Exploring the atmosphere

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Titan’s Methane Cycle



The detection of C4N2 ice by Voyager at the north pole constitutes the first direct evidence of condensation on Titan. Later, clouds were observed in the disk-average spectra in the near-infrared and shown lofted around 16+/- 5 km, and at 27+/- 3 km. A large convective cloud was imaged near the south pole at ~25 km. Cassini images as well as the telescope observations also show frequent clouds scattered around 40o the southern hemisphere.


A general circulation model with cloud microphysical model simulate a 25-year cloud climatology on Titan. The model is able to identify several cloud features that were observed: (i) widespread permanent ethane clouds, or mist, formed above the tropopause and in the troposphere polar regions; (ii) sporadic methane clouds at altitudes between 15 and 18 km. (iii) frequent and thick sporadic methane clouds at 15 km altitude around 40 in the summer hemisphere; (iv) thick and frequent methane clouds at both poles below 30 km altitude. The model also predicts clouds that have not been observed. In the model, methane condenses at ~15 km at 60 latitude and 18 km near the equator. Clouds are formed at these levels anywhere methane is transported upward by the circulation. Methane clouds frequently appear in the ascending branch of the tropospheric Hadley cell at ~ 40o in the summer hemisphere. The mixing process due to inertial instabilities also produces thin clouds between +/- 30o (about 10% of the time) and more important clouds (10~50% of the time) at winter mid-latitudes (between 40o and 60o).

Clouds and Rains on Titan

Clouds have been observed on Titan, through its thick haze, using near-infrared spectroscopy and images near the south pole and in temperature regions near 40S. The telescope and Cassini observations provide an insight into Titan’s cloud climatology. A general circulation model of Titan that includes cloud microphysics identify several type of cloud that is observed.



The morphology of the south polar clouds suggest that they are convective clouds and could be precipitating. Huygens In situ data on the CH4 concentration and the temperature profile in Titan’s troposphere point to the present of layered optically thin stratiform clouds, consist of an upper CH4 ice cloud and a lower liquid CH4-N2 cloud. The lower cloud produces drizzle that reaches the surface. Theses non-convective clouds are quasi-permanent features supported by both global atmospheric circulation and Earth based near-infrared observation, which both indicate that CH4 drizzle is a persistent component of Titan’s methane hydrological cycle.


Near-infrared spectra from the Very Large Telescope and W. M. Keck Observatories reveal an enhancement of opacity in Titan’s troposphere on the morning side of the leading hemisphere. Retrieved extinction profiles are consistent with condensed methane in clouds at altitude of 30 km and concomitant CH4 drizzle below. The moisture encompasses the equatorial region over Titan’s brightest continent, Xanadu. The contributing factor to the condensation mechanisms might be the diurnal temperature gradients that caused variations in the CH4 relative humidity, winds, and topography.


Cloud extinction (m-1) at 620nm, averaged over on terrestrial year around the Cassini –Huygens arriving time. Shown along with the stream function (109 kg s-1) averaged over 7 years (one Titan season) before the arrival (continuous line denote clockwise motion). [Rannou et al., 2006]

Difference imaging of the condensed phase scattering at specific latitudes in the atmosphere. Left: troposphere; Right: Stratosphere. The lower troposphere can be probed by subtracting images at wavelengths sensitive to the variations in the surface reflectivity from those of surface and lower troposphere.  (A) surface, (B) lower troposphere. © the difference. (D)~(E) same as (A) to (c) but for stratosphere.