Constraining Thermal Emission of Pluto's Haze From Infrared Rotational Lightcurves

TL;DR

This study uses infrared lightcurves and Bayesian analysis to investigate Pluto's atmospheric haze emission, finding it consistent with observations but not definitively constrained, and suggests future JWST observations could clarify haze contributions.

Contribution

First to employ Bayesian retrieval on infrared lightcurves to constrain Pluto's haze emission, integrating surface maps and exploring haze's role in thermal emission.

Findings

Haze emission flux around 1.63 mJy consistent with Titan-like tholins.

Current data cannot definitively constrain haze due to degeneracy with surface emission.

Surface properties like thermal inertia and methane emissivity are well constrained.

Abstract

The rotational lightcurves of the Pluto-Charon system were previously believed to be solely attributed to their surfaces. However, a proposed scenario of haze cooling \citep{2017Natur.551..352Z} suggests that the atmospheric haze of Pluto could significantly contribute to mid-infrared emission, which calls for a revisit of previous analyses. In this study, we employ a Bayesian retrieval approach to constrain the haze emission from the rotational lightcurves of the Pluto-Charon system. The lightcurves were observed by the Spitzer and Herschel telescopes at 24 and 70 $\mu$m, and were combined with the latest surface albedo maps of Pluto and Charon from the New Horizons spacecraft. Our results show that including the haze emission is consistent with all current observations, with the best-fit haze flux around 1.63 mJy. This is in agreement with the composition of Titan-like tholins. However, the ``surface only" scenario, which excludes the haze contribution, can still explain the observations. We conclude that the current data at 24 $\mu$m cannot constrain Pluto's haze emission due to the degeneracy with Charon's surface emission. Regardless, some surface properties of Pluto are well constrained by the shape of the lightcurves, with a thermal inertia of approximately 8--10 MKS and a relatively low CH$_4$ emissivity of 0.3--0.5. We suggest that observations by the JWST telescope at 18 $\mu$m, which can resolve Pluto from Charon, could directly probe the haze emission of Pluto due to the low surface emission at that wavelength.