Modeling Pluto’s rotational thermal emission from its atmosphere and surface

The first in-situ observations of the Pluto-Charon system during New Horizons flyby revolutionized our understanding of their atmosphere and surfaces. Pluto’s atmosphere, made of various gases and globally distributed haze particles, is much colder than expected due to the strong haze cooling effect. It implies haze particles, rather than gas molecules, dominate the energy equilibrium of Pluto’s upper atmosphere. In this work, we find that eddy diffusion of the potential temperature is more significant in the lower atmosphere (i.e., below 50 kilometers), which was neglected in precious models. Our modeling temperature profile is close to the diffusional equilibrium and is consistent with the New Horizons measurements.

Multi-terrain surfaces of Pluto and Charon strongly influence the outgoing thermal emission, which has been observed by the Spitzer telescope and shown as rotational light curves. Pluto’s surface has complex distributions of volatile ices caused by sublimation and condensation, and Charon has a relatively uniform surface with a reddish northern polar region. We investigated their surface compositions and albedo properties. We then included them in our surface thermophysical model, where major factors (e.g., thermal inertia, diurnal and seasonal changes of insolation, and emissivity) were systematically explored. We conclude that the peak-to-trough amplitude of our modeling surface emission light curves is comparable with Spitzer observations in the infrared, but the mean flux is much smaller. This indicates additional flux from Pluto’s atmosphere significantly contributes to the outgoing emission flux in the mid-infrared. This emission flux likely comes from the atmospheric haze and might be as large as Charon’s surface thermal emission. Finally, by using our new atmosphere-surface model of Pluto and Charon including eddy diffusion, gas and haze radiative transfer, and surface thermal evolution, we predict the mid-infrared emission spectra. Especially, we predict rotational light curves at featured wavelengths to be detected by future JWST observations.