When NASA’s New Horizons probe sent back pictures of Pluto’s largest moon, Charon, researchers were surprised to see a big red spot on its north pole. More than a year later, they’ve published their best explanation for its origin.
The process begins when molecules of methane escape from Pluto’s thin atmosphere. Those molecules are drawn to Charon, a mere 12,200 miles away, by the moon’s gravitational pull. The rarefied methane gas freezes out and settles onto the surface as ice.
Methane ice piles up when it’s winter in the north, but when the season turns toward spring, the northern polar region is exposed to sunlight. The sun’s ultraviolet rays cook the methane into a mix of hydrocarbons.
As the ice warms up, any methane that remains thaws back into gas. But the heavier hydrocarbons stick around on the surface, and get cooked into reddish organic compounds known as tholins.