TITANS ROTATIONAL LIGHT-CURVE

Recent observations demonstrate that near-infrared spectroscopy can pr obe Titan's surface through its haze. In a 5-week period during Septem ber and October 1993 we observed Titan's methane windows at 1.1, 1.3, 1.6, and 2 mu m. At 1.1 and 1.3 mu m, observations were consistent wit h observations in 1992 at the same phases reported by Lemmon et al. (1 993, Icarus 103, 329-332). Our new observations indicate that Titan wa s brighter near eastern elongation than near western elongation by 23 +/- 2% at 1.6 mu m and 32 +/- 3% at 2 mu m With almost daily observati ons at 2 mu m during one orbit, we observed Titan to be dark near west ern elongation, to brighten as it approached eastern elongation, and t o darken as it returned to western elongation. We determine that the o bserved light-curve is due to surface albedo variations and Titan's ro tational period is 15.950 +/- 0.025 days. By considering the work of o ther observers we constrain the rotational period to be 15.949 +/- 0.0 06 days; this constraint is consistent with synchronous rotation. Mode ls of Titan's surface reflectivity are inconsistent with the presence of a strong 2-mu m water ice absorption feature and do not require the presence of any surface absorption features; however, we cannot rule out models of the surface as dirty water ice or silicates that have be en suggested by others. (C) 1995 Academic Press, Inc.