Randy Gladstone of SwRI, a member of the New Horizons science team, said: “Before New Horizons, the best Hubble images of Pluto showed only a blurred blob of reflected light. In addition to all the fascinating features found on Pluto’s surface, The flyby also uncovered an unusual feature on Charon, a surprising red cap centered on its North Pole.”
Shortly after the 2015 encounter, New Horizons scientists proposed that there was a reddish “mint-leaf-like” substance at Charon’s poles — possibly synthesized by ultraviolet light breaking down methane molecules. The molecules were captured after escaping Pluto and then frozen in the moon’s polar regions during long winter nights. Tholins are sticky organic residues formed by light-driven chemical reactions, in this case, by the Lyman-alpha ultraviolet glow scattered by interplanetary hydrogen atoms.
Dr Ujjwal Raut, SwRI, said: “Our findings suggest that a sharp seasonal surge in Charon’s thin atmosphere and light-splitting condensed methane frosts are key to understanding the origin of Charon’s red polar region. This is what has been observed so far on a planetary body. One of the most telling and stark examples of surface-atmosphere interaction to date.”
The research team faithfully replicated Charon’s table at SwRI’s new laboratory, the Centre for Astrophysics and Space Science Experiments (CLASSE)NoodlePieces to measure the composition and color of hydrocarbons produced in Charon’s winter hemisphere as methane freezes under the Lyman-Alpha glow. The team fed the measurements into a new atmospheric model in Charon to show that methane breaks down into remnants at Charon’s North Pole.
Raut said: “Our team’s novel ‘dynamic photolysis’ experiment provides new constraints on the contribution of interplanetary Lyman-alpha to synthesizing the red species of Charon. Alpha photons condensed methane, thereby replicating the conditions at the Charon poles in high fidelity.”
SwRI scientists also developed a new computer simulation to model Charon’s thin methane atmosphere.
The team fed SwRI’s hyperrealistic experimental results into atmospheric models to estimate the distribution of complex hydrocarbons produced by the decomposition of methane under the influence of ultraviolet light. The polar regions of the model mainly produce ethane, a colorless substance that does not contribute to the red color.
“We think that ionizing radiation from the solar wind breaks down the Lyman-Alpha polar frost to synthesize increasingly complex and redder materials responsible for the unique albedo on this enigmatic moon,” Raut said. It is less volatile than methane and freezes on Charon’s surface long after sunrise in spring. Exposure to the solar wind may convert ethane into persistent red surface deposits, contributing to the formation of Charon’s red cap .”
SwRI’s Dr Josh Kammer said the team would investigate the role of the solar wind in the formation of the red pole.