A visible-light image of the geologically active trailing hemisphere of the Jovian moon Io, taken on 15 March 1992, when Io was 414 million miles from Earth. HST resolves features as small as 150 miles across.
An ultraviolet light (UV) picture of the same hemisphere show's Io's surface. Regions which look bright In visible light are dark in UV. The most likely explanation is that large areas of Io are covered with a sulfur dioxide frost. Because sulfur dioxide is a strong absorber of UV radiation sulfur dioxide-rich areas are dark in the UV though they are bright in visible light.
An image of Io taken 13 years ago by the Voyager spacecraft at a distance of approximately 250 000 miles.
A "synthetic" Voyager Image modified to match HST's resolution, due to the fact that HST is one thousand times farther from Io then Voyager was at closest approach. A direct comparison of the visual FOC visible light image with this synthetic Voyager image shows no change in the large scale distribution of surface materials in the 13 years which have elapsed between the two observations.
HST/FOC images: Francesco Paresce (ESAI STScI) Paola Sartoretti, University of Padova
Voyager Image: NASA