(Top row, from left) Titan, Earth’s moon, Europa and Enceladus. (Bottom row, from left) Callisto, Charon, Ariel and lo.

(Top row, from left) Titan, Earth's moon, Europa and Enceladus. (Bottom row, from left) Callisto, Charon, Ariel and lo.

(Top row, from left) Titan, Earth’s moon, Europa and Enceladus. (Bottom row, from left) Callisto, Charon, Ariel and lo.

(Top row, from left) Titan, Earth’s moon, Europa and Enceladus. (Bottom row, from left) Callisto, Charon, Ariel and lo.