Titan's 2 μm surface albedo and haze optical depth in 1996-2004

We observed Titan in 1996–2004 with high‐resolution 2 μm speckle and adaptive optics imaging at the W. M. Keck Observatory. By observing in a 2 μm broadband filter we obtain images that have contributions from both Titan's surface and atmosphere. We have modeled Titan's atmosphere using a plane‐parallel radiative transfer code that has been corrected to agree with 3‐D Monte Carlo predictions. We find that Titan's surface albedo ranges from ≤0.02 in the darkest equatorial region of the trailing hemisphere to ≃0.1 in the brightest areas of the leading hemisphere. Over the past quarter of a Saturnian year haze optical depth in Titan's Southern hemisphere has decreased substantially from a value of 0.48 in 1996 down to 0.18 in 2004, while the northern haze has been increasing over the past few years. As a result of these changes, in 2004 the North/South haze asymmetry at K' band has disappeared.