Physics
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.p43b..03s&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #P43B-03
Physics
5400 Planetology: Solid Surface Planets, 5464 Remote Sensing, 5470 Surface Materials And Properties, 6062 Satellites, 6280 Saturnian Satellites
Scientific paper
The Cassini Composite Infrared Spectrometer (CIRS) has three focal planes that together cover the thermal spectrum from 10 to 1420 cm-1 (1 mm to 7 μ m). Focal plane 1 (FP1) covers 10 to 600 cm-1 with a circular field of view of 3.9 milliradians diameter. Focal plane 3 (FP3) covers 600 to 1100 cm-1, and focal plane 4 (FP4) covers 1100 - 1420 cm-1, both with a linear array of 10 pixels, each 0.3 milliradians square. Spectral resolution is selectable from 0.5 to 15.5 cm-1 (apodized). During the Cassini flyby of Phoebe on June 11 th 2004, CIRS obtained numerous observations of thermal emission from its surface with all three detectors, though FP4 only detected emission from the very warmest regions. FP3 achieved 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 signal to noise ratio (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 Cassini visible-wavelength 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. FP1 had lower spatial resolution than FP3 (near 9 km at closest approach), but higher S/N, allowing extraction of spectral information. At 50 cm-1 (200 μ m) brightness temperatures varied from 68K in predawn locations, to 101K near the subsolar point. However, strong spectral gradients were apparent in all spectra, with brightness temperatures at 400 cm-1 (25 μ m) in the above instances varying from 76K to 111K. No strong emissivity variations are present. Consequently, these spectral characteristics are due to a combination of unresolved thermal inertia and slope (shadowing) variations. These effects will be discussed.
Pearl John C.
Segura M.
Spencer John Robert
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