Why Pluto's Big Moon Charon Has a Red Polar Cap

The north pole of Pluto's big moon Charon likely gets its reddish color from radiation-altered shreds of Pluto's atmosphere, scientists say. Charon's surface is dominated by water ice, and the 750-mile-wide (1,200 kilometers) moon is mostly a solid grayish-white as a result. New Horizons found that Pluto's surface is reddish-brown as well, and that's no coincidence: The red on both bodies is likely caused by complex compounds called tholins, researchers say.