In June 2015, the cameras of NASA's approaching New Horizons spacecraft first spotted the intriguing large reddish polar region on Pluto's largest moon, Charon, which stretches more than 1,000 miles. Mission scientists by that time never saw anything like it yet in anywhere in the Solar System and thus, they conducted a study to know the story behind it.
Over the past year, after conducting a study and analyzing the images and other data that the New Horizons spacecraft had sent from its July 2015 flight going to the Pluto system, scientists think they have solved the puzzle behind the mystery.
The researchers published their findings this week in the international scientific journal Nature, where they concluded that Charon's polar red spot comes from Pluto itself. As the methane gas that escapes from Pluto's atmosphere and becomes "trapped" by the moon's gravity and it freezes at the moon's pole.
After that, it is followed by chemical processing by ultraviolet light that comes from the sun and transforms the methane into heavier hydrocarbons and it will eventually turn into reddish organic materials called tholins.
"Who would have thought that Pluto is a graffiti artist, spray-paint in its companion with a reddish stain that covers an area the size of New Mexico?" asked Will Grundy, one of the New Horizons co-investigator from Lowell Observatory in Flagstaff, Arizona and the lead author of the paper.
The research team combined analyses from the detailed images of Charon with computer models of how the ice evolves on the moon's poles and using the models of Pluto and Charon which is 248-orbit around the sun, it showed some extreme weather at the moon's poles where 100 years of continuous sunlight and then another century of continuous darkness. During these long winters, it can actually reach up to -430 degrees Fahrenheit or -257 degrees Celsius, causing the freezing of methane gas.
"The methane molecules bounce around on Charon's surface until they either escape back into space or land on the cold pole where they freeze into solid forming a thin coating of methane ice that lasts until sunlight comes back in the spring," Grundy said. While the methane ice quickly sublimates away, the heavier hydrocarbons create from it remains on the surface.
The models also suggested that in the moon's springtime, the returning sunlight triggers conversion of the frozen methane back to gas but while the methane ice quickly sublimates away, the heavier hydrocarbons created from the evaporative process remain on the surface. Sunlight then further exposes those leftovers into reddish material which is called tholins, which, in turn,slowly accrued on the Charon's poles over millions of years.
New Horizon's observations of the moon's other pole which is currently in winter darkness and can only be seen through the reflecting light from Pluto.
"This study solves one of the greatest mysteries we found on Charon, Pluto's giant moon and it opens up the possibility that other small planets in the Kuiper Belt with moons may create similar event, or even more extensive 'atmospheric transfer' features on their moons," said Alan Stern, New Horizons principal investigator from the Southwest Research Institute and one of the study co-author.