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J. R. Spencer (Southwest Research Institute), J. C. Pearl, M. Segura (NASA-Goddard), Cassini CIRS Team
Cassini's Composite Infrared Spectrometer (CIRS) obtained numerous observations of Phoebe's thermal emission during the June 12th 2004 flyby. The highest spatial resolution was achieved with CIRS's FP3 detector, which measured the thermal emission from the surface between 9 and 17 \mum with a spatial resolution as small as 12 km for full-disk observations, and 600 m for local observations near closest approach. Spectral resolving power for most observations was about 50. The S/N of the FP3 observations allows measurement of brightness temperatures as low as ~ 75 K but is not sufficient to detect any deviations from blackbody behavior in the Phoebe data.
There is strong topographic control of temperature, particularly around the prominent large crater seen in ISS images, where temperatures were observed over nearly half a Phoebe rotation. Low-latitude temperatures on Phoebe vary between 82 K before dawn to 112 K near the subsolar point. The diurnal variation can be matched with a thermal inertia near 3 x 104 erg cm-2 s-1/2 K-1, about half the thermal inertia derived from diurnal temperature variations on the Galilean satellites but similar to that of Rhea, Dione, and Tethys. This low thermal inertia implies that the upper centimeter of Phoebe's surface is covered in very porous material.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.