Images of Titan at 1.3 and 1.6 μm with Adaptive Optics at the CFHT

Abstract

Titan was observed with the Adaptive Optics Bonette at the Canada–France–Hawaii Telescope during October 27th 1998 (UTC), when the satellite was at greatest eastern elongation (GEE) with respect to Saturn and its leading hemisphere was seen from the Earth. The seeing was excellent during these observations (with peaks at 0.3″ in the visible), and this allowed us to successfully image Titan for the first time in the J band, where the adaptive optics correction is highly dependent on the atmospheric conditions. Images were obtained in the center of the so-called “methane windows,” where the absorption is weak (at 1.29 –J1– and 1.6 μm –H1–) and also in the wings of the CH 4 bands (at 1.18 μm –J2– and 1.64 μm –H2–) with narrowband filters. The latter wavelengths yield information on Titan's stratosphere and allow us, by subtracting its contribution from the J1 and H1 images, to infer furthermore direct information on the surface properties. The resolution on the J and H diffraction-limited images is about 0.08–0.1″, translating into about 10 independent resolution elements or 20 pixels on the Titan disk. We have obtained reconstructed PSFs of very high signal-to-noise ratio, with associated Strehl ratios of about 35% in J and 50% in H. Hence, new and more efficient deconvolution methods (such as MISTRAL) were applied to the images, reducing ringing defects and restoring the initial photometry while enhancing the contrast on the observed features. Thus, the main features of the Titan atmosphere (bright south pole) and surface (large bright equatorial region and darker areas)—as previously observed with ADONIS/ESO, as well as with the HST and the Keck telescopes—are apparent, but with a much higher level of detail and contrast (about 30%) at these wavelengths. In addition, on the J1 and J2 images, along with the north–south asymmetry (which is probably due to seasona l effects), another bright feature is reported for the first time seen on Titan's morning limb (anti-Saturn during GEE). This feature may be diagnostic of diurnal effects (such as morning fog) at altitudes of 70–90 km (due to the cycle of the condensable species), but requires further investigation before its origin can be firmly identified. On the surface images (corrected for limb effects and atmospheric contribution), the large equatorial feature is found to be bright also at 1.3 μm (thus further constraining models of the surface composition). The high quality of the data allows us to resolve this area into three or more individual peaks, possibly towering over a mountainous plateau covered with ice (a plausible candidate being ethane ice). In any event, our images show this part of Titan's leading hemisphere to be more complex than previously suggested by models of Titan's surface. From our albedo maps, it appears that the darker areas have reflectances that are about three times lower than the bright equatorial region.