2025A&A...696A.147L

Context. Pluto possesses a tenuous, time-variable, N₂-dominated atmosphere, with extensive haze. Previous spectroscopic observations from the ground at near-infrared (NIR) and submillimeter (submm) wavelengths and New Horizons in the ultraviolet (UV) have revealed a Titan-like atmosphere with rich N₂-CH₄ photochemistry. The mid-infrared (MIR) range of Pluto's spectrum, however, has remained out of reach thus far.
Aims. Taking advantage of JWST sensitivity, our goal is to obtain new insights into Pluto's atmospheric gas and haze composition using mid-IR spectroscopy.
Methods. In May 2023, we used JWST/MIRI MRS to acquire a high signal-to-noise (S/N) spectrum of Pluto over 4.9-27 μm, finally opening up the MIR spectral range for Pluto. The data were analyzed with a standard radiative transfer code, including the solar and thermal components, and the gas and haze emission, using gas vertical distributions from photochemical models as templates.
Results. The Pluto spectrum consists of the superposition of solar light reflected off Pluto's surface, surface thermal emission, gas thermal and non-thermal emission, and haze emission. The solar reflected component shows absorption by CH₄, CH₃D, and C₂H₄ surface ices. Spectral signatures of C₂H₆, C₂H₂, CH₃C₂H, and C₄H₂ gases are strongly detected over 12-16 μm, broadly probing the stratopause region (altogether spanning 15-100 km). Unexpectedly, we also detect fluorescence (non-LTE) emission from gas CH₄ (ν₄ and hot bands) and CH₃D (ν₆ band) in the 7-9 μm range, indicating excitation temperatures that are much higher than Pluto's atmosphere kinetic temperature. The C₂H₆ abundance agrees very well with photochemical models, but those of CH₃C₂H, and C₄H₂ are about five and ten times lower than model predictions, respectively. The C₂H₂ 12.9-14.7 μm emission (ν₅) is imperfectly fit and may point to a less steep C₂H₂ profile than in model predictions or (perhaps more likely) to non-LTE effects on this band. Remarkably, C₂HD is detected at 14.75 μm, yielding a (D/H)_C2H2 ratio equal to (3±1) terrestrial. Also, HCN has been tentatively observed and upper limits on several other gases (C₂H₄, C₃H₈, C₆H₆, HC₃N, and CO₂) are obtained. The haze emission is clearly present over 13-20 μm and characterized by emission peaks at 15.45 μm, 14.58 μm, and maybe 13.60 μm. The haze spectrum is very different from Titan's and points to the presence of pure or mixed ices (e.g., C₄H₂, C₆H₆), as previously proposed.
Conclusions. The spectacular JWST MIRI spectrum is giving us a new look at Pluto's atmosphere. An improved non-LTE modeling of the fluorescent emissions (CH₄, CH₃D, and possibly C₂H₂) and of the ice features is expected to yield a broader view of Pluto's D/H ratio in different gases (CH₄, C₂H₂) and phases (gas and solid). This would bear key information on Pluto's ice origin and evolution.