  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.
