Other
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.p11b0109m&link_type=abstract
American Geophysical Union, Fall Meeting 2005, abstract #P11B-0109
Other
6020 Ices, 6280 Saturnian Satellites
Scientific paper
We will present near IR spectra of ice mixtures and review the differences between spectra of mixtures and those of pure solids: the creation of new features, weakening and shifting of bands, dependence on concentration, and changes with temperature. The forbidden CO2 (2ν3) overtone at 2.134 microns (4685 cm-1) is absent from the spectrum of pure CO2, but present in all of the following mixtures: H2O/CO2 = 5 and = 25, H2O:CH3OH:CO2 = 100:2.5:1, and CH3OH:CO2 = 5. Also, in mixtures of H2O and any other material, we see a feature at 1.89 microns (5290 cm-1) that is possibly related to the "dangling OH" feature at 2.73 microns (3360 cm-1). The features of a material in H2O are generally weaker and shifted to longer wavelength in comparison to the pure substance. For example, the largest near-IR absorption of pure solid CH4 is located at 2.324 microns (4303 cm-1) but is broader and at slightly longer wavelength in samples mixed with H2O. The degree of shifting and weakening depends on the ratio of the mixture. The mixture mentioned above was at a ratio of H2O/ CH4 = 3. When the ratio rises to H2O/ CH4 = 87, the CH4 feature at 2.324 microns is shifted to shorter wavelength and is much broader and weaker. In CH4/ H2O mixtures the peaks shift to higher frequency and become increasingly broad, but this trend is reversible on re-cooling, even though the phase transitions of H2O are irreversible. In short, mixtures created in the lab produce spectra that are very different from modeled combinations of end member species. Recent Cassini VIMS observations show the CO2 fundamental at 4.255 microns (2350 cm-1) on Iapetus [1] and at 4.26 microns (2347 cm-1) on Phoebe [2], while Galileo NIMS observed it at 4.25 microns (2353 cm-1) on Ganymede [3]. Since pure CO2 is located at 4.266 (2344 cm-1), the CO2 must be mixed with something else to produce the shift. A mixture of CH3OH:CO2 = 5 at 90 K shifts the fundamental to 4.262 microns (2346 cm-1). The shifts in the feature between satellites could indicate variations in concentration and temperature, or the introduction of a new compound, encouraging further lab investigation. 1. Buratti, B.J., Astrophys. J., 2005. 622: p. L149-L152. 2. Clark, R.N., Nature, 2005. 435: p. 66-69. 3. McCord, T.B., Science, 1997. 278: p. 271-275.
Bernstein Max P.
Mastrapa Rachel M.
Sandford Scott A.
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