Probing Titan's Atmosphere with the 1995 August Stellar Occultation

Abstract

On 1995 August 21, Saturn's moon Titan occulted the 13th magnitude star GSC5254-00997. The predawn event was observed at 2.3 μm with the NASA 3.0-m Infrared Telescope Facility. We measured the average atmospheric scale height between latitudes 50°S and 67°S by fitting isothermal models to the immersion portion of the occultation lightcurve. Within the altitude range of 300 to 500 km, we find an isothermal scale height of Titan's sunset atmosphere of 55±9 km. Our derived temperature of 180±30 K is consistent with modeled temperature profiles derived from Voyager 1 IRIS observations (A. Coustenis and B. Bézard 1995, Icarus 115, 126–140) and the 28 Sagittarii occultation event (W. B. Hubbard et al. 1993, Astron. Astrophys. 269, 541–563). The atmospheric albedo asymmetry due to low level haze (Caldwell et al. 1992, Icarus 96, 1–9) is inferred from the difference in Titan's center-of-light (determined from precise astrometric measurements of images of Titan and the star before and after the occultation) and the actual center of Titan (determined from the occultation analysis). We find the southern hemisphere to be brighter than the northern hemisphere, a result consistent with Combes et al. (1997, Icarus 129, 482–497), Gibbard et al. (1999, Icarus 139, 189–201), and Smith et al. (1996 Icarus, 119, 336–349).