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December 13th, 1999

Thermal Inertia of Saturn's Ring Particles

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Thermal Inertia of Saturn's Ring Particles
Credit: Image courtesy of C. Ferrari, P. Galdemard, P.O. Lagage, & E. Pantin
Equipe GammaG, Université Paris 7, France
Service D'Astrophysique, DAPNIA/DASM/CEA Saclay, France

(Presented at the 31st Annual Meeting of the Division of Planetary Science of the American Astronomical Society

Planet Saturn is seen here in the mid-infrared, at 20.5 microns. The image was obtained with CAMIRAS attached to the CFHT. As opposed to images taken in the optical where the rings reflect the sunlight, Saturn's rings are visible here because of the thermal emission of the dust particles they contain.

This CAMIRAS image allows to estimate the grain properties in Saturn's rings. Clearly, the rings on one side of Saturn are warmer (red), while the particles on the opposite side are cooler (yellow). Notice the color-coded temperature scale at the bottom of the image. The sun is shining on Saturn almost from behind you (the reader) on the image, and the particles are orbiting the planet clockwise. The particles warm up on the sunny side and cool down in the eclipse zone. The eclipse zone is behind Saturn on this image, it is at the bottom and not visible directly. The particles come out of the eclipse zone from the bottom left where they appear the coolest. They rapidly warm up along their orbit as they receive energy from the Sun. They finally enter the eclipse zone again (at bottom right this time) where they cool down again to start a new cycle.

The thermal inertia and the mean particle size in Saturn's ring A, B & C can be estimated from the cooling and heating rates measured by CAMIRAS at CFHT. The observed temperature gradient between both sides is 3 degrees Kelvin, suggesting a low thermal inertia. The same effect (and gradients) is seen in all three rings (A, B, and C). The rings are not distinguishable on this image but their positions are indicated on this one. The images of Saturn obtained by the CAMIRAS group will help prepare the CASSINI space mission.

Technical description: This images was obtained on July 29th, 1999, with CAMIRAS attached the to Canada-France-Hawaii telescope. CAMIRAS is based on a Rockwell 256X256 mid-infrared detector, cooled down to liquid Helium temperature, and operating in the 20 microns atmospheric window. At these wavelengths, the thermal emisson by cold dust is images in Saturn's ring is nicely revealed. This image was obtained at 8h00AM Hawaii time, i.e., during day time, long after sunrise!

next week: A Christmas deconvolution artefact?
available December 23rd only

editors: François Ménard & Jean-Charles Cuillandre
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CFHT is funded by the Governments of Canada and France, and by the University of Hawaii.