Titan’s Atmosphere at High Resolution

Fig. 1.—Example images of some of the phenomena discovered and observed during this work. (a) Accumulation of ethylene in Titan’s south polar stratosphere is apparent in this 10.7 mm image, which is sensitive to thermal emission from haze and ethylene (C2H4) in Titan’s stratosphere. (b) Titan’s troposphere and surface are obscured from view because of strong methane absorption in this mm filter image. The global stratospheric haze and the bright south polar 1.702 0.008 collar of haze are seen in reflected sunlight. (c) and (e) In sections of the K filter (1.95–2.30 mm), Titan’s atmosphere is relatively transparent and numerous surface features are seen in reflected sunlight. (d) and ( f ) In a 2.108–2.140 mm filter, the surface is blocked from view because of moderate methane absorption. This filter probes down into the troposphere, and cloud activity near the south pole is apparent. The resolution in (b)–( f ) is ∼300 km. Titan’s apparent diameter is 0 .83 in (a) and is 0 .87 in (b)–( f ). Titan stands out in our solar system as the sole moon with a substantial atmosphere. Until recently, the only spatially resolved observations of Titan were from the Voyager spacecraft flybys in 1980 and 1981, when the season was early northern spring on Titan. The advent of large ground-based telescopes and adaptive optics allows us to study Titan and its atmosphere with spatial resolutions of a few hundred kilometers. We report nearand mid-infrared observations of Titan’s atmosphere with the Keck and Gemini telescopes in late southern spring on Titan. These observations represent the start of a long-term ground-based monitoring campaign that will extend over a significant fraction of Titan’s 30 Earth-year seasonal cycle, and in doing so will lead us to a better understanding of the seasonal variations. Observing at mid-infrared wavelengths (8–13 mm) with the Long-Wavelength Spectrometer (LWS) on the Keck I Telescope, we found a large buildup of ethylene (C2H4) in the south polar stratosphere (see Fig. 1a, which is from H. G. Roe, I. de Pater, & C. P. McKay 2003, Icarus, submitted). We link this buildup to chemistry that occurs in the shadow of the winter pole. We also developed a new line-by-line radiative-transfer model for Titan at 8–13 mm. This model can easily be extended to longer wavelengths and will in the future be useful for several Titan-related projects, including long-term monitoring of south polar ethylene. At near-infrared wavelengths (1–2.5 mm) using adaptive optics on the Keck II Telescope, we found a collar of haze near the tropopause at 70 –75 south latitude (see Fig. 1b, which is from H. G. Roe et al. 2002, Icarus, 157, 254). This collar is most likely the remnant of stratospheric nitrile chemistry during the long polar winter night. We report a spatially resolved detection of clouds on Titan (see Figs. 1c–1f, which are from H. G. Roe et al. 2002, ApJ, 581, 1399). These observations were made using adaptive optics on the Gemini North and Keck II telescopes. We found the discrete cloud features to vary on timescales of hours to days. All clouds were near the south pole, with the most northern cloud appearing at 61 south. We find each cloud covers an area of Titan’s surface of to km. From a limited 4 5 2 # 10 3 # 10 temporal sample, these clouds appear to be common in the south polar region in late southern spring. We discuss several possible seasonal triggering mechanisms for these clouds. Finally, in appendices we present a new telluric transmission model and discuss the implications of differential atmospheric refraction for adaptive-optics observations (H. G. Roe 2002, PASP, 114, 450).