Possible tropical lakes on Titan from observations of dark terrain.

Titan has clouds, rain and lakes--like Earth--but composed of methane rather than water. Unlike Earth, most of the condensable methane (the equivalent of 5 m depth globally averaged) lies in the atmosphere. Liquid detected on the surface (about 2 m deep) has been found by radar images only poleward of 50° latitude, while dune fields pervade the tropics. General circulation models explain this dichotomy, predicting that methane efficiently migrates to the poles from these lower latitudes. Here we report an analysis of near-infrared spectral images of the region between 20° N and 20° S latitude. The data reveal that the lowest fluxes in seven wavelength bands that probe Titan's surface occur in an oval region of about 60 × 40 km(2), which has been observed repeatedly since 2004. Radiative transfer analyses demonstrate that the resulting spectrum is consistent with a black surface, indicative of liquid methane on the surface. Enduring low-latitude lakes are best explained as supplied by subterranean sources (within the last 10,000 years), which may be responsible for Titan's methane, the continual photochemical depletion of which furnishes Titan's organic chemistry.