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

Hanel, R., et al. 1981. Infrared Observations of the Saturnian System from Voyager 1 Science 212 (4491), 192-200. (Excerpt from pp. 198-199.)

Copyright AAAS, April 10, 1981.

Infrared Observations of the Saturnian System from Voyager 1

Rings. A pair of observations made 2 hours apart have been used to determine the temperature and opacity of a portion of the C ring. Centered at 1.40 Saturn radii, the instrument footprint decreased from one-quarter to one-eighth of the annular width of the ring between the observations; in the same interval, the emission angle decreased from 85.8 degrees to 82.6 degrees. If scattering is ignored, the observed intensity from the ring may be written I(upsilon) = B(upsilon,T)(1 - e^[tau(upsilon)sec theta]), where I(upsilon) and tau(upsilon) are the observed radiance and the optical depth normal to the ring, respectively, as functions of wave number upsilon; B(upsilon,T) is the Planck function; T is the ring temperature; and theta is the emission angle, measured from the ring-plane normal. The derived temperature, 85 +/- 1 K, and the optical depth of the ring, 0.09 +/- 0.01, are shown in Fig. 11. They are quite consistent with the values obtained by Froidevaux and Ingersoll (40) from Pioneer data. The lack of an increase in opacity with wave number indicates that the ring cross section is not dominated by particles with radii of the order of 1 micrometer.

Diagnostic spectral features of such components as water ice, with an absorption near 227 cm^-1, and silicates, many of which have SiO bending modes between 400 and 500 cm^-1, are absent from the spectrum. If either of these materials is present in the ring, this lack of features is also consistent with the absence of a dominant fraction of micrometer-sized particles. A similar lack of spectral features in the thermal infrared was observed for Europa (2), al though the satellite is almost entirely covered with water ice. The absence of an infrared signature is due to the low thermal contrast established across the porous surface layer. This, in turn, suggests that a "regolith" exists on the ring particles.

Additional information on the C ring particles was obtained from two observations of the passage of ring material into the planetary shadow. An observation at low phase angle showed a drop in temperature as the particles passed from light to dark (determination of the actual cooling rate requires pointing data that are not yet available). A similar observation at high phase angle showed a nearly constant low temperature under the same conditions. This indicates that the ring particle size distribution is not dominated by particles less than a few millimeters in size and that the particles are not rapidly rotating.

Table 3 summarizes preliminary brightness temperatures for the illuminated and unilluminated sides of the classical rings. Except for the C ring data, these data are uncorrected for optical depth and the geometry of the observations, and so represent lower limits to actual temperatures. All the results appear consistent with values obtained from Pioneer (40).

Following occultation of the sun by the planet, additional ring data were obtained by viewing the sun as it was occulted by the rings. The observation, made with the off-axis calibration port of IRIS (1.6 degree diameter field of view), began when the sun was behind the B ring and extended until it was well beyond the F ring; the data, taken from the radiometer channel, are shown in Fig. 12. The Cassini division, Encke division, and F ring are all apparent. Comparison of the observations with those from the Pioneer 11 photopolarimeter (41) shows good agreement. The apparent reversal of features in the Encke division and the F ring is due to the difference between direct transmission (IRIS) and diffuse transmission (Pioneer) in these optically thin regions. The structure outside the F ring must be carefully scrutinised; pointing offsets which place the sun near the edge of the instrument field of view could cause spurious intensity changes. Analysis of the data is further complicated because the projection of the sun in the rings is greater than the width of the Cassini division. With estimated uncertainties on the order of a factor of 2, the normal optical depth of the Cassini division is 0.05; assuming that the data are not contaminated by pointing difficulties, the normal optical depth near 160,000 km is 2e-3.

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