The Albedo Problem and Cloud Cover on Hot Jupiters

TL;DR

This paper investigates the variability of albedo and phase integral on hot Jupiters across different wavelengths, proposing that phase integral measurements can reveal cloud cover properties with upcoming space missions.

Contribution

It introduces a method to identify partial cloud cover on hot Jupiters using phase integral measurements and discusses how upcoming missions can constrain atmospheric properties.

Findings

Phase integrals vary significantly across wavelengths.

Cloud cover can be inferred from phase integral uncertainties.

Ariel mission will enable detailed cloud cover studies.

Abstract

Observations of transiting hot Jupiters have revealed a mismatch between the values of the Bond versus geometric albedos. In the planetary science literature, the ratio of these quantities is known as the phase integral. It has been extensively measured for the Solar System planets and shown to generally be non-unity in value. We use existing Cassini data of Jupiter to derive bandpass-integrated geometric albedos and phase integrals in the CHEOPS, TESS and Ariel bandpasses, demonstrating that these quantities vary markedly across these different wavelength ranges. By performing a population study of geometric albedos and phase integrals, we demonstrate that atmospheres with partial cloud cover may be identified using measurements of the phase integral if its measured uncertainty is $\sim 0.1$, which corresponds to an uncertainty of $\sim 3\%$ on the optical/visible secondary eclipse depth. The upcoming Ariel space mission will conduct an unprecedented statistical survey of cloud cover on hot Jupiters via the simultaneous measurement of $\sim 100$ infrared phase curves and optical secondary eclipses. Whenever available, the shape of optical phase curves of reflected light will directly constrain the phase integral, spherical albedo, degree of cloud cover and scattering asymmetry factor.