TRANSIENT CLOUDS IN TITANS LOWER ATMOSPHERE

The 1980 encounter by the Voyager I spacecraft with Titan, Saturn's la rgest moon, revealed(1,2) the presence of a thick atmosphere containin g nitrogen and methane (1.4 and similar to 0.05 bar, respectively). Me thane was found to be nearly saturated at Titan's tropopause, which, w ith other considerations, led to the hypothesis that Titan might exper ience a methane analogue of Earth's vigorous hydrological cycle, with clouds, rain and seas(3-7). Yet recent analyses of Voyager data indica te large areas of supersaturated methane, more indicative of dry and s tagnant conditions(8,9), A resolution to this apparent contradiction r equires observations of Titan's lower atmosphere, which was hidden fro m the Voyager cameras by the photochemical haze (or smog) in Titan's s tratosphere. Here we report near-infrared spectroscopic observations o f Titan within four narrow spectral windows where the moon's atmospher e is ostensibly transparent. We detect pronounced flux enhancements th at indicate the presence of reflective methane condensation clouds in the troposphere, These clouds occur at a relatively low altitude (15 /- 10 km), at low latitudes, and appear to cover similar to 9 per cent of Titan's disk.