Jupiter's Phase Variations from Cassini: a testbed for future direct-imaging missions

TL;DR

This study presents detailed phase curves of Jupiter from Cassini data, revealing its brightness and color variability at different phase angles, which are crucial for interpreting future exoplanet direct imaging observations.

Contribution

The paper provides the first detailed phase curves of Jupiter across multiple wavelengths from Cassini, highlighting deviations from ideal models and implications for exoplanet characterization.

Findings

Jupiter is about 25% darker at partial phases than a Lambertian model.

Jupiter's color varies more with phase angle than models predict.

Analytic fits for Jupiter's phase function in multiple filters are provided.

Abstract

We present phase curves of Jupiter from 0-140 degrees as measured in multiple optical bandpasses by Cassini/ISS during the Millennium flyby of Jupiter in late 2000 to early 2001. Phase curves are of interest for studying the energy balance of Jupiter and understanding the scattering behavior of Jupiter as an exoplanet analog. We find that Jupiter is significantly darker at partial phases than an idealized Lambertian planet by roughly 25% and is not well fit by Jupiter-like exoplanet atmospheric models across all wavelengths. We provide analytic fits to Jupiter's phase function in several Cassini/ISS imaging filter bandpasses. In addition, these observations show that Jupiter's color is more variable with phase angle than predicted by models. Therefore, the color of even a near Jupiter-twin planet observed at a partial phase cannot be assumed to be comparable to that of Jupiter at full phase. We discuss how WFIRST and other future direct-imaging missions can enhance the study of cool giants.