Titan, Saturn’s largest moon, has a dense atmosphere that makes it difficult to study its surface using spectroscopy. It has a complex topography, including lakes of methane and vast fields of dunes. Pluto, which is colder and has a much thinner atmosphere, has a surface covered in ice made of nitrogen, methane and carbon monoxide.
Despite their differences, these two worlds share a complex chemistry fuelled by solar radiation and energetic particles, which produce a rich diversity of organic molecules, with both atmospheres consisting mainly of dinitrogen and methane.
By utilising the unprecedented sensitivity and spectral coverage of the JWST’s NIRSpec and MIRI instruments, the researchers explored a spectral window around a wavelength of 5 micrometres, which had previously been little studied. There, they detected an absorption band centred at 5.11 micrometres on both Titan and Pluto.
This signature, which is absent from current models and laboratory spectrum databases, appears to originate directly from the surface of both bodies. Whilst it appears narrower on Titan than on Pluto, its origin remains unknown at present.
The team compared this observation with the infrared signatures of numerous ices likely to exist in these extreme environments. Several candidates were considered, but none accurately reproduced the observed characteristics. The authors therefore favour the hypothesis of a compound (or family of compounds) that is as yet poorly characterised, or of a molecule within a mixture of ices, whose spectral properties are profoundly altered by its physical environment.
This discovery opens up a new avenue for understanding the organic chemistry that shapes the surfaces of icy worlds. Future observations by the JWST will enable this mysterious signature on Titan to be mapped, whilst new laboratory experiments will need to replicate the temperature and compositional conditions of Titan and Pluto in order to identify its origin. This work will also pave the way for future explorations of Titan, notably NASA’s Dragonfly mission, expected in the coming decade.
Scientific reference
The article “An unidentified absorption feature at 5.11 μm on the surfaces of Titan and Pluto from JWST spectroscopy” by B. Bézard, E. Lellouch and al., Astronomy & Astrophysics, manuscript no. aa60810-26.