Scientists identify possible source of Charon’s red cap

(ORDO NEWS) — Scientists at the Southwestern Research Institute have combined data from NASA’s New Horizons mission with new laboratory experiments and exosphere modeling to figure out the likely composition of the red cap on Pluto’s moon Charon and how it formed.

Previously, scientists had suggested that the reddish tholin-like material that enveloped Charon’s pole could be synthesized by ultraviolet radiation splitting methane molecules. They are captured after fleeing Pluto and then freeze in the moon’s polar regions during the long winter nights.

Tholins are sticky organic residues formed as a result of chemical reactions under the influence of light, in this case ultraviolet Lyman-alpha, scattered by interplanetary hydrogen molecules.

The team realistically reproduced conditions on Charon’s surface to measure the composition and color of hydrocarbons. The scientists fed the measurements into a new atmospheric model to show how methane breaks down into remnants at Charon’s north polar spot.

“In our experiment, methane was condensed in an ultra-high vacuum chamber under the influence of Lyman-alpha photons, which made it possible to reproduce conditions at the poles of Charon with high accuracy,” said Randy Gladstone, a member of the New Horizons science team.

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 caused by extreme changes in conditions during Pluto’s long journey around the Sun.

The team fed the results of ultra-realistic experiments into an atmospheric model to estimate the distribution of complex hydrocarbons produced when methane decomposes under ultraviolet light. In the model, the polar zones mainly generate ethane, a colorless substance that does not contribute to the reddish color.

“We believe that ionizing radiation from the solar wind is decomposing the polar frost with Lyman alpha radiation to synthesize increasingly complex, redder materials responsible for the unique albedo on this mysterious moon.

Ethane is less volatile than methane and remains frozen on Charon’s surface long after the sun has risen in spring. The impact of the solar wind can convert ethane into persistent reddish surface deposits that contribute to the formation of Charon’s red cap, ”the scientists report.

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