To this end, we have developed an iterative coregistration procedure to register and ratio images taken at different times, to highlight any differences which may be present. For Europa, we are in the process of a systematic search for any changes which may have occurred in the almost 20 years between the Voyager and Galileo images. If changes due to recent geologic activity can be identified, this would support the theory that a layer of liquid water currently exists beneath Europa's icy surface. Only subtle differences are apparent, which are most likely due to the variations in photometric behavior of different surface units at these scales. We cannot confidently identify any changes due to current geologic activity.
Further work is needed to account for the photometric behavior of different surface units.
Changes on Io are frequent and widespread, so we have performed comparisons between images taken on different Galileo orbits to document the style and rate of volcanic resurfacing and other geologic activity. Io's photometric function is more complex than Europa's, meaning that comparisons are best made between images taken at similar phase angles. The most pronounced changes on Io seen during the Galileo mission occur around known active volcanic centers such as Kanehekili, Pillan, and Zamama. Intriguing changes are also apparent near the North and South Poles.