Approximate-natural-colour (left) and false-colour (right) pictures of Callisto, one of Jupiter's satellitesNear the centre of each image is Valhalla, a bright area surrounded by a scarp ring (visible as dark blue at right).Valhalla was probably caused b
Science

This or That?: Moon vs. Asteroid

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Photo NASA/JPL/Caltech (NASA photo # PIA01298)
Photo NASA/JPL/Caltech (NASA photo # PIA01298)