The thin methane atmosphere of Pluto’s biggest moon, Charon, undergoes ‘explosive’ pulsations owing to the Pluto-Charon’s system’s near sideways tilt to the Sun, according to new research led by Southwest Research Institute scientists. Spring sunrise may drive polar methane frozen during the centuries long winter night back into Charon’s atmosphere, causing the whole atmosphere to briefly and drastically surge in pressure by a factor of almost 1,000 every equinox. During these exceedingly brief episodes, taking place just a few years out of the Pluto-Charon system’s 248 year orbit around the Sun, polar caps of methane frost tens of microns thick may be suddenly swapped between north and south, evaporated and then re-frozen from the spring to the autumnal polar zones. Charon’s polar red spot, seen by NASA’s New Horizons spacecraft, was suspected to be material synthesized from frozen methane by backscattered solar ultraviolet (UV) light. However, the authors found that Charon’s polar caps are frozen too fast and thick for synthesis of much material more complex than ethane. Nevertheless, ethane, being less volatile than methane, stays frozen to Charon’s surface for decades after spring sunrise, and may under exposure to solar wind be converted to permanent red-colored surface deposits that contribute to the origins of Charon’s red spot.

This impressive view of Charon was captured on July 14, 2015. Charon’s color palette is not as diverse as Pluto’s; most striking is the reddish north (top) polar region, informally named Mordor Macula. Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute.

“Prior to New Horizons, the best Hubble images of Pluto revealed only a fuzzy blob of reflected light,” said New Horizons team member Dr. Randy Gladstone, a researcher with the Southwest Research Institute and the Department of Physics and Astronomy at the University of Texas at San Antonio.

“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 north pole.”

Soon after the 2015 encounter, New Horizons scientists proposed that a reddish tholin-like material at Charon’s pole could be synthesized by UV light breaking down methane molecules.

These are captured after escaping from Pluto and then frozen onto 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 UV 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 Dr. Ujjwal Raut, a researcher at the Southwest Research Institute and the Department of Physics and Astronomy at the University of Texas at San Antonio.

“This is one of the most illustrative and stark examples of surface-atmospheric interactions so far observed at a planetary body.”

The researchers realistically replicated Charon surface conditions to measure the composition and color of hydrocarbons produced on Charon’s winter hemisphere as methane freezes beneath the Lyman-alpha glow.

They fed the measurements into a new atmospheric model of Charon to show 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,” Dr. Raut said.

“Our experiment condensed methane in an ultra-high vacuum chamber under exposure to Lyman-alpha photons to replicate with high fidelity the conditions at Charon’s poles.”

The 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 over Pluto’s long journey around the Sun,” said Dr. Ben Teolis, a researcher with the Southwest Research Institute and the Department of Physics and Astronomy at the University of Texas at San Antonio.

The team input the results from ultra-realistic experiments into the atmospheric model to estimate the distribution of complex hydrocarbons emerging from methane decomposition under the influence of UV light.

The model has polar zones primarily generating ethane, a colorless material that does not contribute to a reddish color.

“We think ionizing radiation from the solar wind decomposes the Lyman-alpha-cooked polar frost to synthesize increasingly complex, redder materials responsible for the unique albedo on this enigmatic moon,” Dr. Raut said.

“Ethane is less volatile than methane and stays frozen to Charon’s surface long after spring sunrise.”

“Exposure to the solar wind may convert ethane into persistent reddish surface deposits contributing to Charon’s red cap.”

The results appear in two papers published the journal Geophysical Research Letters and the journal Science Advances.

Ben Teolis et al. Extreme Exospheric Dynamics at Charon: Implications for the Red Spot. Geophysical Research Letters, published online April 15, 2022; doi: 10.1029/2021GL097580

Ujjwal Raut et al. 2022. Charon’s refractory factory. Science Advances 8 (24); doi: 10.1126/sciadv.abq5701