Jupiter as an Exoplanet Analog: Understanding the Effect of Atmospheric Variability from Cassini ISS Observations

The Coronagraph Instrument (CGI) on board NASA’s upcoming Nancy Grace Roman Space Telescope will directly image true Jupiter-analog exoplanets – planets with masses, semi-major axes, and ages similar to those of Jupiter – in reflected light for the first time. Individual observations of Jupiter-analog exoplanets (previously detected by radial velocity) by CGI will require ~1-2 days of observing time with spacing between these observations of weeks to months. The goal of these CGI observations is to recover fundamental parameters of the exoplanet, including temperature, composition, and radius, using detailed atmospheric models and by combining observations made at multiple phase angles and planet-star separations. Imaging observations of Jupiter show that gas giant planets have large-scale high-contrast features in their atmospheres that can evolve on the same timescales over which Roman will be characterizing these exoplanets. Therefore, it is imperative that the effect of color variability on observed photometry and spectroscopy of exoplanets is understood in order to accurately retrieve planet parameters. In this work, we treat Jupiter as a test exoplanet and measure its disk-averaged absolute reflectivity in the visible and near infrared regimes from imaging observations by the Imaging Science Subsystem (ISS) on Cassini during the Millennium flyby of Jupiter in late 2000 to early 2001. We measure the magnitude of the variability in the phase curve for multiple photometric bandpasses as Jupiter rotates and as the spacecraft flies by the planet. We discuss the contributions to variability in the phase curve as a function of color from Jupiter’s rotation, such as the Great Red Spot rotating on and off the disk of the planet, and long term color effects, such as the South Equatorial Belt fade in 2009 - 2010. We present preliminary results of our analysis, a necessary first step towards quantifying the extent that atmospheric color variability on these timescales could bias Roman CGI determinations of fundamental exoplanet parameters.