An unidentified absorption feature at 5.11 $\mu$m on the surface of Titan and Pluto from JWST spectroscopy
Titan possesses a thick N$_2$-CH$_4$ atmosphere that makes it difficult to study its surface spectroscopically. The chemical composition of the solid surface of Titan thus remains very uncertain. By leveraging JWST's high sensitivity and large spectral coverage, we searched for any signature from Titan's surface in the broad and less explored 5-$\mu$m atmospheric window. We also investigated the JWST spectrum of Pluto which has a thin Titan-like atmosphere. We made selections of JWST NIRSpec and MIRI spectra around Titan's disk center and compared the NIRSpec average spectrum with a radiative transfer model including gas and haze opacity. We detected an unidentified absorption in both NIRSpec and MIRI spectra of Titan centered at 5.113 $\mu$m (1956 cm$^{-1}$) and 6-7% deep. The width of the feature is 0.024$\pm$0.0008 $\mu$m (9.2$\pm$0.3 cm$^{-1}$) in the NIRSpec spectrum recorded on the trailing side and is possibly 25% narrower in the MIRI spectrum of the leading side. This absorption most likely originates from the surface. We could not identify this signature among published laboratory spectra of ices relevant to Titan's atmospheric compounds but present a few plausible candidates. A 4-5% deep absorption is also present in the MIRI spectrum of Pluto but is about 3 times broader than on Titan's trailing side.