As mentioned earlier, we usually don't see surface changes while they're taking place on other planets because they occur over long periods of time. In a few cases, though, we have been able to observe changes almost as they're happening.

The image on the left is from the Viking lander on the surface of Mars. The image on the right show Jupiter's moon Io, as seen by the Voyager and Galileo spacecraft.

  !   Move your cursor over each image to see another view of the same place at a different time. Examine the before-and-after images, and consider what might be responsible for the observed changes.

Need some hints? On the left, the white coating on the ground was visible just after Martian sunrise, and eventually disappeared as the sun climbed higher in the sky. On the right, the dark circular feature that appears in the "after" image has some kind of hole or crater near its center.

9. What processes have we seen in action on Mars? What processes have we seen on Io?