Cassini Spacecraft Reveals Global Energy Imbalance of Saturn

Global energy budget is pivotal to understanding planetary evolution and climate behaviors. Assessing the energy budget of giant planets, particularly those with large seasonal cycles, however, remains a challenge without long-baseline observations. Evolution models of Saturn cannot explain its estimated Bond albedo and internal heat flux [Mankovich and Fortney, 2020], probably because previous estimates were based on limited observations [Hanel et al., 1983]. Here, we analyze the long-term observations recorded by the Cassini spacecraft and find notably higher Bond albedo (0.41 ± 0.02) and internal heat flux (2.84±0.20 Wm-2) values than previous estimates. Surprisingly, Saturn’s global energy budget is not in a steady state. The global radiant energy deficit at the top of the atmosphere, indicative of the planetary cooling of Saturn, reveals remarkable seasonal fluctuations with a magnitude of 16.0±4.2%. Further analysis of the energy budget of the upper atmosphere including the internal heat suggests seasonal energy imbalances at both global and hemispheric scales, possibly contributing to the development of storms on Saturn. Similar seasonal variabilities of planetary cooling and energy imbalance probably exist in other giant planets within and beyond the Solar System, a prospect currently overlooked in existing evolutional and atmospheric models.