Jupiter as an Exoplanet: Insights from Cassini Phase Curves

TL;DR

This study analyzes Cassini phase curves of Jupiter using advanced atmospheric models to infer properties of cloud particles, demonstrating the potential to extract detailed atmospheric information from exoplanet phase curves.

Contribution

It introduces a Bayesian fitting method with double Henyey-Greenstein law to derive aerosol properties from phase curves, applicable to exoplanet observations.

Findings

Aerosols are large, irregular, polydisperse particles causing strong forward scattering.

Single-scattering albedos are near unity, indicating significant multiple scattering.

Narrow backscattering lobes suggest coherent backscattering effects.

Abstract

Due to its proximity to Earth, Jupiter of the Solar System serves as a unique case study for gas-giant exoplanets. In the current study, we perform fits of ab initio, reflective, semi-infinite, homogeneous model atmospheres to 61 phase curves from 0.40 to 1.00 $\mu$m, obtained from the Cassini spacecraft, within a Bayesian framework. We reproduce the previous finding that atmospheric models using classic reflection laws (Lambertian, Rayleigh, single Henyey-Greenstein) provide poor fits to the data. Using the double Henyey-Greenstein reflection law, we extract posterior distributions of the single-scattering albedo and scattering asymmetry factors and tabulate their median values and uncertainties. We infer that the aerosols in the Jovian atmosphere are large, irregular, polydisperse particles that produce strong forward scattering together with a narrow backscattering lobe. The near-unity values of the single-scattering albedos imply that multiple scattering of radiation is an important effect. We speculate that the observed narrow backscattering lobe is caused by coherent backscattering of radiation, which is usually associated with Solar System bodies with solid surfaces and regolith. Our findings demonstrate that precise, multi-wavelength phase curves encode valuable information on the fundamental properties of cloud/haze particles. The method described in this Letter enables single-scattering albedos and scattering asymmetry factors to be retrieved from James Webb Space Telescope phase curves of exoplanets.