Other
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p23d1737b&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P23D-1737
Other
[6223] Planetary Sciences: Solar System Objects / Callisto
Scientific paper
The analysis of the mid-infrared spectrum of Callisto described in the following was carried out with the intention to remove ambiguities in the identification of spectral features at shorter wavelengths. These concerned in particular the presence of sulfur dioxide on the surface of this moon. A detection of this molecule was first claimed by Noll et al. on the basis of an absorption band centered at 280 nm (36,000 cm-1) in a spectrum taken of Callisto's leading hemisphere with HST, but OH and organics also absorb in this region of the UV. Callisto's near-infrared spectrum, taken at a later date with NIMS, did not provide a clear answer, either, although it showed an absorption band at 4 μm (2490 cm-1), the wavelength of the ν1 + ν3 combination of SO2. The reason was in this case that the presence of carbonates provided an unlikely, but nevertheless possible, alternative explanation. Searching for features in the mid-infrared, we analyzed the spectrum of Callisto taken with the Photo-Polarimeter (PHT) aboard the ISO satellite (PI: Th. Encrenaz). Disk-integrated spectra of Callisto were recorded on 1997-05-23, between 2.5 and 4.9 μm (PHT-SS) and between 5.8 and 11.6 μm (PHT-SL). As data with much better signal-to-noise ratio are now available in the near-infrared wavelength range from NIMS on the Galileo spacecraft and VIMS on Cassini, we only present the data from PHT-SL. The astronomical observing template PHT40 was started on Callisto at 4:41 (UT). At that time the apparent planetographic longitude of its center amounted to 72° west, close to the center of the leading hemisphere. The figure shows the emission spectrum of Callisto obtained with PHT-SL. Reflected sunlight contributes only a small part to the measured flux density. The positions of the strongest SO2 bands are marked. We note that at wavenumbers smaller than 1150 cm-1 the large error bars render the spectrum almost useless. Given the possibility of an absorption feature at the exact position of the ν3 band of sulfur dioxide at 1306 cm-1 it seems conceivable that the thermal infrared provides useful information about the composition of Callisto's surface. An in-depth study of the thermal emission from the (sub)surface of Callisto must be performed, however, before firm conclusions about the presence of SO2 or other molecules can be drawn. It is also desirable to obtain spectra of Callisto over a larger wavelength range than was accessible to ISOPHOT, for example with SOFIA.
Burgdorf Martin J.
Cruikshank Dale P.
Dalle Ore Cristina Morea
Encrenaz Th.
Quirico Eric
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