Polar methane accumulation and rainstorms on Titan from simulations of the methane cycle
Titan has a methane cycle akin to Earth's water cycle. It has lakes in polar regions, preferentially in the north; dry low latitudes with fluvial features and occasional rainstorms; and tropospheric clouds mainly (so far) in southern middle latitudes and polar regions. Previous models have explained the low-latitude dryness as a result of atmospheric methane transport into middle and high latitudes. 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.
Additional Information© 2012 Macmillan Publishers Limited. Received 04 August 2011. Accepted 20 October 2011. Published online 04 January 2012. 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. Author 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. The authors declare no competing financial interests.
Supplemental Material - nature10666-s1.pdf