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 salt and sulphur. Europa is less strongly colored, although still relatively dark at short wavelengths. Markings on Europa are less evident than on other satellites, although this picture shows darker regions toward the trailing half of the visible disk.