Dimming Titan Revealed by the Cassini Observations

Here we report the temporal variation of Titan's emitted energy with the Cassini/CIRS observations. In the northern hemisphere, the hemispheric-average emitted power decreased from 2007 to 2009 and increased from 2009 to 2012-13, which make the net change insignificant (0.1 +/- 0.2%) during the period 2007-2013. The decrease from 2007 to 2009 is mainly due to the cooling around the stratospause, and the increase from 2009 to 2012-13 is probably related to temporal variation of atmospheric temperature around the tropopuase in the northern hemisphere. In the southern hemisphere, the emitted power continuously decreased by 5.0 +/- 0.6% from 2.40 +/- 0.01W/m(2) in 2007 to 2.28 +/- 0.01 in 2012-13, which is mainly related to Titan's seasonal variation. The asymmetry in the temporal variation between the two hemispheres results in the global-average emitted power decreasing by 2.5 +/- 0.6% from 2.41 +/- 0.01 W/m(2) in 2007 to 2.35 +/- 0.01 W/m(2) in 2012-13. The solar constant at Titan decreased by,13.0% in the same period 2007-2013, which is much stronger than the temporal variation of emitted power. The measurements of Titan's absorbed solar power are needed to determine the temporal variation of the global energy budget.