12031996 View of Callisto from Voyager and Galileo View of Callisto, most distant of the four large moons of Jupiter.
North is to the top of the picture.
This could be dark ejecta from a small impact crater. This is the first optical navigation picture of Callisto taken on the approach to the first Callisto encounter. The spacecraft can either transmit a signal based on an onboard oscillator or coherrently transpond a signal received from a tracking station. The second of these modes is more useful for navigation because the returning signal is measured against the same frequency reference at the tracking station that created the originally transmitted signal.