Titan's optical limb from Voyager 1 images, although no wavelength dependence was investigated. Using a high
We measured the location, size, and shape of Titan's shadow in five images taken with the Hubble Space Telescope in 1995. phase-angle image in the clear filter (480 nm) from Voyager We inferred the altitude of Titan's optical limb at wavelengths 2, Rages and Pollack (1983) found the altitude of the optiof 337-954 nm to an accuracy of 15 km. At each wavelength, cal limb higher at the equator than at the poles.
altitudes are constant north of ؊5؇ and also constant but lower
Imaging of Titan by the Hubble Space Telescope has south of ؊50؇ latitude, with a linear transition region inbetween. the advantage of a wide wavelength range. However, limb The amplitude of the variation of altitude with latitude inprofiles of these images are greatly affected by the point creases from close to zero at wavelength 337 nm to 130 km at spread function of the telescope and by the limb darkening 954 nm. We conclude that Titan's aerosols are larger (0.3 m profile, both of which make measurements of the altitude mean radius) at northern latitudes where we probe the detached of the optical limb difficult. haze layer than at southern latitudes (0.1 m south of ؊50؇) Hubbard et al. (1993) constrained Titan's haze properties where we probe the main haze layer below. The geometric cross by a stellar occultation in 1989. They determined an aerosol section of aerosols at probed altitudes (ȁ300 km) does not show a significant latitudinal variation. The wavelength dependence size of 0.2 Ȑm, or a smaller mean size if the variance of of the size of Titan's shadow is not consistent with a spherical the size distribution is large. They found a north-south shape of aerosols, but gives evidence of their fractal asymmetry of the altitude of the optical limb with the nature. © 1997 Academic Press transition at Ϫ20Њ latitude. Since occultation data are very sensitive to the gas density and density gradient, haze properties were constrained after gas density and density gradi-I. INTRODUCTION ent had been modeled and accounted for.
Here, we report on a unique way to constrain the alti-Titan's aerosol sizes have been inferred by different techtudes and sizes of Titan's aerosols. We use the location, niques. Tomasko and Smith (1982) used Pioneer 11 polarsize, and shape of Titan's shadow cast on Saturn in images imetry to derive a mean aerosol radius of 0.1 Ȑm. Rages over a wide wavelength range (337-954 nm). Our images et al. (1983) found a lower bound of 0.2 Ȑm based on taken in 1995 are the only well-resolved images of Titan's Voyager images at high phase angles. Both determinations shadow so far. Hubbard et al. (1996) report imaging Titan's assumed spherical aerosols, and the disagreement is an shadow on Mimas and Saturn's rings, but seeing from the indication that Titan's aerosols are not spherical.
1.8-m Vatican Advanced Technology Telescope was 1.3 The variation of altitude at a certain optical depth with arc-seconds. The next opportunity to image Titan's shadow wavelength yields information about the wavelength dewill not come before 2008. pendence of optical depth, and this constrains aerosol sizes.
Previous methods of determining the size of Titan's aero-Smith (1980) measured Titan's radius at 440 and 640 nm sols required sophisticated modeling of Titan's atmosphere wavelength using Pioneer 11 data. His data give a decrease with the need to assume the values for poorly known paof altitude of the optical limb by 40 Ϯ 33 km from the rameters about Titan's atmosphere. Only recently has blue to the red wavelength. A decreasing altitude with there been any attempt to model a latitudinal variation of increasing wavelength indicates aerosols smaller than 0.3 aerosol sizes (Hutzell et al. 1996). On the other hand, by Ȑm radius. Smith et al. (1981) reported the altitude of looking at the shape of Titan's shadow at different wavelengths, one can almost ''see'' that aerosol sizes must con-1 Based on observations with the NASA/ESA Hubble Space Telescope siderably vary with latitude without knowledge of any paobtained at the Space Telescope Science Institute, which is operated by rameter of Titan's atmosphere.