NASA discovers the cause of red spot on Pluto’s moon Charon

Researchers may have finally found the cause of the large red spot at the north pole of Pluto’s largest moon, Charon: the satellite and the planet which it orbits share an atmosphere, and methane gas escaping from Pluto is being trapped and frozen by Charon, according to a new study.

The red spot, discovered by NASA’s New Horizons spacecraft during its historic July 2015 flyby of Pluto and its moons, is caused when trapped methane gas from Pluto’s atmosphere is frozen to Charon’s icy surface at the pole, then chemically transformed first into hydrocarbons, then into a reddish type of organic materials known as tholins, by ultraviolet radiation.

As lead author Will Grundy, a planetary scientist at the Lowell Observatory in Arizona and part of the New Horizons mission team, told via email, “Methane is volatile enough that it can only stick to the surface during the long, cold polar winters.” Models created by Grundy and his colleagues found that the UV rays eventually stripped the methane of its hydrogen.

That activity left behind carbon, which paired with other molecules to produce a substance which persisted even when surface temperatures increased. Those remaining hydrocarbons continued to accumulate and become increasingly carbon-rich, and over a period of several million years, they become tholins that produce the reddish hue, the authors wrote in the journal Nature.

Polar temperatures, length of winter led to methane accumulation

Based on data collected by New Horizons following its flyby of Charon, Grundy’s team wasted little time speculating that the red spot was created by tholins produced by the transfer of Pluto’s atmosphere to its moon – a phenomenon resulting from the dwarf planet’s relatively tiny size and inability to maintain its atmosphere, according to’s report.

While that atmosphere escapes outward in all directions, Grundy told the website that Charon’s gravity is strong enough to capture some of that lost methane. He and his colleagues then used a system to simulate how the moon’s temperature changed throughout history, and found that due to the extremely cold temperatures at its poles (-459.67 degrees Fahrenheit; -273.15 Celsius) and the length of the winter (one Earth century), the captured gas has plenty of time to freeze.

“For the most part, Charon’s surface is too warm for methane to stick, so the methane molecules that make it to Charon’s surface just bounce around there until they either escape back to space again or find a place cold enough to stick,” Grundy told via email. “The winter pole does get cold enough for methane to stick; so methane will accumulate there, but only until the sun rises again in the spring and warms it back up.”

“This study solves one of the greatest mysteries we found on Charon, Pluto’s giant moon,” Alan Stern, New Horizons principal investigator at the Southwest Research Institute (SwRI) as well as a co-author of the new study, added in a press release. “And it opens up the possibility that other small planets in the Kuiper Belt with moons may create similar, or even more extensive ‘atmospheric transfer’ features on their moons.”


Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute