1. Department of Physics and Astronomy, Northern Arizona University, PO Box 6010, Flagstaff, Arizona 86001-6010, USA
2. Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, Hawaii 96822, USA
3. Joint Astronomy Centre, 660 N. Aohoku Place, Hilo, Hawaii 96720, USA

Correspondence to: Caitlin A. Griffith¹ Correspondence and requests for materials should be addressed to C.A.G. (e-mail: Email: griffith@vela.phy.nau.edu).

Abstract

The 1980 encounter by the Voyager 1 spacecraft with Titan, Saturn's largest moon, revealed¹,² the presence of a thick atmosphere containing nitrogen and methane (1.4 and 0.05 bar, respectively). Methane was found to be nearly saturated at Titan's tropopause, which, with other considerations, led to the hypothesis that Titan might experience a methane analogue of Earth's vigorous hydrological cycle, with clouds, rain and seas^{3, 4, 5, 6, 7}. Yet recent analyses of Voyager data indicate large areas of supersaturated methane, more indicative of dry and stagnant conditions⁸,⁹. A resolution to this apparent contradiction requires observations of Titan's lower atmosphere, which was hidden from the Voyager cameras by the photochemical haze (or smog) in Titan's stratosphere. Here we report near-infrared spectroscopic observations of Titan within four narrow spectral windows where the moon's atmosphere is ostensibly transparent. We detect pronounced flux enhancements that 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 9 per cent of Titan's disk.

1. Department of Physics and Astronomy, Northern Arizona University, PO Box 6010, Flagstaff, Arizona 86001-6010, USA
2. Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, Hawaii 96822, USA
3. Joint Astronomy Centre, 660 N. Aohoku Place, Hilo, Hawaii 96720, USA

Correspondence to: Caitlin A. Griffith¹ Correspondence and requests for materials should be addressed to C.A.G. (e-mail: Email: griffith@vela.phy.nau.edu).