Researchers are constantly sifting through new spectral data gathered by powerful telescopes, like the James Webb Space Telescope (JWST). Most of the time, when they identify spectral features—specific absorption or emission lines from different types of light gathered from a planet, moon or star—these features are known to be caused by certain atoms or molecules. For example, the emission line at 426.7 nanometers is known to come from singly ionized carbon, representing a specific atomic transition between energy states of a carbon ion.
But recently, a team of scientists identified an absorption line coming from Saturn’s moons Titan and Pluto that has not yet been seen anywhere else. This means scientists don’t know which atom or molecule it originates from, but whatever it is, it might indicate a unique shared chemical pathway between the two worlds, according to a new research published on the arXiv preprint server.
Atmospheric similarities and differences
The differences between Titan and Pluto may be clear. Titan is a large moon and one of the only bodies in the solar system with clear evidence of stable surface liquid. Pluto is a frozen dwarf planet at the edge of the solar system, far colder than Titan and about half the diameter. However, they do have similar atmospheres. Both possess a nitrogen-rich atmosphere with a significant amount of methane. Because of this, they both have prominent, hazy atmospheres caused by reactions between ultraviolet light and nitrogen and methane. But since Pluto is much smaller, its atmosphere is much thinner.
Titan’s thicker, hazy atmosphere has made it difficult to read surface chemistry using reflected light. In previous studies, data from the Cassini-Huygens probe showed that Titan has rivers, lakes, dunes and complex terrain, but researchers were unable to pin down specific surface chemistry beyond hints of possible water ice. Earlier JWST Titan studies have gained insight into atmospheric chemistry, but surface composition is still largely unknown because surface signatures are obscured by the thicker atmosphere. On the other hand, Pluto’s atmosphere is extremely thin, so surface signatures are easier to detect.
An unidentified absorption feature
The team involved in the new study recently analyzed the JWST spectra of Titan from NIRSpec in 2022 and MIRI in 2023, focusing on wavelengths in the atmospheric window of 4.9–5.4 micrometers. In both data sets, they identified an absorption feature at 5.11 micrometers. They note that it’s unlikely to be a glitch or instrumental artifact because it was detected on two separate instruments. MIRI data from Pluto also showed a 5.11-micrometer absorption feature at essentially the same wavelength, although the feature is about three times broader on Pluto than on Titan.
The team searched past studies and other published lab spectra, but could not find a match for the 5.11-micrometer absorption feature anywhere. They write, “We did not find any band referenced in these publications that corresponds to the location of the observed absorption in Titan and Pluto. However, a signature may shift if the compound is mixed with other species.”
Considering the possibility of a shift from another wavelength, the team identified some candidates, such as C2H2 ice and benzene mixed with other molecules. Additional testing is needed to verify whether these are likely to cause the absorption feature.
The researchers also say the evidence suggests that the signal is coming from the surfaces of both Titan and Pluto, and not their atmospheres. Modeling of Titan’s atmosphere produced no absorption dips at 5.11 micrometers, while other atmospheric features were accurately depicted in the model.
The search continues
The team suspects that the methane–nitrogen environments of Titan and Pluto, along with the effects of irradiation, likely play a big role in the chemical pathway leading to the 5.11-micrometer feature. They write, “A more likely mechanism is related to the physical state of the molecular species involved, and more specifically to the diversity of its environment at the molecular scale.”
Although the absorption feature remains unidentified, the team plans to continue its search. Additional JWST observations can be used to map where on Titan the 5.11-micrometer feature is strongest, which could narrow down its chemical source.
New laboratory measurements of the selected candidate molecules in realistic mixtures and matrices are also needed to find matches for the feature under Titan- and Pluto-like conditions. And in the mid-2030s, NASA’s new Dragonfly spacecraft could help identify candidate molecules directly on Titan’s surface. However, a lack of infrared spectroscopy will prevent direct detection of the spectral feature on the surface.
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Publication details
B. Bézard et al, An unidentified absorption feature at 5.11 𝜇m on the surface of Titan and Pluto from JWST spectroscopy, arXiv (2026). DOI: 10.48550/arxiv.2606.13350
Journal information:
arXiv
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Titan and Pluto exhibit the same mysterious spectral feature—and researchers can’t figure out its origin (2026, June 22)
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