Back in 2015, the science community’s focus turned toward the New Horizons flyby of Pluto. The images the craft sent back early on were amazing, revealing not only the heart of Sputnik Planitia that has become iconic of the mission but also an intriguing red cap at the northern pole of Pluto’s companion Charon. Ever since that discovery, researchers have been trying to figure out just what mechanics produced that red cap.

The earliest proposed answer is not going to excite anyone – tholins. Per the latest press release: Tholins are sticky organic residues formed by chemical reactions powered by light, in this case, the Lyman-alpha ultraviolet glow scattered by interplanetary hydrogen atoms.

Okay, so the name ‘tholin’ is cool, and the concept is interesting. Basically, the tholin-like material seen at Charon could be created when ultraviolet light breaks down the methane molecules that are captured after they escape from Pluto’s thin atmosphere. Those methane molecules then freeze at the poles during winter, and when sunlight hits them, they break down. We’ve talked before about how sunlight breaks apart methane in stories about Mars where the debate over methane replenishment continues.

But testing out this theory about Charon was going to take time and effort since New Horizons was only a flyby mission. No going back for samples or second looks here. And to top it off, due to the distance and data speeds of the spacecraft, we are still receiving information collected during the flyby.

So scientists at the ​​Southwest Research Institute replicated the conditions of Charon’s surface in the lab, as one does. This experiment allowed them to measure the composition and color of the hydrocarbons produced by methane freezing in that Lyman-alpha glow. Lead author Ujjwal Raut explains: 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.

Of course, no experiment would be complete without a computer simulation, so the team also developed a new program that modeled Charon’s very thin methane atmosphere. And with all that data from the lab experiment, the model produced polar zone generating ethane, which doesn’t make anything red. Raut notes: 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.

And that means more experimentation and simulation are needed. Science, especially when done at a distance, is not easy.

More Information

SwRI press release

“Extreme Exospheric Dynamics at Charon: Implications for the Red Spot,” Ben Teolis et al., 2022 April 15, Geophysical Research Letters