The T18 fly-by on 23 September 2006 was at an altitude of 960 kilometres. On this occasion, the Visual and Infrared Mapping Spectrometer showed a vast tropospheric cloud occupying the observed range of longitudes (10° to 190°W) and latitudes (51° to 68°N) that appeared to be composed of ethane.287 It is believed to be the result of the concentration of organic haze in the descending north polar vortex. Ethane raining into the lakes of methane would dissolve. If the temperature dipped low enough in the dead of winter, ethane would undoubtedly fall as snow, and might even build up an ice cap. The localisation of methane and ethane at the winter pole implied a seasonal cycle in which the volatile hydrocarbons migrated from one pole to the other over a Titanian year (which lasts 29.5 terrestrial years) and this, in turn, offered an explanation for the current paucity of clouds and absence of liquid on the surface at other latitudes.
All in all, with the evidence mounting for a subsurface ammonia-water ocean, the ongoing release by cryovolcanism of methane, and a seasonal cycle for atmospheric volatiles, the Cassini spacecraft and its Huygens probe had, between them, lifted the veil on Titan. By the end of the primary tour in June 2008 the highresolution radar map will have covered some 20 per cent of the surface, and if the mission is extended it should be possible to monitor the onset of the northern summer, with storms forming in the tropics as the volatiles migrate south, possibly temporarily submerging the now inert Huygens probe on the floor of a lake of rainfall runoff.
Detail of the ring system as imaged on 15 September 2006, together with a dramatic exposition of the jets on the south pole of Enceladus replenishing the densest section of the 'E' ring (lower).
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