CICLOPS: Cassini Imaging Central Laboratory for OPerationS

Spotlight on Penelope
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Spotlight on Penelope
PIA 11624

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  Cassini spies the large Penelope crater on Saturn's moon Tethys.

Penelope crater lies near the center of the image. See PIA11495 to learn more about the prominent features on Tethys. This view looks toward the trailing hemisphere of Tethys (1062 kilometers, 660 miles across). North on Tethys is up.

The image was taken with the Cassini spacecraft narrow-angle camera on Oct. 14, 2009 using a spectral filter sensitive to wavelengths of ultraviolet light centered at 338 nanometers. The view was obtained at a distance of approximately 256,000 kilometers (159,000 miles) from Tethys and at a Sun-Tethys-spacecraft, or phase, angle of 1 degree. Image scale is 2 kilometers (5,022 feet) per pixel.

The Cassini Equinox Mission is a joint United States and European endeavor. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter was designed, developed and assembled at JPL. The imaging team consists of scientists from the US, England, France, and Germany. The imaging operations center and team lead (Dr. C. Porco) are based at the Space Science Institute in Boulder, Colo.

For more information about the Cassini Equinox Mission visit, and

Credit: NASA/JPL/Space Science Institute
Released: November 16, 2009 (PIA 11624)
Image/Caption Information

Alliance Member Comments
PeterDarmady (Dec 21, 2009 at 5:55 AM):
Tethys looks as if it made of some kind of sponge - or sorbet! Now back on line and loving the images! Well done you guys!
Red_dragon (Nov 20, 2009 at 7:38 AM):
Great explanation and image, thanks.
20tauri (Nov 16, 2009 at 4:23 PM):
Wow, neat. I had a feeling there might be some sort of optical reason, as I've seen plenty of icy bodies showing up way darker than this (or at least with higher contrast) in images. Thanks for that thorough explanation!
carolyn (CICLOPS) (Nov 16, 2009 at 3:24 PM):
20Tauri and PolishBear: The reason why Tethys is so bright is BECAUSE the phase angle is near zero. All the solid, airless bodies in the solar system, even the rings, show an increase in brightness near zero phase called `the opposition surge'. (See the opposition effect on the rings in , and ignore the `rainbow' because that's an artifact.) It's a well known observational phenomenon and has its origin in 3 different effects. One, the roughness of the surface will not produce shadows at zero phase (or thereabouts), as it would at other phase angles, because the sun is directly behind the observer: that's what zero phase means. Second, one particle or body on the surface won't cast a shadow on its neighbors at or near zero phase, either. Third, there is something called 'coherent backscatter' which is multiple light scattering that occurs between microscopic grains on a surface that is observable very close to zero phase. The opposition effect can be seen in images taken of the Apollo astronauts of their own shadows: the surface immediately surrounding the astronaut's shadow is peculiarly bright. Anyway...more than you probably bargained for, but that is why Tethys is so bright in this nearly fully illuminated geometry.
PolishBear (Nov 16, 2009 at 10:25 AM):

The density of Tethys is 0.97 g/cm³, indicating that it is composed almost entirely of water-ice. Its surface is one of the most reflective (at visual wavelengths) in the solar system, with a visual albedo of 1.229.
20tauri (Nov 16, 2009 at 9:25 AM):
Ooh, pretty. Seems particularly bright...does Tethys have a naturally high albedo or is that an imaging byproduct?

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