Scientists identify a possible source for Charon’s red hat

Scientists at the Southwest Research Institute combined data from NASA’s New Horizons mission with new lab experiments and exospheric modeling to reveal the likely composition of the red cap on Pluto’s moon Charon and how it may have formed. This first-ever description of Charon’s dynamic methane atmosphere using new experimental data provides a fascinating glimpse into the origins of this moon’s red spot, as detailed in two recent papers.

“Prior to New Horizons, the best Hubble images of Pluto revealed only a faint blob of reflected light,” said Randy Gladstone of SwRI, a member of the New Horizons science team. “In addition to all the fascinating features discovered on Pluto’s surface, the flyby revealed an unusual feature on Charon, a surprising red cap centered on its Arctic†

Shortly after the meeting in 2015, New Horizons scientists proposed that a reddish “tholin-like” material at the pole of Charon could be synthesized by ultraviolet light to break down methane molecules. These are captured after escaping Pluto and then frozen on the moon’s polar regions during their long winter nights. Tholins are sticky organic residues formed by chemical reactions powered by light, in this case the Lyman alpha ultraviolet glow scattered by interplanetary hydrogen molecules.

“Our findings indicate that drastic seasonal variations in Charon’s thin atmosphere, as well as light breaking off the condensing methane frost, are key to understanding the origin of Charon’s red polar zone,” said Dr. Ujjwal Raut of SwRI, lead author of a paper entitled “Charon’s Refractory Factory” in the journal scientific progress† “This is one of the most illustrative and stark examples of surface-atmosphere interactions yet observed in a planetary body.”

The team realistically replicated Charon’s surface conditions in SwRI’s new Center for Laboratory Astrophysics and Space Science Experiments (CLASSE) to measure the composition and color of hydrocarbons produced in Charon’s winter hemisphere as methane freezes beneath the Lyman- alpha glow. The team fed the measurements into a new atmospheric model of Charon to show that methane breaks down into residue at Charon’s Arctic spot.

“Our team’s new ‘dynamic photolysis’ experiments have set new limits for the contribution of interplanetary Lyman alpha to the synthesis of Charon’s red material,” Raut said. “Our experiment condensed methane in a ultra-high vacuum chamber under exposure to Lyman-alpha photons to replicate the conditions at Charon’s poles with high fidelity.”

SwRI scientists also developed a new computer simulation to model Charon’s thin methane atmosphere.

“The model points to ‘explosive’ seasonal pulsations in Charon’s atmosphere due to extreme shifts in conditions during Pluto’s long journey around the sun,” said Dr. Ben Teolis, lead author of a related paper titled “Extreme Exospheric Dynamics at Charon: Implications.” for the red spot” in Geophysical Survey Letters†

The team fed the results of SwRI’s ultra-realistic experiments into the atmospheric model to estimate the distribution of complex hydrocarbons resulting from the breakdown of methane under the influence of ultraviolet light. The model has polar zones that mainly generate ethane, a colorless material that does not contribute to a reddish color.

“We think that ionizing radiation from the solar wind dissects the Lyman-alpha-boiled arctic frost to synthesize increasingly complex, redder materials responsible for the unique albedo on this enigmatic moon,” Raut said. Charon. Exposure to the solar wind can convert ethane into persistent reddish surface deposits that contribute to Charon’s red cap.”

“The team plans to investigate the role of solar wind in the formation of the red pole,” said Dr. Josh Kammer of SwRI, who has received continued support from NASA’s New Frontier Data Analysis Program.