5.09.2025

Observations with the James Webb Space Telescope have shown the new interstellar Comet 3I/ATLAS is surprisingly rich in carbon dioxide.

The James Webb Space Telescope imaged the interstellar Comet 3I/ATLAS on August 6, 2025. The left panel shows the overall infared image. The other two panels highlight emission from carbon dioxide (center) and water vapor (right). The insets show spectral line profiles that confirm these molecular signatures.
NASA / ESA / CSA / M. Cordiner (NASA-GSFC) / CC BY 4.0 INT

Observations with the James Webb Space Telescope have shown the new interstellar Comet 3I/ATLAS is surprisingly rich in carbon dioxide. JWST’s Near-Infrared Spectrograph (NIRSpec) found the comet’s coma contains eight times more carbon dioxide (CO₂) than water vapor. That’s in striking contrast to the gaseous halos around solar system comets, almost all of which contain more water vapor than CO₂.

"I have never seen such a strong CO₂ peak in a comet spectrum," says Martin Cordiner (NASA Goddard Space Flight Center), lead author of a paper describing the results posted August 25th on the astronomy arXiv preprint server. The results suggest a potentially puzzling difference between comets that arrived in the solar system from interstellar space and those that formed here, but with an important qualifier: Webb wasn’t in orbit in 2019, when the only other interstellar comet, 2I/Borisov, passed through the solar system. (The first interstellar object, known as 1I/‘Oumuamua, was not a comet.)

Cordiner’s team received only 10 minutes on August 6th to collect data on the comet using Webb’s NIRSpec, but that was enough for significant new finds. Although many other telescopes had observed 3I/ATLAS by then, none of them had infrared vision around 4.3 microns, where CO₂emits most strongly; our atmosphere blocks these wavelengths from reaching the ground. With sensitivity from 0.6 to 5.3 micrometers, NIRSpec also provided the range needed to measure water vapor.  

“The detection of such a high level of CO₂ is quite exciting,” says Bryce Bolin (Eureka Scientific, Inc.), who worked with Cordiner in the past but was not part of the Webb study. He adds that Comet 3I/ATLAS’s brightness had already suggested that something besides water must be driving its activity. “What is surprising is just how much CO₂ there is in the coma,” he adds.

In late August, a different team turned to another NASA space telescope to study 3I/ATLAS. Called SPHEREx, the telescope has broadband coverage of the whole sky, including at infrared wavelengths. SPHEREx, too, found abundant CO₂ in the comet. Cordiner says the amount of carbon seen by the two telescopes is “compatible.” Webb’s result is more precise, though, because it was designed to pinpoint a specific target, while SPHEREx covers a large area because it was designed to map the whole sky. Other space telescopes might also be turned toward the interstellar comet, including the Juno mission at Jupiter and Europa Clipper, on its way to the Jovian moon.

The discovery of the high concentration of CO₂ emphasizes the importance of observing the comet's appearance as it zips sunward at 60 kilometers per second (130,000 mph). Increasing solar heat will bulk up the coma by vaporizing water and other ices. Measuring the coma’s content will give new insight into the comet’s composition, its origins, and its passage through interstellar space and the solar system.

“We don't have a huge amount of time” Cordiner says. The comet’s nearest approach to the Sun comes in October, when it will come close to Mars but will be close to the Sun in the terrestrial sky. Terrestrial telescopes will find the comet hidden in solar glare, but Mars-based telescopes might focus on 3I/ATLAS while it’s relatively near.

Terrestrial telescopes will have to wait until December to get their second view, with a final one in March. “By next April [the comet will be] on its way out,” Cordiner adds. Although it might be trackable until late next year, he expects detailed observations to wind down by June.

Much remains to be learned about three interstellar objects known to have passed through the solar system. The first, 1I/‘Oumuamua, was small, dark, and lacked a detectable coma when it was observed in 2017. We still don’t know exactly what shape it had. We know more about 2I/Borisov’s appearance but little about its composition.

“It is still an open question how frequent these objects are in the galaxy and how likely they are to pass within our solar system,” says Bolin. “I think we will need more accurate physical information about these comets such as their size to answer this question.” He hopes to learn that information in the near future.

Even if we cannot predict the arrival of the next interstellar comet, we are well equipped to study it when it arrives. “Now, with JWST,” Bolin adds, “we can thoroughly test interstellar comets for the presence of CO₂ gas in their comas.”

Quelle: Sky&Telescope