Saturn's largest satellite, Titan, has a dense atmosphere of nitrogen with a few per cent of methane¹. At visible wavelengths its surface is hidden by dense orange-brown smog, which is produced in the stratosphere by photochemical reactions following the dissociation of methane by solar ultraviolet light. The most abundant of the products of these reactions is ethane, and enough of it should have been generated over the life of the Solar System to form a satellite-wide ocean one kilometre deep². Radar observations³ have found specular reflections in 75 per cent of the surface spots observed, but optical searches for a sun-glint off an ocean have been negative⁴. Here I explain the mysterious absence or rarity of liquid ethane: it condenses onto the smog particles, instead of into liquid drops, at the cold temperatures in Titan's atmosphere. This dusty combination of smog and ethane, forming deposits several kilometres thick on the surface, including the observed dunes and dark areas, could be named 'smust'. This satellite-wide deposit replaces the ocean long thought to be an important feature of Titan.

1. Lunar and Planetary Laboratory, The University of Arizona, Tucson, Arizona 85721, USA

Correspondence to: D. M. Hunten¹ Correspondence and requests for materials should be addressed to the author (Email: dhunten@lpl.arizona.edu).

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