Seasonal Variations of Titan's Brightness

Abstract

The absolute brightness of astronomical bodies can be represented by the emitted power, which plays important roles in their radiated energy budgets. The Cassini observations include three seasons of Titan, which provides an unprecedented opportunity to examine the seasonal variations of Titan's emitted power. Our analyses show that Titan's emitted power displays different seasonal behaviors between the Northern Hemisphere and the Southern Hemisphere. The global-average emitted power decreased by 6.8 ± 0.4% during the Cassini period (2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017). Such a temporal variation represents the magnitude of the seasonal cycle of Titan's emitted power, which is at least one order of magnitude stronger than the seasonal variation of Earth's emitted power (<0.5%). More importantly, the~6.8% decrease of the emitted power is much smaller than the~18.6% decrease of the solar flux from the change of Sun-Titan distance, implying a significantly dynamical energy budget on Titan.Key Points:• The seasonal variations of Titan's emitted power are examined for the first time. • The seasonal cycle of the global-average emitted power is at least one order of magnitude stronger on Titan than on Earth. • The comparison of seasonal cycle between emitted power and solar flux suggests a significantly dynamical energy budget on Titan.

Variability in Titan’s Mesospheric HCN and Temperature Structure as Observed by ALMA

“…These predictions, along with inferences of temperature from in situ density profiles obtained with the Cassini Ion and Neutral Mass Spectrometer (INMS) and Huygens Atmospheric Structure Instrument (HASI) and measurements derived through stellar occultations from the ground and the Cassini Ultraviolet Imaging Spectrograph (UVIS), revealed large variability in Titan's middle and upper atmospheric thermal structure on both diurnal and seasonal timescales. Further, observations from the Cassini Composite Infrared Spectrometer (CIRS), Imaging Science Subsystem (ISS), and Visible and Infrared Mapping Spectrometer (VIMS) instruments covering roughly half a Titan year indicated that Titan's radiative energy budget may not be fully balanced on both temporal (seasonal) and spatial (hemispheric) scales (Li 2015;Creecy et al 2019Creecy et al , 2021, though further observations are required to determine the nature and effects of this imbalance on timescales of a Titan year or more.…”

Titan's Global Radiant Energy Budget During the Cassini Epoch (2004–2017)

“…Figure 3 shows the comparison between the absorbed power from this study and the emitted power measured in our previous study (Creecy et al., 2019). The decrease in the absorbed power (18.7% ± 0.5%) is much stronger than the decrease of emitted power (6.8 ± 0.4%) for the Cassini epoch.…”

“…(a) Comparison between the absorbed solar power (red line) and the emitted thermal power (blue line). The measurements of Titan's emitted power are from our previous study (Creecy et al., 2019). (b) The ratio between the net radiant energy (i.e., absorbed power‐emitted power).…”

“…The combination of the dramatic change of solar flux and the relatively weak variation of Bond albedo means that the absorbed solar power varies strongly (Figure 2). On the other hand, the temporal variation of the emitted power is relatively weak because of the large thermal inertial and long radiative time constant of the atmospheric layers mainly contributing to Titan's emitted power (Creecy et al., 2019; Li et al., 2011). The change of emitted power cannot match the change in the absorbed solar power at the timescales of Earth years and Titan's seasons.…”

“…We discuss these uncertainties in Supporting Information (Figures S32–S47 in Supporting Information ). We also discuss the uncertainties of Titan's emitted power (the other component of Titan's radiant energy budget), even though such uncertainties were already investigated in our previous studies of Titan's emitted power (Creecy et al., 2019; Li et al., 2011). It should be mentioned that we have consider all possible uncertainty sources to the best of our ability but it is still possible that there are more uncertainty sources.…”