  The strength of Titan's methane cycle, as measured by precipitation and
evaporation, is key to interpreting fluvial erosion and other indicators of the
surface-atmosphere exchange of liquids. But the mechanisms behind the
occurrence of large cloud outbursts and precipitation on Titan have been
disputed. A gobal- and annual-mean estimate of surface fluxes indicated only 1%
of the insolation, or $\sim$0.04 W/m$^2$, is exchanged as sensible and/or
latent fluxes. Since these fluxes are responsible for driving atmospheric
convection, it has been argued that moist convection should be quite rare and
precipitation even rarer, even if evaporation globally dominates the
surface-atmosphere energy exchange. In contrast, climate simulations that allow
atmospheric motion indicate a robust methane cycle with substantial cloud
formation and/or precipitation. We argue the top-of-atmosphere radiative
imbalance -- a readily observable quantity -- is diagnostic of horizontal heat
transport by Titan's atmosphere, and thus constrains the strength of the
methane cycle. Simple calculations show the top-of-atmosphere radiative
imbalance is $\sim$0.5-1 W/m$^2$ in Titan's equatorial region, which implies
2-3 MW of latitudinal heat transport by the atmosphere. Our simulation of
Titan's climate suggests this transport may occur primarily as latent heat,
with net evaporation at the equator and net accumulation at higher latitudes.
Thus the methane cycle could be 10-20 times previous estimates. Opposing
seasonal transport at solstices, compensation by sensible heat transport, and
focusing of precipitation by large-scale dynamics could further enhance the
local, instantaneous strength of Titan's methane cycle by a factor of several.

Earth and Planetary Astrophysics