"Prior to New Horizons, the finest Hubble photographs of Pluto revealed only a fuzzy blob of reflected light," Randy Gladstone of SwRI, a member of the New Horizons research team, stated. "The flyby showed a unique feature on Charon, a startling red cap centered on its north pole, in addition to all the remarkable characteristics observed on Pluto's surface."
Soon after the 2015 flyby, New Horizons scientists speculated that UV radiation breaking down methane molecules may produce a reddish "tholin-like" substance at Charon's pole. After fleeing Pluto, they are trapped and frozen in the polar regions of the moon during their lengthy winter nights. Tholins are sticky organic residues that arise as a result of chemical processes fueled by light, in this instance the Lyman-alpha ultraviolet illumination dispersed by interplanetary hydrogen molecules.
"Our findings show that drastic seasonal surges in Charon's thin atmosphere, as well as light breaking down the condensing methane frost, are critical to understanding the origins of Charon's red polar zone," said SwRI's Dr. Ujjwal Raut, lead author of a paper titled "Charon's Refractory Factory" published in the journal Science Advances. "This is one of the most demonstrative and striking examples of surface-atmospheric interactions yet seen on a planetary body," said the team.
The researchers measured the composition and color of hydrocarbons formed on Charon's winter hemisphere when methane freezes beneath the Lyman-alpha light by simulating Charon surface conditions at SwRI's new Center for Laboratory Astrophysics and Space Science Experiments (CLASSE). The results were incorporated into a new Charon atmospheric model, which showed methane breaking down into residue on the planet's north polar region.
"New limitations on the contribution of interplanetary Lyman-alpha to the synthesis of Charon's red material were provided by our team's innovative 'dynamic photolysis' tests," Raut added. "To mimic the circumstances at Charon's poles with high fidelity, we condensed methane in an ultra-high vacuum chamber while exposing it to Lyman-alpha photons."
In addition, SwRI scientists created a novel computer simulation to mimic Charon's thin methane atmosphere.
"The model suggests 'explosive' seasonal pulsations in Charon's atmosphere due to extreme shifts in conditions over Pluto's long journey around the Sun," said Dr. Ben Teolis, lead author of a related paper published in Geophysical Research Letters titled "Extreme Exospheric Dynamics at Charon: Implications for the Red Spot."
The researchers used the results of SwRI's ultra-realistic experiments to predict the distribution of complex hydrocarbons produced by methane breakdown under the impact of UV light in an atmospheric model. Polar zones in the model largely produce ethane, a colorless gas that does not contribute to the reddish hue.
"We believe that ionizing radiation from the solar wind decomposes the Lyman-alpha-cooked polar frost, resulting in the synthesis of progressively complex, redder compounds that are responsible for the mysterious moon's peculiar albedo," Raut stated. "Because ethane is less volatile than methane, it remains frozen to Charon's surface well after the spring daybreak. Ethane may be converted to permanent reddish surface deposits by exposure to the solar wind, leading to Charon's red cap."
"The team is prepared to examine the role of solar wind in the development of the red pole," SwRI's Dr. Josh Kammer stated, noting that NASA's New Frontier Data Analysis Program has continued to fund the project.
