Errors in calculated planetary phase functions and albedos due to neglecting polarization

Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands e-mail: dstam@science.uva.nl

Received: 27 June 2005
Accepted: 13 August 2005

Abstract

Accurate calculations of disk-integrated quantities such as planetary phase functions and albedos will be crucial for the analysis of direct observations of light that is reflected by extrasolar planets. We show that adopting a scalar representation of light and thus neglecting the polarized nature of light leads to significant, wavelength dependent, errors in calculated planetary phase functions and geometric albedos of homogeneous giant planets. The errors depend on the planetary model atmosphere. For planets with little to no aerosol/cloud particles in their atmosphere, these errors can reach more than 9%. For cloud covered planets, the errors are generally smaller, but can still reach several percent. The errors in the planetary phase function and geometric albedo also depend on the atmospheric absorption optical thickness. Neglecting polarization thus influences the absolute and relative depth of absorption bands in albedos and phase functions, and thus indirectly e.g. a gaseous mixing ratio that is derived from the depth of a band. Specifically, we find that neglecting polarization when deriving the methane mixing ratio from numerically simulated reflection spectra of a giant planet can lead to values that are too large by several tens of percent. Neglecting polarization generally leads to (wavelength and absorption optical thickness dependent) errors smaller than 0.5% in calculated (monochromatic) planetary Bond albedos. The errors in phase functions and albedos due to neglecting only circular polarization appear to be smaller than 0.0005%. When calculating phase functions and albedos of homogeneous planets, describing light by its intensity (or flux) and its state of linear polarization should thus suffice.