EFFECTS OF TIME-VARYING HAZE PRODUCTION ON TITANS GEOMETRIC ALBEDO

Two versions of the Toon et al. (1992, Icarus 95, 24-53) Titan haze model were used to simulate the observed blue and yellow geometric albedo variations using a time-dependent haze production. Steady-state calculations were used to suggest that the range of production rate changes necessary to explain the observations was a factor of between 2 and 4. However, the blue and yellow geometric albedo variations were about 10 times smaller than the observations, when computed with a time-varying production rate with this same range. Long haze residence times were the cause of this decreased response. This result was insensitive to the details of the functional representation of the production. In addition to the amplitude of the geometric albedo, we found that the phase between albedo response and the production forcing is a critical discriminate when comparing model results to the observations. Long haze time scales also produced a phase delay in the albedo response to seasonal solar forcing which can agree with the observations (Sromovsky et al. 1981, Nature 292, 698-702). Unfortunately, the size of the phase delay was dependent on model parameters. We found that raising the level of rainout well into the stratosphere (90 km) decreased the residence time of the haze and allowed us to fit the magnitude of the albedo variation. However, the resulting phase delay (-60°) was not in agreement with observations (+90°) and such a high altitude for rainout is probably un-physical. Our results suggest that a time-varying mass production cannot explain Titan's geometric albedo variations. © 1993 by Academic Press, Inc.