JWST has detected an identical, unexplained infrared signature on both Titan and Pluto, indicating an unknown compound or family of compounds exists on both worlds despite their vastly different environments. The discovery came from observations of Titan in November 2022 and Pluto in May 2023, with the signal confirmed by two independent JWST instruments (NIRSpec and MIRI), ruling out instrumental error. Planetary scientist Bruno Bézard of the Paris Observatory led the research, which was accepted for publication in Astronomy & Astrophysics.
The mystery molecule absorbs light at precisely the same infrared wavelength on both worlds but matches no known spectral fingerprint in existing catalogs. Researchers believe the compound likely belongs to the hydrocarbon family known as allenes, or possibly a known molecule whose spectral signature has shifted due to mixing with uncatalogued planetary ices. The feature appears only at the 5-micron infrared range where allenes exhibit strong absorption bands.
Evidence points to the mystery compound forming in the atmosphere through photochemical reactions before snowing onto the surface. Both Titan and Pluto have nitrogen and methane-dominated atmospheres where ultraviolet sunlight breaks apart these molecules, triggering reactions that produce complex organic compounds. The signal weakens across Titan's disk from center to limb, consistent with a surface origin rather than atmospheric source. Pluto's near-vacuum atmosphere is too thin to generate such a deep absorption feature on its own, leaving the surface as the only plausible source. When researchers checked Jupiter's moon Ganymede, which lacks a nitrogen-methane atmosphere, they found no trace of the mystery feature, supporting the theory that it is tied to specific atmospheric chemistry.
Upcoming JWST observations will map the signature across Titan's surface to determine whether it associates with specific geologic features like dune fields. NASA's Dragonfly mission, launching in 2028 and arriving at Titan in the mid-2030s, carries a mass spectrometer that could identify surface compounds and help scientists narrow down candidates to test in laboratories.