C4N2 ice in Titanʹs north polar stratosphere

The 478 cm−1 ν8 band of condensed dicyano-acetylene (C4N2) associated with Titanʹs north polar hood has been modeled using a radiative transfer program developed for tangent viewing in spherical atmospheres. Absence of the associated 471 cm−1 band of gaseous C4N2 leads to an upper limit of 4 × 10−10 for the vapor mole fraction, and a corresponding upper limit of 90 km for the cloud top. The condensate/vapor abundance ratio ranges between 50 and 200 at the cloud top, depending on the cloud scale height. Corresponding cloud optical thicknesses are ∼0.1 and 0.006, respectively, while the majority of particles appear to have radii between 5 and 10 μm. Because of delayed response to the changing seasons, Titanʹs polar stratosphere appears to be cooling down as it becomes more illuminated by sunlight during the progression of winter into spring. As a result, photolytic decomposition of C4N2 vapor occurs above the advancing sunlight/shadow boundary at the same time that condensation increases below the boundary. This time-dependent cyclic process appears to give rise to a C4N2 condensate/vapor ratio approximately two orders of magnitude greater than that expected under steady-state conditions.