaces of the Galilean and Saturnian satellites

May 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agusm.p24a..07h&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #P24A-07

Mathematics

Logic

5410 Composition (1060, 3672), 5464 Remote Sensing, 5470 Surface Materials And Properties, 6218 Jovian Satellites, 6280 Saturnian Satellites

Scientific paper

CO2 has been detected in the surfaces of several of the Galilean and Saturnian satellites [1-6]. However, it's physical state is poorly understood. On the Galilean satellites the CO2 that is detected is bound to, or trapped within, the non-ice materials preventing it from sublimating or otherwise escaping from the surface. Although CO2 is always present in the nonice material on Callisto, it is a trace material, with only a few hundred molecules responsible for the deepest absorption features and an estimated molar abundance of 0.1 percent [2; 7-9]. The abundance of CO2 in the Saturnian satellites Phoebe and Iapetus is similar, but whether the host is the ice and/or the nonice material is uncertain. Physisorbtion may be able to explain the spectral characteristics of the CO2 and apparently limited stability. We have begun to investigate the spectral properties of CO2 adsorbed onto non-ice materials at cryogenic temperatures to determine if they are consistent with the spectral characteristics of CO2 on the icy satellites. An additional mechanism of trapping CO2 on the Saturnian satellites, may be trapping or mixing of CO2 with water-ice. We therefore compare our results with results from others'laboratory investigations of spectral signatures of water-ice/CO2 mixtures [10- 14]. We have measured the mid-IR absorption feature of CO2 physisorbed onto pellets of Ca-montmorillonite, Na-montmorillonite, (Mg,Li)-montmorillonite, palagonite, Mg-serpentine, goethite, and kaolin at room temperature and, in some cases, at 125K using transmission spectroscopy. CO2 only adsorbed onto montmorillonite and kaolin and did not appear to react with any of the materials. Montmorillonite absorbs more CO2 than kaolin, with the position of the ~4.26m CO2 absorption band dependent on the cation charge density in a similar manner observed for the adsorption of CO2 onto zeolites [15]. The IR absorption band of CO2 in montmorillonite tends to shift toward longer wavelengths as the density of the electric field associated with the principle cation decreases, with the exception that the IR absorption band of the Na-rich endmember occurs at a shorter wavelength than for the Li-rich endmember. Also, CO2 physisorbed onto montmorillonite at cryogenic temperature has a different shape than for CO2 physisorbed at room temperature and appears similar to the absorption feature of CO2 in the nonice materials on the Galilean and Saturnian satellites. It is likely the presence of charge-compensating ions, and the resulting negative charge of the remaining structure, enable CO2 to physisorb by inducing a dipole in the CO2 molecule. However, physisorption of CO2 in unirradiated laboratory samples is too weak to account for the long-term stability of CO2 in the satellites' surfaces. It remains to be determined if radiation damage of non-ice materials could alter them so physisorbed CO2 would be stable over geologic time. References: [1] Carlson et al., (1996) Science; [2] McCord et al., (1998) J. Geophys. Res.; [3] Buratti et al., (2005) Astrophys. J.; [4]Clark et al., (2005) Nature; [5] Brown et al., (2006) , Icarus; [6] Filacchione et al., (2006) , Icarus; [7] Hibbitts et al., (2000) J. Geophys. Res.; [8] Hibbitts et al., (2002) , J. Geophys. Res.; [9] Hibbitts et al., (2003) J. Geophys. Res.; [10] Prialnik and Bar-Nun, (1990) Astrophys. J.; [11] Whittet et al., (1996) Astron. & Astrophys.; [12] Gerakines et al., (1999) Astrophys. J.; [13] Ehrenfreund et al., (1999) Astron. & Astrophys.; [14] Sandford et al., (2001) Astrophys. J.; [15] Hibbitts and Szanyi, (2006), Icarus.

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