Why Charon, Pluto’s largest moon, wears a red cap

Charon, Pluto’s largest moon, wears a red cap. Charon’s north pole has a dark red color, starkly different from its otherwise gray-white surface, and scientists have finally found out why.

Using a healthy dose of math and data from NASA’s New Horizons mission, planetary scientists calculated how this red cap formed. They report that Charon steals gases from neighboring Pluto, which supports a popular theory that Pluto and Charon exist as a binary planet.

“This is the first example we’ve seen of a planet’s escaping atmosphere polluting the surface of its moon in colorful ways,” Will Grundy, co-author of the report and Lowell Observatory planetary scientist, told the NewsHour. “Over the past half-century of spacecraft exploration, we’ve seen numerous worlds with all kinds of surprising things, sculpting their surfaces, and yet we hadn’t seen something like this before.”

Grundy and his colleagues found Pluto releases colorless methane gas, which gets trapped in Charon’s gravity. The gas freezes onto Charon’s northern pole during its hundred year-long winters. When the icy pole finally gets some sunlight, it creates a chemical reaction, transforming the methane into red-colored molecules called tholins that are too heavy to escape back into space.

Grundy said this finding, reported today in the journal Nature, reinforces the idea that Pluto and Charon are a “double planet.” Most moons in our solar system are really small compared to their planets. But Charon is almost the same size as Pluto, leading to exchanges in surface materials (like the methane in Charon’s red cap) or even shifts in the center of mass for the entire planet. If Charon wasn’t already orbiting Pluto, it could potentially be seen as its own planet. The Earth and the moon are the next closest thing to a “double planet” in our solar system, to scientists’ knowledge.

Other dwarf planets exist in our solar system, like Pluto’s neighbors Eris and Makemake, which may have a similar relationship. But it’ll be several years before a spacecraft reaches the dwarf planets to get close enough to look.

“Every time we explore something new, we find new surprises,” Grundy said. “Nature is amazingly inventive in using the basic laws of physics and chemistry to create spectacular and distinctive landscapes.”