Bruce Murray Space Image Library
The colors of Saturn's moons
Brightness is a function of "phase angle" -- they will look brighter if the Sun is more "overhead" -- so Ugarkovic attempted to choose images lit at phase angles of approximately 50 degrees. (Prometheus and Pandora are much more fully lit at 10 and 19 degrees, so they appear brighter than they should.) Enceladus is clearly the brightest of the moons. Mimas, which is closer to the ring system, is rather darker. Tethys and Rhea are similar in color and brightness, with a slightly reddish or orangish case to their gray surfaces, while Dione is much grayer and is stained dark near the "wispy terrain" at the bottom of the image. Iapetus' Cassini Regio -- its dark-stained leading hemisphere -- is practically invisible. Hyperion is nearly as dark, and even Titan appears dim (though it makes up for dimness by its large size). Unlike the raw images available immediately on the Internet, the data released a year or so later to the PDS includes information on the proper calibration of the images. Calibration allows images taken at different times and of different targets to be compared in this fashion.