By combining data obtained from several sources, scientists managed to unravel the mystery behind the red cap on Pluto’s moon Charon and its composition. NASA’s New Horizons interplanetary space probe captured the reddish region atop Charon and collected data in 2015. After the encounter, scientists quickly speculated that tholin-like material on Pluto’s moon could be synthesized by breaking down methane molecules using ultraviolet light.
However, now scientists from the Southwest Research Institute (SwRI) have combined data from the New Horizons mission, exospheric modeling and laboratory experiments to describe the red spot and shed light on its origin.
“Before New Horizons, the best Hubble images of Pluto revealed only a fuzzy blob of reflected light. In addition to all the fascinating features discovered on Pluto’s surface, the flyby revealed an unusual feature on Charon, a startling red cap centered on its north pole,” said Randy Gladstone of SwRI, a member of the New Horizons science team.
The tholins, on the red spot, tend to escape from the surface of Pluto and freeze upon reaching the polar region of Charon during the long winter nights. They are basically sticky organic residues that are the result of chemical reactions fueled by light.
“Our results indicate that drastic seasonal surges in Charon’s thin atmosphere, along with light breaking down the condensed methane gel, are key to understanding the origins of Charon’s red polar zone,” said Dr Ujjwal Raut of SwRI, lead author of the study published in Science Advances.
For the study, the team replicated conditions on the surface of Charon at SwRI’s new Center for Laboratory Astrophysics and Space Science Experiments (CLASSE). They analyzed the composition and color of hydrocarbons produced in Charon’s winter hemisphere and fed the data into a new model of Charon. This helped them demonstrate how methane decayed into tailings at Charon’s north pole. According to Raut, their “dynamic photolysis” experiment provided insight into the role of interpolar Lyman-alpha in the formation of Charon’s red material.
After conducting experiments, the researchers used a model to estimate the distribution of complex hydrocarbons from the decomposition of methane under ultraviolet light. “We believe that ionizing radiation from the solar wind breaks down Lyman-alpha baked polar frost to synthesize increasingly complex and redder materials responsible for the unique albedo on this enigmatic moon,” Raut said. He added that because ethane is less volatile than methane, it stays frozen on Charon’s surface long after sunrise in the spring. “Exposure to solar wind can convert ethane into persistent reddish surface deposits contributing to Charon’s red cap,” Raut said.