Seasonal variation of Titanʹs stratospheric ethylene (C2H4) observed

All previous observations of seasonal change on Titan have been of physical phenomena such as clouds and haze. We present here the first observational evidence of chemical change in Titanʹs atmosphere. Images taken during 1999–2002 (late southern spring on Titan) with the W.M. Keck I 10-meter telescope at 8–13 μm show a significant accumulation of ethylene (C2H4) in the south polar stratosphere as well as north–south stratospheric temperature variation (colder at poles). Our observations restrict this newly discovered south polar ethylene accumulation to latitudes south of 60° S. The only other observations of the spatial distribution of C2H4 were those of Voyager I, which found a significant north polar accumulation in early northern spring. We see no build-up in the north, although the highest northern latitudes are obstructed from view in the current season. Our observations constrain any unobserved north polar accumulation of C2H4 to north of 50° N latitude. Comparison of the Voyager I results with our new results show seasonal chemical change has occurred in Titanʹs atmosphere.