Saturn's moon Dione, in the foreground of this Cassini spacecraft image, appears darker than the moon Tethys.
Tethys appears brighter because it has a higher albedo than Dione, meaning Tethys reflects more sunlight. This higher albedo is due to Tethys being closer to the moon Enceladus and the E ring. Bright debris spews from Enceladus, feeding the E ring. This debris then coats Enceladus and Tethys with bright material. See Casting a Shadow and Bursting at the Seams to see images of Enceladus's plume and the E ring.
Because of the viewing geometry, lit terrain seen here is on the anti- Saturn side of Dione (1,123 kilometers, or 698 miles across) and the leading hemisphere of Tethys (1,062 kilometers, or 660 miles across).
The image was taken in visible green light with the Cassini spacecraft narrow-angle camera on March 23, 2010. The view was obtained at a distance of approximately 1.2 million kilometers (746,000 miles) from Dione and at a Sun-Dione-spacecraft, or phase, angle of 88 degrees. The view was obtained at a distance of approximately 1.8 million kilometers (1.1 million miles) from Tethys and at a Sun-Tethys-spacecraft, or phase, angle of 88 degrees. Image scale is 7 kilometers (4 miles) per pixel on Dione and 11 kilometers (7 miles) per pixel on Tethys.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. 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 and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo.
Credit: NASA/JPL/Space Science Institute