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

Simulations reproduce previously unexplained features of Titan’s methane cycle, attributing them to atmospheric instabilities and cold-trapping of methane in the polar regions. Operation of the methane cycle on Titan The atmosphere on Saturn's moon Titan features an Earth-like cycle, but with methane rather than water taking the pivotal role. The Cassini mission has discovered numerous lakes, dunes and clouds on Titan, and planetary scientists are developing models to explain the atmospheric dynamics involved. Here Schneider et al . report simulations of the methane cycle using a general circulation model that successfully reproduces observations of Titan's methane clouds and lakes. They find that methane is cold-trapped and accumulates in polar regions, mainly in the north. At low latitudes, rare but intense storms occur around the equinoxes, producing enough precipitation to carve surface features. Tropospheric clouds form primarily in mid and high latitudes. The model predicts that prominent clouds are likely to form in the northern polar region within about two (Earth) years and that lake levels are set to rise. Titan has a methane cycle akin to Earth's water cycle. It has lakes in polar regions 1 , 2 , preferentially in the north 3 ; dry low latitudes with fluvial features 4 , 5 and occasional rainstorms 6 , 7 ; and tropospheric clouds mainly (so far) in southern middle latitudes and polar regions 8 , 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 latitudes 16 . 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 sum