Acknowledgement. We thank Science for their permission to use an excerpt from:

Hanel, R., et al. 1982. Infrared Observations of the Saturnian System from Voyager 2 Science 215 (4532), 544-548. (Excerpt from pp. 547-548.)

Copyright AAAS, January 29, 1982.

Infrared Observations of the Saturnian System from Voyager 2

Rings. Approximately 9 hours before closest approach to Saturn a scan was made from outside the A ring to within the C ring (Fig. 8). The width of the IRIS field of view projected on the ring plane was about 3500 km, or approximately the width of the Cassini division. The intensity scan in reflected sunlight (Fig. 8a) shows radial structure virtually identical to that in a solar transmission observation by IRIS from Voyager l; in that scan, which extended from the middle of the B ring to well beyond the F ring [see figure 12 in (3)], the projected solar disk diameter was also approximately equal to the width of the Cassini division.

The ring particle temperatures and the mean ring optical depth in the infrared can be estimated from the thermal spectra obtained simultaneously with the reflected solar radiation data. A simple model considers the ring as a thin, homogeneous, nonscattering slab that radiates as a blackbody. The observed thermal emission as a function of wave number, I_nu, will then be related to the blackbody emission of the material, B_nu(T_ring), by

I_nu = (1 - e^-[tau_ring/cos(theta)]) B_nu(T_ring)

where T_ring and tau_ring are the blackbody temperature and normal infrared optical depth of the ring material and theta is the emission angle measured from the ring plane normal.

From averages of pairs of spectra in the ring scan, intensities at 200 and 400 cm^-1 were used to derive normal infrared optical depths and ring temperatures (Fig. 8, b and c). If the system is optically thick the optical depth is poorly determined because of sensitivity to noise, as is evident for the B ring (Fig. 8b). The results are summarized in Table 1. The results for the C ring are in agreement with those from Pioneer (15) and are identical to those derived from Voyager 1 data by applying the above model to observed variations of infrared intensity with emission angle (3). No similar calculations were made for the A and B rings with Voyager 1 data. The normal optical depths of the A and C rings and of the Cassini division are in agreement with values at similar spatial resolution at shorter wavelengths (11, 15, 16). Discrepancies may exist with longer wave length observations (15, 17). The sensitivity of the simple model to poise for an optically thick system and the neglect of phase angle corrections and of mutual shadowing among ring particles make detailed quantitative comparison of these results with other data premature.

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