Caitlin A. Griffith 1, T. Owen 2, B. Schulz 3, T. Encrenaz 4, T. Roush 5, & T. Geballe 6
1 N. Ariz, U., Flagstaff, AZ, 86011-6010 USA
2 U. Hawaii, Honolulu, HI; A. Coustenis, Paris Obs.;
3 ISO Science Operation Center, Madrid; E. Lellouch, Paris Obs.;
4 Paris Observatory
5 NASA Ames Res. Ctr., Mountain View, CA
6 J. Astron. Ctre., Hilo, HI
PHT-S took a spectrum of Titan in January 1997, covering the 2.5-5 and 6-12 micron region, with a resolving power of about 100. The signal-to-noise is about 10. Two peaks are observed in the flux density at 2.7 and 2.8 micron at levels of 0.13 Jy. The associated geometric albedo is about 0.01.
The 2.7-2.9 um region is an atmospheric window on Titan, discovered with the CGS4 spectrometer on the United Kingdom Infrared Telescope on Mauna Kea. From this site it is possible to record the 2.87-2.93 wing of the window, the shorter wavelengths being obscured by telluric absorption (Griffith et al. 1998a). ISO was able to capture the entire wavelength interval of this window at lower resolution. The extreme blackness of the surface of Titan at 2.8 um enabled Griffith et al. (1998a) to find the first clear evidence of clouds in the satellite's lower atmosphere. Observations taken with no clouds present on Titan (e.g., with ISO) measured the satellite's surface albedo which, when combined with surface albedos obtained in 6 other windows, provides a crude spectrum that constrains models of Titan's surface composition. A water ice terrain darkened by some other material is the most evident solution that explains the observations in all spectral windows to date (Griffith et al 1991,1998a, 1998b, Coustenis et al. 1995,1998, Noll et al. 1993,1996, Lemmon et al. 1995). We will present Titan's 2.7-2.9 um spectrum augmented by the results of new monitoring programs (Coustenis et al. 1998) and new data from observations of Titan's 5 micron window (Griffith et al 1998b).
Griffith, C.A, T. Owen, G. Miller, and T. Geballe 1998a Nature In Press.