Spectro-polarimetry of the bright side of Saturn’s moon Iapetus^⋆

¹ Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Strasse 2, 37191 Katlenburg-Lindau, Germany
e-mail: ejeta@mps.mpg.de
² Institut für Geophysik und extratrrestishe Physik, TU Braunschweig, Germany
³ Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland, UK
⁴ INAF – Oss. Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

Received: 10 August 2011
Accepted: 18 October 2011

Abstract

Context. Measurements of the polarized reflected sunlight from atmosphereless solar system bodies, over a range of phase angles, provide information about the surface structure and composition.

Aims. With this work, we provide analysis of the polarimetric observations of the bright side of Iapetus at five different phase angles, and over the full useful wavelength range (400–800 nm), so as to assess the light scattering behaviour of a typical surface water ice.

Methods. Using FORS2 of the ESO VLT, we have performed linear spectro-polarimetric observations of Iapetus’ bright side from 2009 to 2011 at five different phase angles, in the range from 0.80−5.20°, along with circular spectro-polarimetric observations at one phase angle.

Results. By measuring, with high accuracy (~0.1% per spectral bin for each Stokes parameter), the spectral polarization of the bright trailing hemisphere of Saturn’s moon Iapetus, we have identified the polarimetric characteristics of water ice, and found that its linear degree of negative polarization decreases with increasing phase angle of observation (varying from  −0.9% to  −0.3%), with a clear dependence on wavelengths of observation.