Although both satellites have about the same brightness, Io's color is very different from Europa's. The poles are darker and reddish.
Preliminary evidence suggests color variations within and between the polar regions.
Io's surface composition is unknown, but scientists believe it may be a mixture of salts and sulfur. Europa is less strongly colored, although still relatively dark at short wavelengths. Markings on Europa are less evident than on the other satellites, although this picture shows darker regions toward the trailing half of the visible disk. This image was obtained before JPL started copyrighting their images and is in the public domain. All rights reserved.