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H. Imanaka, B.N. Khare, E.L.O. Bakes (SETI Inst./NASA ARC), M.A. Cannady, C.P. McKay, D.P. Cruikshank (NASA ARC), J.E. Elsila, R.N. Zare (Stanford University), S. Sugita, T. Matsui (Univ. of Tokyo)
We have conducted an experimental simulation of Titan's atmospheric chemistry, which forms haze and condensation clouds at various altitudes.
The chemical and optical properties of the tholin formed from a methane/nitrogen (=10/90) gas mixture at various pressures (13 to 2300 Pa) in cold plasma were examined. The amount and size of aromatic ring structures and the effective nitrogen incorporation increased in tholins formed at low pressures. Reddish-brown films formed at low pressures showed larger k-values in the UV/VIS range than those of the yellowish films formed at high pressures. These results indicate that the haze layers at various altitudes may have different chemical and optical properties.
The gas products were condensed onto a window cooled to a temperature in the range of 70 to 180 K, and the infrared spectrum of the ice was measured. This simulates the possible condensation process in Titan's lower stratosphere. Experimental results show that the IR spectra of ice mixtures change considerably with condensation temperature. This change was probably caused by a variation in the ice composition. These results strongly suggest that Titan has several condensation cloud layers with distinct chemical compositions.
When the ice formed at 70 K was heated to 94 K (Titan's surface temperature), little change in infrared spectra was observed. These ice mixtures could accumulate on Titan's surface. Red-brownish residues are formed during the phase change from ice to liquid, when the white ice is heated to room temperature. These reactions, forming complex organics, could occur in the melted surface regions after meteoritic impacts, forming dark colored craters in the bright icy surface.
Our results can be compared with the forthcoming observations of the Cassini/Huygens mission regarding the variations of chemical/optical properties of the haze, clouds and surface of Titan.
This work has been supported by NASA Exobiology Program.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.