Seasonal trends in Titan's atmosphere: haze, wind, and clouds

Abstract

I present an analysis of visible and near-infrared adaptive optics images and spectra of Titan taken over 43 nights between October 1997 and January 2003 with the AEOS 3.6-m, Palomar Hale 5-m, and W.M. Keck 10-m telescopes. These observations reveal a seasonally changing stratospheric haze layer, two distinct regions of condensate clouds in the southern hemisphere, the albedo of Titan's surface, and the zonal wind field of the stratosphere. Transient convective CH4 clouds are identified near Titan's south pole, rising to 16±5 km above the surface. These clouds have been continuously present south of 70°S since at least December 2001, currently account for 0.5-1% of Titan's 2μm flux, and appear to be gradually brightening or thickening as the insolation of the south polar region increases. Above the polar clouds, an extensive but optically thin (τ≈0.05 at 2μm) cloud layer is noted near the tropopause south of 30°S. This cirrus-like structure has remained unchanged in extent and thickness since September 1999 despite seasonal changes in the underlying convective clouds and the overlying stratospheric haze. Aside from the convective CH4 clouds near the south pole, Titan's troposphere is free of aerosols with an upper limit of τ The albedo of Titan's surface at 2.0μm is derived from the radiative transfer analysis of spatially resolved spectra and images, and presented in the form of a ~600 km resolution global surface albedo map. At this resolution, the 2.0μm albedo ranges from 0.05 to 0.17, consistent with extensive exposure of clean water ice in some regions, while hydrocarbons and atmospheric sediments blanket others. The zonal wind field of Titan's stratosphere near southern summer solstice is derived from adaptive optics observations of the occultation of a binary star on 20 December 2001. Multiple refracted stellar images were detected on Titan's limb during the each successive occultation, allowing the angular deflection of the starlight at two altitudes over both hemispheres to be measured with an uncertainty of ~2 milliarcseconds. The zonal wind field derived from this measurement of the shape of Titan's limb exhibits strong but asymmetric high latitude jets, with peak wind speeds of 230±20 m s-1 at 60°N and 160±40 m s-1 at 40°S, and lower winds of 110±40 m s-1 at the equator. The direction of the wind is not constrained.