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Investigating the overall brightness of planets (and moons) provides insightinto their envelopes and energy budgets [1, 2, 3, 4]. Titan phase curves (arepresentation of overall brightness vs. Sun-object-observer phase angle) havebeen published over a limited range of phase angles and spectral passbands [5,6]. Such information has been key to the study of the stratification,microphysics and aggregate nature of Titan's atmospheric haze [7, 8], and hascomplemented the spatially-resolved observations first showing that the hazescatters efficiently in the forward direction [7, 9]. Here we present CassiniImaging Science Subsystem whole-disk brightness measurements of Titan fromultraviolet to near-infrared wavelengths. The observations reveal that Titan'stwilight (loosely defined as the view when the phase angle 150deg) outshinesits daylight at various wavelengths. From the match between measurements andmodels, we show that at even larger phase angles the back-illuminated moon willappear much brighter than when fully illuminated. This behavior is unique toTitan in our solar system, and is caused by its extended atmosphere and theefficient forward scattering of sunlight by its atmospheric haze. We infer asolar energy deposition rate (for a solar constant of 14.9 Wm-2) of(2.84+/-0.11)x10^14 W, consistent to within 1-2 standard deviations withTitan's time-varying thermal emission spanning 2007- 2013 [10, 11]. We proposethat a forward scattering signature may also occur at large phase angles in thebrightness of exoplanets with extended hazy atmospheres, and that thissignature has valuable diagnostic potential for atmospheric characterization.

Titan brighter at twilight than in daylight