Pluto and its large moon Charon have already given planetary scientists some surprises. Now the James Webb Telescope has revealed new details about Charon’s chemistry. The first observations at wavelengths larger than 2.5 micrometers demonstrated for the first time the presence of carbon dioxide ice and hydrogen peroxide on the surface of Pluto’s moon. Both molecules had previously been suspected to occur on Charon, but this was only proven by the current data from the near-infrared spectrometer on the Webb telescope. The new results not only expand the range of molecules found on Charon, they also provide insight into the chemical processes occurring on the surface of this distant Kuiper Belt object, the team reports.
Pluto and its largest moon Charon form an exceptional duo in the solar system. Because they are so similar in size and behavior that they look less like a dwarf planet-moon team and more like a double planet. With a diameter of around 2,370 kilometers, Pluto is just twice as big as Charon with its 1,212 kilometers. And the masses of the two celestial bodies also have a ratio of only eight to one. For comparison: For the Earth and the Moon this is at least 81:1. While our moon moves around the Earth as its central reference point, Pluto and Charon orbit around the shared center of gravity between them. More details about Pluto and Charon were only provided by NASA’s New Horizons space probe, which flew past both and revealed surprising things about these supposedly “dead”, icy worlds. Unlike Charon, Pluto has an atmosphere and winds as well as a surprisingly dynamic geology. Both celestial bodies are also chemically closely linked to one another. The reddish coating in the polar regions of Charon was only created by emissions from Pluto.
Extended look at Charon’s chemistry
“Thanks to New Horizons, Charon is the only medium-sized Kuiper Belt object that has been geologically mapped,” says lead author Silvia Protopapa from the Southwest Research Institute in Boulder. “Unlike many of the larger celestial bodies in the Kuiper Belt, Charon’s surface is not covered by very volatile ices such as methane ice. It therefore provides us with valuable insights into how solar radiation, meteorite impacts and other processes shape these distant objects.” As a result, planetary scientists have already discovered that Charon’s surface is largely covered by crystalline water ice and compounds containing ammonia. Organic deposits and the reddish tholins in the polar regions of Charon have already been detected. However, spectral analyzes have so far been limited to wavelengths of visible light and near-infrared below 2.5 micrometers, as Protopapa and her colleagues explain. As a result, data on some important molecules such as carbon dioxide (CO2) and hydrogen peroxide (H2O2) and also on the photochemical effects of solar and cosmic radiation on Charon’s surface.
Now the James Webb Space Telescope has provided some of that missing data. Protopapa and her team used the telescope’s near-infrared spectrometer (NIRSpec), which collected high-resolution spectral data from the Charon Northern Hemisphere in the 2.5 to 5.2 micrometer wavelength range. The analysis of this data now confirms the presence of two additional molecules on the surface of Pluto’s moon. The first signature confirms the presence of frozen CO2. “We have the CO2“Ice on the surface of Charon was identified by its characteristic absorption features, specifically a combined line at 2.7 micrometers,” the researchers report. To find out what form the carbon dioxide is in on Charon, they supplemented this data with comparative measurements in the laboratory, in which they produced different forms of this ice as well as mixtures with other components.
(Video: Silvia Protopapa (SwRI)/ Ian Wong (STScl))
Crystalline CO2 and hydrogen peroxide produced by radiation
The research suggests that the carbon dioxide exists as a thin layer of pure, crystalline ice. “Our preferred interpretation is that the CO2“Ice exists inside Charon and that this surface layer was exposed by impact events,” reports Protopapa. “Carbon dioxide is known to occur in the region of the protoplanetary disk where the Pluto system once formed.” A second spectral signature in the NIRSpec data indicates the presence of hydrogen peroxide H2O2 ) there. Comparison with laboratory experiments showed that this molecule is formed on Charon when UV radiation from the sun and cosmic rays hit the water ice on the surface and cause the water molecules to break down. The OH radicals generated can then react with each other and form hydrogen peroxide. “The proof of H2O2 “clearly demonstrates the influence radiolysis and photolysis have on Charon’s surface,” the scientists state.
The new results provide further valuable insights into the chemistry of celestial bodies in the distant Kuiper Belt – a region of our solar system that is still poorly researched.
Source: Silvia Protopapa (Southwest Research Institute, Boulder) et al., Nature Communications, doi: 10.1038/s41467-024-51826-4