Particle size and abundance of HC₃N ice in Titan’s lower stratosphere at high northern latitudes

Abstract

Up to now, there has been no corroboration from Cassini CIRS of the Voyager IRIS-discovery of cyanoacetylene (HC ₃N) ice in Titan's thermal infrared spectrum. We report the first compelling spectral evidence from CIRS for the ν₆ HC ₃N ice feature at 506 cm ^-1 at latitudes 62°N and 70°N, from which we derive particle sizes and column abundances in Titan's lower stratosphere. We find mean particle radii of 3.0 μm and 2.3 μm for condensed HC ₃N at 62°N and 70°N, respectively, and corresponding ice phase molecular column abundances in the range 1-10 × 10 ¹⁶ mol cm ^-2. Only upper limits for cloud abundances can be established at latitudes of 85°N, 55°N, 30°N, 10°N, and 15°S. Under the assumption that cloud tops coincide with the uppermost levels at which HC ₃N vapor saturates, we infer geometric thicknesses for the clouds equivalent to 10-20 km or so, with tops at 165 km and 150 km at 70°N and 62°N, respectively.