Seasonal photometric variability of Titan, 1972–2006

Measurements at Lowell Observatory of Titan in the b (472 nm) and y (551 nm) filters of the Strömgren photometric system at thirty four consecutive apparitions (282 nights) from 1971/72 to 2006 show a 10% sinusoidal variation that lags seasonal extremes by about 1/8 of a Titan year. The seasonal variations are asymmetric: the autumn lightcurve maxima of the northern and southern hemispheres differ significantly as do the spring lightcurve minima. Changes also occur from one Titan year to the next: Titan was ∼3% fainter in b and ∼1% fainter in y following the 2002 southern summer solstice than it was one Titan year earlier in 1973. These changes appear to be intrinsic to Titanʹs atmosphere and cannot be explained by instrumental effects and changing geometries. Orbital variations visible in recent Hubble Space Telescope images at 673 nm and Voyager orange images (590–640 nm) may have a small ( 0.002 ± 0.001 mag ) counterpart in the b, y photometric record (eastern elongation brighter, consistent with the Cassini near-infrared albedo map).