An interesting relationship is the cause of Pluto's moon, Charon's strange red spot, scientists say. Will Grundy, planetary scientist of the Lowell Observatory in Arizona, explained how he and his colleagues (a group that is part of NASA's New Horizons mission, which involved a fly-by of Pluto in 2015) modeled the co-opted atmosphere of Pluto and its moon Charon, and how this shared atmosphere has created this peculiar reddish mark on Charon's surface. What causes the red spot on Pluto's companion? Grundy provides a very good, scientific answer in the article at the above link...but I'm going to simplify it. Imagine you have two yoyos (Pluto and Charon), each on the end of a single string (atmospheric molecules). Charon yoyo goes round and round Pluto yoyo, and Charon yoyo's strong gravitational pull causes the string to drag on Charon's surface and wrap tightly around the (north) pole. The atmospheric molecules condense and freeze, and the result is the rust-colored stain across a large portion of the north pole.