Physical properties of the organic aerosols and clouds on Titan

Abstract

Titan's haze is optically thick in the visible, with an optical depth at 0.5 μm of about three. The haze varies with latitude in a seasonal cycle and has a detached upper layer. Microphysical models, photochemical models, and laboratory simulations all imply that the production rate of the haze is in the range of 0.5– 2×10 −14 g cm −2 s −1 . Given the rate of sedimentation, the total mass loading is about 250 mg m −2 . The transparency of the haze is high for wavelengths above 1 μm because the haze material becomes almost purely scattering and the optical depth decreases with increasing wavelength. The particles in the main haze deck are probably fractal in structure with an equivalent volume radius of 0.2 μm . The haze material is organic and, if similar to laboratory tholin, has a C/N ratio in the range of 2–4 and a C/H ratio of about unity. The haze significantly affects the thermal balance of Titan, causing an antigreenhouse effect that cools the surface by 9 K. Titan's faintly banded appearance suggests strong zonal winds in the lower stratosphere. Condensate clouds of ethane or methane, if present, are thin, patchy, or transient. Stratospheric clouds of condensed nitriles and (possibly) hydrocarbons appear to be associated with, though not contained entirely in, the polar shadow, suggesting abundances may vary with the season. Precipitating condensate particles from the stratosphere probably act as nucleating centers for the formation and rapid growth of methane ice particles in the troposphere, where the gas phase appears to be highly supersaturated. Once formed, fallout times for these hailstones are ∼2 h or less. Melting, and possible subsequent fragmentation of methane raindrops should occur at ∼12 km and below. Almost complete evaporation should occur just above the surface. A thin residue of ethane-enriched fog particles would then slowly settle to the surface, steadily modifying an existing surface or subsurface residue of liquid hydrocarbons. The optical properties of the haze in the 1 to 3 μm spectral region and the implications for the visibility of the surface are probably the most pressing current research questions. Other key questions include the nature of the high altitude detached haze layer, altitude and seasonal changes in composition of the haze, the role of haze particles as condensation nuclei for clouds, and the nature of any condensate clouds.