Seasonal Variations of Titan's Brightness

Brightness is a fundamental metric of astronomical bodies. The brightness can be represented by the emitted power, which plays important roles in the radiated energy budgets of astronomical bodies. Here, we report the seasonal variations of Titan's emitted power based on the observations recorded by the Cassini spacecraft from 2004 to 2017. The Cassini observations cover three seasons of Titan, which provides an unprecedented opportunity to examine the seasonal variations of Titan's atmosphere. Our measurements show that Titan's emitted power displays different seasonal behaviors between the Northern Hemisphere (NH) and the Southern Hemisphere (SH). The SH-average emitted power decreased ~9.1 ± 0.4%, which is roughly twice the decrease of the NH-average emitted power ~4.6 ± 0.4% during the Cassini period (2004-2017). The global-average emitted power decreased by 6.6 ± 0.6% from 2.41 ± 0.01 W/m² in 2004 to 2.25 ± 0.01 W/m² in 2017. Such a temporal variation (~ 6.6%) represents the magnitude of the seasonal cycle of Titan's global-average emitted power, which is at least one order of magnitude larger than the seasonal variation of Earth's global-average emitted power (< 0.5%). More importantly, the ~6.6% decrease of the global-average emitted power is much smaller than the ~18.6% decrease of the solar constant during the Cassini period, implying a possible imbalance of the radiant energy budget on Titan.