Pluto’s friend Charon has a red hat – now we think we know why

Pluto may have been demoted from planetary status in 2006 (it was 16 years ago, Pluto fans – get over it!), but that doesn’t mean we’re not still looking at our little friend on the solar system’s outskirts.

Let’s turn our attention to the dwarf planet’s constant companion, Charon. Pluto and Charon orbit each other, though Charon is about half Pluto’s diameter.

Though Charon was discovered in 1978, it wasn’t until 2015 when NASA’s New Horizons mission to the outer solar system revealed Charon has a red region on its north pole. On an otherwise icy and – let’s be honest – relatively bland-looking rock, questions emerged about what Pluto’s friend’s red beanie is and how it was formed.

Now, New Horizons images plus novel laboratory experiments and modelling have revealed the likely story behind Charon’s cherry headwear. The team at the US Southwest Research Institute (SwRI) behind the research published their results in Geophysical Research Letters.

Read also: Pluto moon Charon’s red head tinted by trapped gas

“Prior to New Horizons, the best Hubble images of Pluto revealed only a fuzzy blob of reflected light,” says SwRI’s Randy Gladstone. “In addition to all the fascinating features discovered on Pluto’s surface, the flyby revealed an unusual feature on Charon, a surprising red cap centred on its north pole.”

Not long after the 2015 flyby, scientists proposed that the reddish patch could be due to ultraviolet light breaking down methane molecules. Methane from Pluto would freeze on the moon’s polar regions during the exceedingly chilly winter nights on Charon, which check in at -273°C. The substance would be like tholin – a sticky organic residue formed by light-powered chemical reactions.

“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,” says the lead author of a related Science Advances paper on the research, SwRI’s Dr. Ujjwal Raut. “This is one of the most illustrative and stark examples of surface-atmospheric interactions so far observed at a planetary body.”

The team attempted to replicate Charon’s surface conditions at SwRI’s new Center for Laboratory Astrophysics and Space Science Experiments (CLASSE).

They were able to measure the composition and colour of hydrocarbons that could be produced over the winter hemisphere of the dwarf planet when methane freezes under Lyman-alpha ultraviolet light – the kind that would be scattered onto the moon by the hydrogen in Pluto’s atmosphere.

“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 under exposure to Lyman-alpha photons to replicate with high fidelity the conditions at Charon’s poles.”

SwRI scientists then fed these results into their newly developed computer simulation to model Charon’s thin methane atmosphere.

Read also: How Pluto’s big frozen heart grew and evolved

“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,” says Dr Ben Teolis, lead author the Geophysical Research Letters paper. From the experimental data and their model, the researchers were able to estimate where the produced hydrocarbons (molecules made of carbon and hydrogen like methane) would end up on Charon’s surface.

But the model shows that Charon’s polar zones primarily generate ethane which is colourless. So, how did Charon get its crimson coloration?

“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,” Raut explains. “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 team is set to investigate the role of solar wind in the formation of the red pole,” says SwRI’s Dr Josh Kammer.

So, Charon’s funky scarlet bonnet may be due to frozen ethane being burned by solar radiation. Now, that’s a fire hat.

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