Southwest Research Institute scientists 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 . This first-ever description of Charon’s dynamic methane atmosphere using new experimental data provides fascinating insight into the origins of this moon’s red spot, as described in two recent papers.
“Before New Horizons, the best Hubble images of Pluto revealed only a fuzzy blob of reflected light,” said New Horizons science team member Randy Gladstone of SwRI. “In addition to all the fascinating features discovered on the surface of Pluto, 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 reddish “tholin-like” material at Charon’s pole could be synthesized by ultraviolet light breaking down methane molecules. These are captured after escaping from Pluto and then frozen on the polar regions of the moon during their long winter nights. Tholins are sticky organic residues formed by chemical reactions fueled by light, in this case the Lyman-alpha ultraviolet glow scattered by interplanetary hydrogen molecules.
“Our results indicate that drastic seasonal surges in Charon’s thin atmosphere as well as light breaking down the condensed methane gel are key to understanding the origins of Charon’s red polar zone,” said Dr Ujjwal Raut of SwRI, main author of an article entitled “Charon’s Usine Réfractaire” in the journal Scientists progress. “This is one of the most illustrative and striking examples of surface-atmosphere interactions observed so far on a planetary body.”
The team realistically replicated Charon’s surface conditions at SwRI’s new Center for Laboratory Astrophysics and Space Science Experiments (CLASSE) to measure the composition and color of hydrocarbons produced on 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 that methane decays into tailings on Charon’s north polar spot.
“Our team’s new ‘dynamic photolysis’ experiments have 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 under exposure to Lyman-alpha photons to reproduce with high fidelity the conditions at the poles of Charon.”
SwRI scientists also developed a new computer simulation to model Charon’s thin methane atmosphere.
“The model indicates seasonal ‘explosive’ 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 the Red Spot”in Geophysical Research Letters.
The team fed the results of ultra-realistic SwRI experiments into the atmospheric model to estimate the distribution of complex hydrocarbons emerging from the decomposition of methane under the influence of ultraviolet light. The model has polar areas generating mostly ethane, a colorless material that does not contribute a reddish color.
“We believe that ionizing radiation from the solar wind breaks down Lyman-alpha baked polar frost to synthesize increasingly complex and 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 spring sunrise. Exposure to solar wind can convert ethane into persistent reddish surface deposits contributing to the red cap of Charon.”
“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 continued support from NASA’s New Frontier data analysis program.
Pluto “paints” its largest moon Charon red
Ben Teolis et al, Extreme Exospheric Dynamics at Charon: Implications for the Red Spot, Geophysical Research Letters (2022). DOI: 10.1029/2021GL097580
Ujjwal Raut et al, Charon Refractory Plant, Scientists progress (2022). DOI: 10.1126/sciadv.abq5701
Provided by the Southwest Research Institute
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