A missing line of light on Pluto and Titan has left researchers with a molecule they cannot identify.

While studying the surfaces of Pluto and Saturn’s largest moon Titan, researchers found an unexplained absorption feature at the 5.11-micrometer line in their spectral signatures. After checking past datasets and published literature, they did not find a suitable match for the molecule behind it.

The results are detailed in a paper accepted for publication in Astronomy & Astrophysics and now available as a preprint on arXiv.

“We have a few candidates, but it will not be a simple compound,” Bruno Bézard, the study’s first author and a researcher at the Paris Observatory, told New Scientist. “Whatever it is, it will be a surprise.”

Pluto and Titan look different, but both have nitrogen-rich atmospheres with some methane. According to the paper, photochemical effects create a “ubiquitous organic haze” that falls to the surface and shapes surface morphology on both bodies.

The researchers chose a narrow spectral window that was less affected by that haze so they could study the surface more clearly. In that window, Pluto and Titan showed the same missing spectral band, but the team could not identify the chemical compound responsible for the absorption.

Data collected by the James Webb Space Telescope showed the absorption line on Pluto is about three times thicker than Titan’s. On Titan, the molecule behind the feature appears to be more unevenly distributed. The study said that difference likely comes from the “physical state of the unknown compound at the molecular scale.”

The researchers searched the literature for similar compounds and found some possible fits, including “allenes and, if mixed with other species, benzene, ketene, or less likely acetylene,” according to the paper.

But the paper said there is not enough information to tell if the mystery compound is one of those candidates, or something else entirely.

“The lack of onboard infrared spectroscopy capabilities prevents any direct observation of the spectral feature itself in surface materials,” the researchers concluded.

The team told New Scientist it had already secured more James Webb Space Telescope data for follow-up work. NASA’s Dragonfly mission is scheduled to reach Titan in the mid-2030s, with hopes it could return new information about Titan’s surface.

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