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Polar methane accumulation and rainstorms on Titan from simulations of the methane cycle

Abstract

Titan has a methane cycle akin to Earth's water cycle. It has lakes in polar regions1,2, preferentially in the north3; dry low latitudes with fluvial features4,5 and occasional rainstorms6,7; and tropospheric clouds mainly (so far) in southern middle latitudes and polar regions8,9,10,11,12,13,14,15. Previous models have explained the low-latitude dryness as a result of atmospheric methane transport into middle and high latitudes16. Hitherto, no model has explained why lakes are found only in polar regions and preferentially in the north; how low-latitude rainstorms arise; or why clouds cluster in southern middle and high latitudes. Here we report simulations with a three-dimensional atmospheric model coupled to a dynamic surface reservoir of methane. We find that methane is cold-trapped and accumulates in polar regions, preferentially in the north because the northern summer, at aphelion, is longer and has greater net precipitation than the southern summer. The net precipitation in polar regions is balanced in the annual mean by slow along-surface methane transport towards mid-latitudes, and subsequent evaporation. In low latitudes, rare but intense storms occur around the equinoxes, producing enough precipitation to carve surface features. Tropospheric clouds form primarily in middle and high latitudes of the summer hemisphere, which until recently has been the southern hemisphere. We predict that in the northern polar region, prominent clouds will form within about two (Earth) years and lake levels will rise over the next fifteen years.

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Figure 1: Annual cycle of zonal-mean climate statistics in Titan GCM.
Figure 2: Relationship between tropospheric cloudiness and atmospheric circulation in southern summer in Titan GCM.
Figure 3: Annual cycle of tropospheric methane-cloud frequency.

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Acknowledgements

We are grateful for support by a NASA Earth and Space Science Fellowship and a David and Lucile Packard Fellowship. We thank I. Eisenman for code for the insolation calculations, and O. Aharonson, A. Hayes and A. Soto for comments on a draft. The simulations were done on the California Institute of Technology’s Division of Geological and Planetary Sciences Dell cluster.

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Authors

Contributions

T.S. and M.E.B. conceived the study; T.S., S.D.B.G. and E.L.S. developed the GCM; E.L.S. and M.E.B. provided data; and T.S. and S.D.B.G. wrote the paper, with contributions and comments from all authors.

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Correspondence to T. Schneider.

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The authors declare no competing financial interests.

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Schneider, T., Graves, S., Schaller, E. et al. Polar methane accumulation and rainstorms on Titan from simulations of the methane cycle. Nature 481, 58–61 (2012). https://doi.org/10.1038/nature10666

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