Saturn's moon Dione, in the foreground of this Cassini 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 which coats these moons in fresh, bright debris spewing from Enceladus. See PIA08921 and PIA11688 to learn more.
Because of the particular viewing geometry here, lit terrain seen here is on the anti-Saturn side of Dione (1123 kilometers, 698 miles across) while lit terrain on Tethys (1062 kilometers, 660 miles across) is on the leading hemisphere of that moon.
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 (745,000 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 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.