This stereo image illustrates the topography of the Tvashtar
Catena region on Jupiter's moon Io. It was created by combining
two different views of Tvashtar taken by NASA's Galileo
spacecraft on November 25,1999 (shown in red) and February 22,
2000 (shown in blue). A raised plateau surrounds the volcanic
depression, or caldera, in the center of the image. To the
northeast of the main caldera, the plateau's inner and outer
margins are scalloped, which may indicate that a process called
sapping is eroding them. Sapping occurs when fluid escapes from
the base of a cliff, causing the material above it to collapse.
Smaller calderas have formed in the floor of the main caldera.
This nesting of calderas is also observed on Earth, at Kilauea
in Hawaii. (The two bright red regions toward the upper left of
this image, which are roughly triangular in shape, are the areas
where the earlier image was overexposed by the brightness of hot
lava fountains). Galileo scientists are in the process of
generating topographic maps