HST observations of the limb polarization of Titan

Abstract

Context. Titan is an excellent test case for detailed studies of the scattering polarization from thick hazy atmospheres. Accurate scattering and polarization parameters have been provided by the in situ measurements of the Cassini-Huygens landing probe. For Earth-bound observations Titan can only be observed at a backscattering situation, where the disk-integrated polarization is close to zero. However, with resolved imaging polarimetry a second order polarization signal along the entire limb of Titan can be measured. Aims. We present the first limb polarization measurements of Titan, which are compared as a test to our limb polarization models. Methods. Previously unpublished imaging polarimetry from the HST archive is presented, which resolves the disk of Titan.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. Results. In the wavelength bands between 250 nm and 2 μm 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 μm. As a first approximation, the polarization seems to be equally strong along the entire limb. The comparison of our data with model calculations and the literature shows that the detected polarization is compatible with expectations from previous polarimetric observations taken with Voyager 2, Pioneer 11, and the Huygens probe. Conclusions. 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.

Titan's atmosphere and climate

“…With the highly successful New Horizons flyby of Pluto, we now have a rich atmospheric data set for another hazy N 2 /CH 4 /CO atmosphere; comparative planetology investigations in the coming years should improve our understanding of both atmospheres. Additionally, since a large number of exoplanets appear to have an aerosol absorber in their atmospheres [see, e.g., Kreidberg et al, 2014;Knutson et al, 2014aKnutson et al, , 2014b, which may be photochemically produced [Marley et al, 2013], there is increasing interest in using Titan as an exoplanet analogue [Lunine, 2010;Bazzon et al, 2014;Robinson et al, 2014].…”

RefPlanets: Search for reflected light from extrasolar planets with SPHERE/ZIMPOL

“…Observations of individual objects have shown that for Rayleigh scattering atmospheres like Uranus and Neptune (Schmid et al 2006b) the fractional polarization can be substantially higher than this value (p(90 • ) > 20 %). For mostly haze scattering atmospheres as found on Titan (Tomasko & Smith 1982;Bazzon et al 2014) or in the polar regions of Jupiter (Smith & Tomasko 1984;Schmid et al 2011;McLean et al 2017) the fractional polarization can even reach values up to p(90 • ) ≈ 50 %. On the other hand, the Mie scattering process in the clouds that dominate the atmospheres of Venus, Saturn or the equatorial regions of Jupiter produces a lower polarization in the visual wavelengths < 10 % (Smith & Tomasko 1984;Hansen & Hovenier 1974).…”

A model grid for the reflected light from transition disks

“…The q(θ pp ) = q(θ, 0 • : 180 • ) curve shows a weakly negative section around the back-scattering directions θ pp = 50 • −80 • . This is a well-known higher order scattering effect (e.g., van de Hulst 1980), which for dust (or haze) scattering in Jupiter and Titan leads to strong "negative" limb polarization effects (Schmid et al 2011;McLean et al 2017;Bazzon et al 2014).…”