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Methane drizzle on Titan

Abstract

Saturn's moon Titan shows landscapes with fluvial features1 suggestive of hydrology based on liquid methane. Recent efforts in understanding Titan's methane hydrological cycle have focused on occasional cloud outbursts near the south pole2,3,4 or cloud streaks at southern mid-latitudes5,6 and the mechanisms of their formation. It is not known, however, if the clouds produce rain or if there are also non-convective clouds, as predicted by several models7,8,9,10,11. Here we show that the in situ data on the methane concentration and temperature profile in Titan's troposphere point to the presence of layered optically thin stratiform clouds. The data indicate an upper methane ice cloud and a lower, barely visible, liquid methane-nitrogen cloud, with a gap in between. The lower, liquid, cloud produces drizzle that reaches the surface. These non-convective methane clouds are quasi-permanent features supported by the global atmospheric circulation, indicating that methane precipitation occurs wherever there is slow upward motion. This drizzle is a persistent component of Titan's methane hydrological cycle and, by wetting the surface on a global scale, plays an active role in the surface geology of Titan.

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Figure 1: Vertical profile of the methane relative humidity at the Huygens entry site under different assumptions.
Figure 2: Thermodynamic diagram at the Huygens entry site.
Figure 3: Diagram of the vertical cloud structure at the Huygens entry site.

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Acknowledgements

T.T. and F.M.N. were supported by the DLR in the HASI project. T.T. received a grant from the DFG. S.K.A. and C.P.M. acknowledge support from NASA's Planetary Atmospheres Program and the Cassini-Huygens Project. M.F. and F.F. acknowledge support from the Italian Space Agency (ASI) for the HASI experiment on board the ESA Huygens probe.

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Correspondence to Tetsuya Tokano.

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Supplementary Notes

In this file we discuss how the surface precipitation rate at the Huygens entry site is estimated based on previous model results and a comparison between the observed and modelled methane humidity profiles. (DOC 31 kb)

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Tokano, T., McKay, C., Neubauer, F. et al. Methane drizzle on Titan. Nature 442, 432–435 (2006). https://doi.org/10.1038/nature04948

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