Scientists identify a possible source for the Red Cap on Pluto’s largest moon Charon

Scientists at the Southwest Research Institute have 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. New findings suggest that drastic seasonal waves in Charon’s thin atmosphere combined with light that breaks the methane frost into condensation could hold the key to understanding the origins of Charon’s polar red zones. Credit: Courtesy NASA / Johns Hopkins APL / SwRI

Research spacecraft data combined with new lab experiments,

Charon is the largest of Pluto’s moons. At half the size of Pluto, it is the largest known satellite relative to its parent body. Charon orbits Pluto every 6.4 Earth days. James Christy and Robert Harrington discovered Charon in 1978 at the US Naval Observatory in Flagstaff, Arizona.

“Before New Horizons, the best Hubble images of Pluto revealed only a diffuse 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 startling red cap centered on its north pole.”

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

“Our findings indicate that drastic seasonal surges in Charon’s thin atmosphere, as well as the light that breaks the methane frost into condensation, are key to understanding the origins of Charon’s red polar zone,” said Dr. Refractory Factory” in the journal Science Advances. “This is one of the most illustrative and glaring examples of surface-atmospheric interactions so far observed on a planetary body.”

The team realistically replicated Charon’s surface conditions at SwRI’s new Space Science Laboratory and Experiments (CLASS) Astrophysics Center 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 of Charon to show methane turning into residue at Charon’s north polar point.

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

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 changes 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 a Red Spot” in Geophysical Survey Letters.

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

“We believe that ionizing radiation from the solar wind breaks down polar frost baked into Lyman-alpha to synthesize increasingly complex, redder materials responsible for the unique albedo on this enigmatic moon,” Raut said. “Ethane is less volatile than methane and remains frozen on Charon’s surface long after the spring sunrise. Exposure to the solar wind can convert ethane into persistent reddish surface deposits, contributing to Charon’s red cap.”

“The team is ready to investigate the role of the solar wind in the formation of the red pole,” said Dr. Josh Kammer of SwRI, who has ensured continued support from

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