Titan is an excellent test case for detailed studies of the scattering
polarization from thick hazy atmospheres. We present the first limb
polarization measurements of Titan, which are compared as a test to our limb
polarization models. Previously unpublished imaging polarimetry from the HST
archive is presented which resolves the disk of Titan. We determine
flux-weighted averages of the limb polarization and radial limb polarization
profiles, and investigate the degradation and cancelation effects in the
polarization signal due to the limited spatial resolution of our observations.
Taking this into account we derive corrected values for the limb polarization
in Titan. The results are compared with limb polarization models, using
atmosphere and haze scattering parameters from the literature.
In the wavelength bands between 250 nm and 2000 nm a strong limb polarization
of about 2-7 % is detected with a position angle perpendicular to the limb. The
fractional polarization is highest around 1 micron. As a first approximation,
the polarization seems to be equally strong along the entire limb. The detected
polarization is compatible with expectations from previous polarimetric
observations taken with Voyager 2, Pioneer 11, and the Huygens probe.
Our results indicate that ground-based monitoring measurements of the
limb-polarization of Titan could be useful for investigating local haze
properties and the impact of short-term and seasonal variations of the hazy
atmosphere of Titan. Planets with hazy atmospheres similar to Titan are
particularly good candidates for detection with the polarimetric mode of the
upcoming planet finder instrument at the VLT. Therefore, a good knowledge of
the polarization properties of Titan is also important for the search and
investigation of extra-solar planets.Comment: 13 pages, 13 figures, accepted for publication in Astronomy &
Astrophysic
