Every element has a unique set of absorption and emission lines that create spectral signatures. These signals are key information for astronomers studying faraway objects—but what happens when there’s no good match for a particular signature?
While studying Pluto and Saturn’s largest moon Titan, researchers spotted a missing band of light in their spectral signatures, specifically at the 5.11-micrometer line. However, after searching past datasets and literature, they weren’t able to find any suitable matches for whatever molecule was responsible for this phenomenon. In other words, Pluto’s and Titan’s surfaces may harbor an unknown molecule—a substance that we’ve never seen anywhere else in our solar system and beyond.
A paper detailing the results has been accepted for publication in Astronomy & Astrophysics and is currently available as a preprint in 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.”
Close but far
Pluto and Titan don’t look much alike externally, but both possess nitrogen-rich atmospheres with some methane. Photochemical effects create a “ubiquitous organic haze” that eventually trickles down to shape the surface morphology of Pluto and Titan, according to the paper. Researchers behind the latest paper selected a narrow spectral window least affected by the haze to investigate the surface beyond the foggy atmosphere.
They probably weren’t expecting to learn of an unidentified substance, however. That Pluto and Titan share the same missing spectral band was fascinating on its own, but the team couldn’t find a good match for the chemical compound responsible for this absorption. According to the data collected by the James Webb Space Telescope (JWST), the absorption line for Pluto is roughly three times thicker than Titan’s, where the molecule causing this gap appears to be more unevenly distributed. This difference likely stems from the “physical state of the unknown compound at the molecular scale,” the study noted.
Waiting for Dragonfly
The researchers scoured the literature to see if anything could be even remotely similar, and, to their credit, they did find some decent fits, such as “allenes and, if mixed with other species, benzene, ketene, or less likely acetylene,” according to the paper. However, there isn’t enough information available to determine whether the mystery compound is one of these candidates or none at all.
“The lack of onboard infrared spectroscopy capabilities prevents any direct observation of the spectral feature itself in surface materials,” the researchers concluded.
That said, the team told New Scientist that it had already secured more JWST data for follow-up investigations. Further down the line, NASA’s Dragonfly mission is slated to reach Titan in the mid-2030s, and the hope is that the spacecraft will send back some new information about Titan’s surface.
