Charon is the largest moon of Pluto. It’s also one of the most significant objects in the Kuiper Belt, the region of the solar system beyond Neptune, filled with icy bodies. Measuring about half the size of Pluto, Charon is unique among moons because its mass is large enough that it and Pluto orbit each other in a gravitational dance, making it almost like a binary planet system.

In some ways, Charon is way more interesting than Pluto. Unlike Pluto, Charon’s surface is not covered by volatile ices like methane, giving scientists a clearer view of the geological processes. Its surface, largely composed of water ice, is marked by deep canyons, icy plains, and a reddish polar region likely coated with organic compounds called tholins.

Now, in a new discovery, researchers from the Southwest Research Institute (SwRI) have also detected carbon dioxide (CO2) and hydrogen peroxide (H2O2) on the surface of Charon. Using the advanced capabilities of the James Webb Space Telescope (JWST), the researchers have now revealed new details about the complex evolutionary processes that shaped Charon’s surface.

A surprisingly active moon

We know a lot about Charon, given how far away it is from us. In 2015, the New Horizons mission already highlighted a fascinating blend of icy features and intriguing geology. Now, thanks to the James Webb Telescope, we also have a decent idea of what’s going on with Charon’s chemistry.

“Charon is the only midsized Kuiper Belt object, in the range of 300 to 1,000 miles in diameter, that has been geologically mapped, thanks to the SwRI-led New Horizons mission, which flew by the Pluto system in 2015,” said SwRI’s Dr. Silvia Protopapa, lead author of a new Nature Communications paper and co-investigator of the New Horizons mission. “Unlike many of the larger objects in the Kuiper Belt, the surface of Charon is not obscured by highly volatile ices such as methane and therefore provides valuable insights into how processes like sunlight exposure and cratering affect these distant bodies.”

Before the JWST observations, scientists knew that Charon’s surface featured a mix of crystalline water ice and ammonia-bearing species, both of which were largely concentrated in regions associated with younger craters. This pointed to the existence of an active subsurface, one that ejects materials through cratering events. However, some significant gaps remained in understanding Charon’s full chemical makeup.

In this work, researchers used spectrographs to detect chemical elements on Charon.

Spectrography is a technique that involves studying how light interacts with matter to identify the composition of objects — in this case, even those located very far away. JWST’s Near-Infrared Spectrograph (NIRSpec) aptures light reflected from Charon’s surface across a range of wavelengths, up to 5.2 microns. When light hits a surface, different chemicals absorb and reflect specific wavelengths of light, creating a unique “fingerprint” or spectrum for each substance. By analyzing these patterns of absorption and emission in the spectral data, researchers can identify the presence of specific compounds, like carbon dioxide (CO2) and hydrogen peroxide (H2O2).

“The advanced observational capabilities of Webb enabled our team to explore the light scattered from Charon’s surface at longer wavelengths than what was previously possible, expanding our understanding of the complexity of this fascinating object,” said Dr. Ian Wong, a staff scientist at the Space Telescope Science Institute and co-author of the paper.

What does this mean?

As exciting as this is, it raises even more questions about Charon. For example, what exactly drives the radiation processes that contribute to these chemical changes, and how do these processes vary across the surface? Furthermore, how does this all link to the frozen water on Charon?

To answer this, the team compared the JWST observations with laboratory measurements, concluding that the carbon dioxide mostly came as a surface veneer on a water-ice substrate.

“Our preferred interpretation is that the upper layer of carbon dioxide originates from the interior and has been exposed to the surface through cratering events. Carbon dioxide is known to be present in regions of the protoplanetary disk from which the Pluto system formed,” Protopapa said.

Despite being so far out in the solar system, Charon is revealing itself to be a fascinating body in our solar system. These new insights into its chemical processes, combined with the detailed geological mapping from New Horizons, suggest an active and vivid moon, shaped by complex forces. As new missions and observations take place, Charon will definitely get even more interesting.

The study “Detection of carbon dioxide and hydrogen peroxide on the stratified surface of Charon with JWST” was published in the journal Nature Communications.