HR: 0800h
AN: P11B-0109    [Abstracts]
TI: Near Infrared Spectra of Mixtures Relevant to Icy Satellites
AU: * Mastrapa, R M
EM: rmastrapa@arc.nasa.gov
AF: NASA Ames Research Center, Mail Stop 245-6, Moffett Field, Ca 94035-1000 United States
AU: Bernstein, M P
EM: mbernstein@arc.nasa.gov
AF: NASA Ames Research Center, Mail Stop 245-6, Moffett Field, Ca 94035-1000 United States
AU: Sandford, S A
EM: ssandford@arc.nasa.gov
AF: NASA Ames Research Center, Mail Stop 245-6, Moffett Field, Ca 94035-1000 United States
AB: 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.
DE: 6020 Ices
DE: 6280 Saturnian satellites
SC: Planetary Sciences [P]
MN: Fall Meeting 2005