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

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.

Polarization of Saturn’s moon Iapetus

“…Moreover, with the aim of detecting chiral signatures on Iapetus, we performed circular spectro-polarimetric observations of its two hemispheres, at a single phase angle for each. The spectropolarimetric measurements of Iapetus' trailing hemisphere have already been reported in Ejeta et al (2012, from here referred to as Paper I). In this paper, we report measurements of Iapetus' leading hemisphere in comparison with that of its trailing hemisphere, to investigate differences in the polarization behavior of the surface materials on its two hemispheres.…”

Polarimetry of Saturnian satellite Enceladus

“…In comparison with the trailing hemisphere of Iapetus [8] having albedo 0.55 in visible light, the icy satellites Rhea and Enceladus show lower asymmetry and less depth of the phase-angle dependence of polarization. Also, Rhea and Enceladus show less depth of the phase-angle dependence of polarization in comparison with large transneptunian objects with icy surfaces [1,2].…”

“…For Saturnian satellites Rhea [31] and bright side of Iapetus [8] POE was detected in a form of asymmetrical negative polarization branch.…”

Influence of the Surface Temperature Evolution over Organic and Inorganic Compounds on Iapetus

“…On the bright side, which is less emissive than the dark one, the diffuse scattering by water ice particles takes place [31]. Analysis of polarization in the bright side performed by [33] shows that this effect is much more pronounced on the surface, making it more sensitive to a diversity of scattering parameters like particle size, shape, surface texture, and reflectance of objects. However, it is still possible to infer the diffusion of methane ice and water ice, since what happened on the surface of Trans Neptunian Objects can be taken as a reference, mainly because both compounds are very abundant in them [34].…”