Charon, Pluto’s largest moon, has a red region over its north pole. We may finally have an explanation seven years after its discovery. When the New Horizons spacecraft first saw the surface of Pluto’s largest moon, Charon, in 2015, the most striking feature was a large red spot. It encompasses Charon’s north pole. Scientists believe it is made up of tholins, which are hydrocarbons.
Tholins are widely distributed throughout the Solar System. They are formed on Charon by methane, which is “blown out” by Pluto’s solar wind. However, the specifics of this procedure were unknown until now. Southwest Research Institute (SwRI) researchers attempted to find them.
Southwest Research Institute scientists used data from NASA’s New Horizons mission, novel laboratory experiments, and exospheric modeling to determine the likely composition and formation of Pluto’s moon Charon’s red cap. This first-ever description of Charon’s dynamic methane atmosphere based on new experimental data offers an intriguing glimpse into the origins of this moon’s red spot, which has been described in two recent papers.
“Prior to New Horizons, the best Hubble images of Pluto revealed only a fuzzy blob of reflected light,” SwRI’s Randy Gladstone, a New Horizons science team member, explained. “The flyby revealed an unusual feature on Charon, a surprising red cap centered on its north pole, in addition to all the fascinating features discovered on Pluto’s surface.”
Our findings indicate that drastic seasonal surges in Charon’s thin atmosphere as well as light breaking down the condensing methane frost are key to understanding the origins of Charon’s red polar zone. This is one of the most illustrative and stark examples of surface-atmospheric interactions so far observed at a planetary body.
Dr. Ujjwal Raut
Soon after the 2015 encounter, New Horizons scientists proposed that a reddish “tholin-like” material at Charon’s pole could be created by breaking down methane molecules with ultraviolet light. These are captured after escaping from Pluto and then frozen onto the polar regions of the moon during their long winter nights. Tholins are sticky organic residues formed by light-powered chemical reactions, in this case the Lyman-alpha ultraviolet glow scattered by interplanetary hydrogen molecules.
“Our findings indicate that drastic seasonal surges in Charon’s thin atmosphere as well as light breaking down the condensing methane frost are key 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” in the journal Science Advances. “This is one of the most illustrative and stark examples of surface-atmospheric interactions so far observed at a planetary body.”
The team measured the composition and color of hydrocarbons produced on Charon’s winter hemisphere as methane freezes beneath the Lyman-alpha glow at SwRI’s new Center for Laboratory Astrophysics and Space Science Experiments (CLASSE). The measurements were fed into a new atmospheric model of Charon, which showed methane breaking down into residue on Charon’s north polar spot.
“Our team’s novel ‘dynamic photolysis’ experiments provided new limits on the contribution of interplanetary Lyman-alpha to the synthesis of Charon’s red material,” Raut said. “Our experiment condensed methane in an ultra-high vacuum chamber while exposed to Lyman-alpha photons in order 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 predicts ‘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 published in Geophysical Research Letters titled “Extreme Exospheric Dynamics at Charon: Implications for the Red Spot.”
The team used the results of SwRI’s ultra-realistic experiments to estimate the distribution of complex hydrocarbons emitted by methane decomposition under the influence of ultraviolet light. The model includes polar zones that primarily produce ethane, a colorless material that does not contribute to the reddish hue.
“We believe that ionizing radiation from the solar wind decomposes the Lyman-alpha-cooked polar frost to synthesize increasingly complex, redder materials responsible for this enigmatic moon’s unique albedo,” Raut explained. “Ethane is less volatile than methane and remains frozen to the surface of Charon long after the spring sunrise. Ethane may be converted into persistent reddish surface deposits as a result of exposure to the solar wind, contributing to Charon’s red cap.”
“The team is ready to investigate the role of solar wind in the formation of the red pole,” said SwRI’s Dr. Josh Kammer, who has secured ongoing funding from NASA’s New Frontier Data Analysis Program.
