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E. Karkoschka, M. Tomasko (Univ. of Arizona)
For centuries, observers of Saturn have noticed its shadow onto its rings. It always appeared as sharp, round, featureless, and without color. Our images taken with the Hubble Space Telescope on March 7, 2003 revealed that the outermost part of the shadow is orange, similar to the color of the moon during a deep total lunar eclipse. While the outer part of Earth's umbra displays a bluish illumination due to stratospheric ozone, the umbra can become black due to stratospheric aerosols of low vertical optical depths, present after volcanic eruptions. The umbral color and brightness is remakably sensitive to weak stratospheric extinction from gases and aerosols. Similarly, we explored weak absorptions in Saturn's stratosphere by measuring its shadow onto the rings.
For each of 36 images in 28 filters, spanning wavelengths 0.26 - 2.2 micrometers, and for each of about 100 ringlets, we measured the location at the edge of Saturn's shadow where the intensity drops to half of the intensity outside the shadow. Theoretical calculations, taking into account refraction and extinction in Saturn's stratosphere, typically agree with our measurements throughout the A-, B-, and C-ring. At continuum wavelengths, the agreement is about 0.1 pixel, indicating a relatively clear stratosphere.
At methane absorption wavelengths, our measurements constrain methane absorption coefficients at 1 mb pressure, 140 K temperature, and 5000 km pathlength, which are very far from any laboratory conditions. Our data will help the analysis of methane band images of the Jovian planets where probed parameters are typically between those in laboratories and those for our shadow measurements.
This work is supported by STScI grant HSTGO0935401A.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.