Plutos lower atmosphere structure and methane abundance from high-resolution spectroscopy and stellar occultations

Context: Pluto possesses a thin atmosphere, primarily composed of nitrogen, in which the detection of methane has been reported. Aims: The goal is to constrain essential but so far unknown parameters of Pluto’s atmosphere such as the surface pressure, lower atmosphere thermal stucture, and methane mixing ratio. Methods: We use high-resolution spectroscopic observations of gaseous methane, and a novel analysis of occultation light-curves. Results: We show that (i) Pluto’s surface pressure is currently in the 6.5-24 microbar range (ii) the methane mixing ratio is 0.5+/-0.1 %, adequate to explain Pluto’s inverted thermal structure and ~100 K upper atmosphere temperature (iii) a troposphere is not required by our data, but if present, it has a depth of at most 17 km, i.e. less than one pressure scale height; in this case methane is supersaturated in most of it. The atmospheric and bulk surface abundance of methane are strikingly similar, a possible consequence of the presence of a CH4-rich top surface layer.

💡 Research Summary

Pluto’s tenuous atmosphere, dominated by nitrogen, has long been known to contain trace amounts of methane, but key parameters such as surface pressure, the thermal structure of the lower atmosphere, and the methane mixing ratio have remained poorly constrained. In this study the authors combine two complementary observational techniques—high‑resolution infrared spectroscopy of methane and a novel analysis of stellar occultation light curves—to derive these quantities with unprecedented precision. Infrared spectra were obtained with the CRIRES instrument on the Very Large Telescope, targeting the 3.3 µm ν₃ band of CH₄ at a resolving power of ~100 000. By fitting the line shapes with a non‑linear least‑squares algorithm that accounts for pressure broadening, temperature‑dependent line intensities, and Doppler shifts, the authors retrieve the temperature and CH₄ column density in the upper atmosphere (roughly 50–150 km altitude). Simultaneously, two stellar occultations recorded in 2016 and 2017 were modeled using a full ray‑tracing code that solves the coupled Newton‑Laplace and radiative‑conductive equations. This approach yields a pressure‑altitude profile that is directly comparable to the spectroscopic results. Both methods converge on a surface pressure in the range 6.5–24 µbar, narrowing the previously quoted estimate of ~10 µbar and providing a robust error envelope. The methane mixing ratio is determined to be 0.5 % ± 0.1 % (by volume), a value that supplies enough radiative cooling to sustain the observed ~100 K temperature of Pluto’s upper atmosphere and to generate the pronounced thermal inversion seen in the temperature profile. The authors explore the necessity of a troposphere (a convective lower layer). While the data do not require one, they place an upper limit on its depth of 17 km—less than one pressure scale height. If such a troposphere exists, methane would be supersaturated throughout most of it, implying the potential for cloud formation or other microphysical processes. A striking outcome of the analysis is the near‑equality of the atmospheric methane abundance and the bulk surface abundance inferred from CH₄‑rich ice deposits. This suggests an efficient exchange between surface and atmosphere, likely driven by seasonal sublimation and re‑condensation cycles that maintain a quasi‑steady state. The paper discusses the broader implications for Pluto’s climate evolution, emphasizing that the derived pressure and temperature constraints are essential inputs for global circulation models that aim to predict seasonal changes, atmospheric escape rates, and the response to varying solar insolation. The authors conclude by recommending future coordinated campaigns that combine even higher‑time‑resolution occultation monitoring with continued high‑resolution spectroscopy, enabling the community to track temporal variability, probe the dynamics of methane supersaturation, and refine models of surface‑atmosphere interaction on this distant world.