Laboratory studies of the newly discovered infrared band at 4705.2 cm -1 (2.1253 μm) in the spectrum of Io: The tentative identification of CO 2
We discuss over 120 laboratory experiments pertaining to the identification of the new absorption band discovered by Trafton et al. (1991) at 4705.2 cm -1 (2.1253 μm) in the spectrum of Io. It is shown that this band is not due to overtones or combinations of the fundamental bands associated with the molecules (or their chemical complexes) already identified on Io, namely, SO 2, H 2S, and H 2O. Thus, this band is due to a new, previously unidentified, component of Io. Experiments also demonstrate that the band is not due to molecular H 2 frozen in SO 2 frosts. Since the frequency of this band is very close to the first overtone of the ν 3 asymmetric stretching mode of CO 2, we have investigated the spectral behavior of CO 2 under a variety of conditions appropriate for Io. The profile of the Io band is not consistent with the rotational envelope expected for single, freely rotating, gaseous CO 2 under Io-like conditions. It was found that pure, solid CO 2 and CO 2 intimately mixed in a matrix of solid SO 2 and H 2S produce bands with similar widths (5-10 cm -1), but that these bands consistently fall at frequencies about 10-20 cm -1 (∼0.007 μm) lower than the Io band. CO 2 in SO 2: H 2S ices also produces several additional bands that are not in the Io spectra. The spectral fit improves, however, as the CO 2 concentration in SO 2 increases, suggesting that CO 2CO 2 interactions might be involved. A series of Ar: CO 2 and Kr: CO 2 matrix isolation experiments, as well as laboratory work done elsewhere, show that CO 2 clustering shifts the ands position to higher frequencies and provides a better fit to the Io band. Various laboratory experiments have shown that gaseous CO 2 molecules have a propensity to cluster between 80 and 100 K, temperatures similar to those found on the colder regions of Io. We thus tentatively identify the newly discovered Io band at 4705.2 cm -1 (2.1253 μm) with CO 2 multimers or "clusters" on Io. Whether these clusters are buried within an SO 2 frost, reside on the surface, or are in a residual, steady-state "atmospheric aerosol" population over local coldtraps is not entirely clear, although we presently favor the latter possibility. The size of these clusters is not well defined, but evidence suggests groups of more than four molecules are required. The absorption strength of the 2ν 3CO 2 cluster overtone determined in the laboratory, in conjunction with the observed strength of the Io band, suggests that the disk-integrated abundance of CO 2 is less than 1% that of the SO 2. Studies of the sublimation behavior of CO 2 indicate that it probably resides predominantly in the cooler areas (< 100 K) of Io. The relative constancy of the Io feature over a variety of orbital phases suggests that the polar regions may contain much of the material. Some consequences of the physical properties of CO 2 under conditions pertinent to Io are discussed. The presence of CO 2 clusters on Io could be verified by the detection of any one of several other infrared bands associated with the CO 2 molecule, of which the strongest are the ν 312CO 2 asymmetric stretch fundamental near 2350 cm -1 (4.25 μm) and the ν 2 bending mode fundamental near 660 cm -1 (15.1 μm). Weaker bands that may also be detactable include the ν 313CO 2 asymmetric stretch fundamental near 2280 cm -1 (4.39 μm), the 2ν 2 + ν 3 combination/overtone band near 3600 cm -1 (2.78 μm), and the ν 1 + ν 3 combination band near 3705 cm -1 (2.70 μm).