of Callisto, the fourth Galilean satellite, looks similar to the dark
regions on the surface of Ganymede. The surface of Callisto is covered
with numerous impact craters, indicating little geologic activity has
taken place here since the intense bombardment associated with the early
history of the solar system. The Valhalla impact structure consists of
a bright central region roughly 600 kilometers (400 miles) across
(near the top of the photograph) with a series of concentric bright rings
surrounding the central region. Voyager 1 obtained this
view of Callisto in March 1979.
image (Press Release P-21287).
click here to download a high-resolution version of the image (9.16 MB)