Latitudinal Variation of Aerosol Sizes Inferred from Titan's Shadow

Abstract

We measured the location, size, and shape of Titan's shadow in five images taken with the Hubble Space Telescope in 1995. We inferred the altitude of Titan's optical limb at wavelengths of 337–954 nm to an accuracy of 15 km. At each wavelength, altitudes are constant north of −5° and also constant but lower south of −50° latitude, with a linear transition region inbetween. The amplitude of the variation of altitude with latitude increases from close to zero at wavelength 337 nm to 130 km at 954 nm. We conclude that Titan's aerosols are larger (0.3 μm mean radius) at northern latitudes where we probe the detached haze layer than at southern latitudes (0.1 μm south of −50°) where we probe the main haze layer below. The geometric cross section of aerosols at probed altitudes (∼300 km) does not show a significant latitudinal variation. The wavelength dependence of the size of Titan's shadow is not consistent with a spherical shape of aerosols, but gives evidence of their fractal nature.