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Titan brighter at twilight than in daylight

Nature Astronomy volume 1, Article number: 0114 (2017) Cite this article

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Abstract

Investigating the overall brightness of planets (and moons) provides insights into their envelopes and energy budgets1–4. Phase curves (a representation of the overall brightness versus the Sun–object–observer phase angle) for Titan have been published over a limited range of phase angles and spectral passbands5,6. Such information has been key to the study of the stratification, microphysics and aggregate nature of Titan’s atmospheric haze7,8 and has complemented the spatially resolved observations showing that the haze scatters efficiently in the forward direction7,9. Here, we present Cassini Imaging Science Subsystem whole-disk brightness measurements of Titan from ultraviolet to near-infrared wavelengths. The observations show that Titan’s twilight (loosely defined as the view at phase angles ≳150°) outshines its daylight at various wavelengths. From the match between measurements and models, we show that at even larger phase angles, the back-illuminated moon will appear much brighter than when fully illuminated. This behaviour is unique in our Solar System to Titan and is caused by its extended atmosphere and the efficient forward scattering of sunlight by its atmospheric haze. We infer a solar energy deposition rate (for a solar constant of 14.9 W m−2) of (2.84 ± 0.11) × 1014 W, consistent to within one to two standard deviations with Titan’s time-varying thermal emission from 2007 to 201310,11. We propose that a forward scattering signature may also occur at large phase angles in the brightness of exoplanets with extended hazy atmospheres and that this signature has a valuable diagnostic potential for atmospheric characterization.

For larger phase angles up to ~140°, the diminishing size of Titan’s visible dayside is compensated by efficient forward scattering of its upper atmosphere haze and the curves show plateaus or mild increases in brightness. As Titan nears back-illumination (α ≥ 150°), the twilight brightness increases steeply and at α ≈ 160–166° it is comparable to, or higher than, the brightness at full illumination. This is particularly true for wavelengths at which absorption by haze (≲600 nm) or atmospheric methane (strong bands occur at 619, 727 and 889 nm; Fig. 2) make Titan’s dayside darker. The brightness surge is unrelated to the central flash caused by atmospheric refraction during stellar occultations19.

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Figure 1: Titan phase curves inferred in this work.
Figure 2: Full disk albedo spectrum at phase angle α = 5.7°.
Figure 3: Radiative transfer modelling.
Figure 4: Rates for incident solar energy and energy scattered by Titan.

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Acknowledgements

A.G.M. gratefully acknowledges correspondence with L.A. Sromovsky and P.M. Fry on Voyager 2 observations. P.L. acknowledges financial support from the Programme National de Planétologie of the Institut National des Sciences de l’Univers/Centre National de la Recherche Scientifique.

Author information

Author notes

  1. e-mail: garciamunoz@astro.physik.tu-berlin.de;

  2. tonhingm@gmail.com

Authors and Affiliations

  1. Zentrum für Astronomie und Astrophysik, Technische Universität Berlin, Berlin, D-10623, Germany.

    A. García Muñoz

  2. Groupe de Spectrométrie Moléculaire et Atmosphérique, UMR 7331, CNRS, Université de Reims Champagne-Ardenne, Reims, 51687, France.

    P. Lavvas

  3. Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, 91109, California, USA.

    R. A. West

Authors
  1. A. García Muñoz
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  2. P. Lavvas
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Contributions

A.G.M. devised the research, performed the data reduction and model simulations and wrote the manuscript. P.L. provided various haze properties. R.A.W. provided insight into the treatment of images. P.L. and R.A.W. provided valuable expertise on Titan’s atmosphere. All authors discussed the content of the manuscript.

Corresponding author

Correspondence to A. García Muñoz.

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The authors declare no competing financial interests.

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Supplementary information

Supplementary Figure 1 and Supplementary Table 1 (PDF 150 kb)

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García Muñoz, A., Lavvas, P. & West, R. Titan brighter at twilight than in daylight. Nat Astron 1, 0114 (2017). https://doi.org/10.1038/s41550-017-0114

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