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

C. Ejeta, H. Boehnhardt, S. Bagnulo, and G.-P. Tozzi

Fig. 5. Circular polarization of the bright side of Iapetus versus wavelength, obtained with FORS2 at the ESO VLT during the night 2011-03-28 at a phase angle of 0.77°. The blue dashed curve shows the Stokes P'V profile obtained at 0.0° instrument position angle (with respect to the north celestial pole), the green dotted curve showing the measurement obtained after rotating the instrument by 90.0° on the sky, and the red solid curve represents the average of the two signals. It can be noted that a change in signal occurs in circular polarization observations that are taken after rotating the instrument by 90 degree with respect to the intial position angle on the sky.

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 (400-800nm), 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 one circular spectro-polarimetric observation 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.

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Last Revised: 2011 October 17th