Observational evidence for active dust storms on Titan at equinox

Abstract

Saturn’s moon Titan has a dense nitrogen-rich atmosphere, with methane as its primary volatile. Titan’s atmosphere experiences an active chemistry that produces a haze of organic aerosols that settle to the surface and a dynamic climate in which hydrocarbons are cycled between clouds, rain and seas. Titan displays particularly energetic meteorology at equinox in equatorial regions, including sporadic and large methane storms. In 2009 and 2010, near Titan’s northern spring equinox, the Cassini spacecraft observed three distinctive and short-lived spectral brightenings close to the equator. Here, we show from analyses of Cassini spectral data, radiative transfer modelling and atmospheric simulations that the brightenings originate in the atmosphere and are consistent with formation from dust storms composed of micrometre-sized solid organic particles mobilized from underlying dune fields. Although the Huygens lander found evidence that dust can be kicked up locally from Titan’s surface, our findings suggest that dust can be suspended in Titan’s atmosphere at much larger spatial scale. Mobilization of dust and injection into the atmosphere would require dry conditions and unusually strong near-surface winds (about five times more than estimated ambient winds). Such strong winds are expected to occur in downbursts during rare equinoctial methane storms—consistent with the timing of the observed brightenings. Our findings imply that Titan—like Earth and Mars—has an active dust cycle, which suggests that Titan’s dune fields are actively evolving by aeolian processes.

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Fig. 1: VIMS colour composite maps of Titan’s singular brightening events that occurred in June 2009 (T56 flyby), January 2010 (T65) and June 2010 (T70).
Fig. 2: VIMS infrared spectra of the brightening events that occurred at T56, T65 and T70 flybys.
Fig. 3: Observed spectra of the brightest pixels compared with best-match calculated spectra.
Fig. 4: Transport thresholds on Titan.

Data availability

VIMS data are available via NASA’s Planetary Data System (PDS): http://pds-atmospheres.nmsu.edu/data_and_services/atmospheres_data/Cassini/vims.html. The data that support the analysis and plots within this paper and other findings of this study are available from the corresponding author upon request.

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Acknowledgements

We thank P. Claudin and B. Andreotti for discussions, especially regarding thresholds and modes of sediment transport. We are also grateful to the Cassini/VIMS team for the calibration and planning of the data. We acknowledge financial support from the UnivEarthS LabEx programme of Sorbonne Paris Cité (ANR-10-LABX-0023 and ANR-11-IDEX-0005-02), the French National Research Agency (ANR-APOSTIC-11-BS56-002 and ANR-12-BS05-001-03/EXO-DUNES) and the CNES. This study was partly supported by the Institut Universitaire de France. T.C. was funded by the ESA Research Fellowship Programme in Space Sciences. Part of this work has been performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA.

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Contributions

S.R., S.L.M., J.W.B., J.B. and G.V. discovered the brightening spots in the VIMS dataset and proposed the dust storm hypothesis. S.R., T.A., M.H., L.M., P.R., C.A.G. and A.C. developed, adapted and ran the radiative transfer model and designed the inversion scheme, including a low atmospheric layer of suspended particles. B.C., J.F.K., R.D.L., J.R., C.N. and S.C.P. helped with the wind and sediment transport calculations. J.F.K., C.N., T.C., O.B. and A.L. participated in the discussion of the geological implications of dust storm occurrence in Titan’s atmosphere. C.S., R.H.B., K.H.B., B.J.B., R.N.C. and P.D.N. designed the planning of VIMS observations. S.R. drafted the manuscript with contributions from all authors.

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Correspondence to S. Rodriguez.

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Supplementary Figures 1–8, Supplementary Tables 1–6 and Supplementary Methods.

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Rodriguez, S., Le Mouélic, S., Barnes, J.W. et al. Observational evidence for active dust storms on Titan at equinox. Nature Geosci 11, 727–732 (2018). https://doi.org/10.1038/s41561-018-0233-2

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