Charon

"The largest of five moons of Pluto, Charon is a very large moon in comparison to its parent body: it has half the diameter and one eighth the mass of Pluto, making it look almost more like a binary planetary system than a planet and moon.In September 2016, astronomers announced that the reddish-brown cap of the north pole of Charon is composed of tholins, organic macromolecules that may be essential ingredients of life. This is one of many mysteries your explorers will be able to investigate upon arrival."