An astonishing discovery comes from the outer reaches of the Solar System: Pluto and Titan, two very different celestial bodies, both display an absorption line never before observed. The James Webb Space Telescope detected this signature at 5.11 micrometers, a wavelength that does not correspond to any known chemical compound elsewhere. This observation indicates that a yet-unidentified molecule exists on the surface of these two bodies.
To grasp the significance of this finding, we must recall the principle of spectroscopy. Each element or molecule absorbs specific wavelengths of light. By analyzing the light reflected by a celestial body, astronomers look for dark lines, called absorption lines, which betray the presence of a given substance. For example, molecular oxygen absorbs at 230 nanometers. This is how the composition of planets and moons is studied.
Titan (top left) and Pluto (bottom right). Image credits: Titan: NASA/JPL/Space Science Institute ; Pluto: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute ; Spectrograph: NOAO/AURA/NSF ; annotations by Harry Baker Thanks to its exceptional sensitivity, JWST was able to probe very short wavelengths, still little explored. By observing Pluto and Titan, scientists highlighted an absorption line at 5.11 micrometers. No previous publication mentions a similar line, whether for other planets in the Solar System or for exoplanets. The researchers therefore believe that this signature comes from an unknown molecule, present only on these two icy bodies.
It is remarkable that Pluto and Titan have very little in common. Titan, Saturn's largest moon, has rivers and oceans of liquid methane on its surface, and is larger than Mercury. Pluto, on the other hand, is a frozen dwarf planet, twice as small as Titan and four times farther from the Sun. Yet the two worlds share atmospheres rich in nitrogen and methane. But this unknown molecule appears to be located on their surfaces, not in their atmospheres.
On Pluto, the absorption line is about three times stronger than on Titan, indicating a higher concentration. On Titan, the distribution is uneven: the trailing hemisphere (the one opposite the direction of rotation around Saturn) shows stronger absorption than the leading hemisphere. This asymmetry intrigues researchers, who are working to understand its origin.
Artist's rendition of Titan's landscape with a hazy atmosphere.
Several explanations have been proposed. It could be benzene, a ring-shaped hydrocarbon, mixed with another unknown molecule. Or else acetylene or ketene ice. But none of these leads are confirmed yet. The study authors note that additional work is needed to definitively identify the compound responsible for this 5.11-micrometer absorption.
The future may bring answers. NASA's Dragonfly mission, scheduled for launch after 2028, is to fly over Titan in 2034. This robotic helicopter will carry a spectrograph capable of analyzing the moon's surface. In the meantime, astronomers remain alert, hoping to solve this mystery.