Context. Physical characterization of trans-Neptunian objects, a primitive population of the outer solar system, may provide constraints on their formation and evolution. Aims. The goal of this work is to characterize a set of 15 scattered disk (SDOs) and detached objects, in terms of their size, albedo, and thermal properties. Methods. Thermal flux measurements obtained with the Herschel-PACS instrument at 70, 100 and 160 μm, and whenever applicable, with Spitzer-MIPS at 24 and 70 μm, are modeled with radiometric techniques, in order to derive the objects' individual size, albedo and when possible beaming factor. Error bars are obtained from a Monte-Carlo approach. We look for correlations between these and other physical and orbital parameters. Results. Diameters obtained for our sample range from 100 to 2400 km, and the geometric albedos (in V band) vary from 3.8% to 84.5%. The unweighted mean V geometric albedo for the whole sample is 11.2% (excluding Eris); 6.9% for the SDOs, and 17.0% for the detached objects (excluding Eris). We obtain new bulk densities for three binary systems: Ceto/Phorcys, Typhon/Echidna and Eris/Dysnomia. Apart from correlations clearly due to observational bias, we find significant correlations between albedo and diameter (more reflective objects being bigger), and between albedo, diameter and perihelion distance (brighter and bigger objects having larger perihelia). We discuss possible explanations for these correlations.

We present a comprehensive analysis of the chemical composition of the Jupiter-family comet and potential spacecraft target 46P/Wirtanen, in the near-IR wavelength range. We used iSHELL at the NASA Infrared Telescope Facility to observe the comet on 11 pre-, near-, and postperihelion dates in 2018 December and 2019 January and February during its historic apparition. We report rotational temperatures, production rates, and mixing ratios with respect to H2O and C2H6 or 3σ upper limits of the primary volatiles H2O, HCN, CH4, C2H6, CH3OH, H2CO, NH3, CO, C2H2, and HC3N. We also discuss the spatial outgassing of the primary volatiles, to understand their sources and the spatial associations between them. The spatial profiles of H2O in 46P/Wirtanen suggest the presence of extended H2O outgassing sources in the coma, similar to the EPOXI target comet 103P/Hartley 2. 46P/Wirtanen is among the few known hyperactive comets, and we note that its composition and outgassing behavior are similar to those of other hyperactive comets in many ways. We note that the analyzed parent volatiles showed different variations (relative mixing ratios) during the apparition. We compared the chemical composition of 46P/Wirtanen with the mean abundances in Jupiter-family comets and the comet population as measured with ground-based near-IR facilities to date. The molecular abundances in 46P/Wirtanen suggest that although they were changing, the variations were small compared to the range in the comet population, with CH3OH showing notably more variation as compared to the other molecules.

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.

Context. The surface properties of atmosphereless solar system objects can be constrained by investigating the nature of the polarized light scattered from their surfaces. Aims. We provide new and precise measurements of the phase angle and the wavelength dependence of linear polarization on the leading side of Iapetus over the maximum phase angle range accessible from Earth (∼0.5−6.0• ) and over a broad spectral range (400-800 nm), thereby identifying the polarimetric characteristics of its dark material. Moreover, we provide circular spectral polarization measurement of the same side of Iapetus at one phase angle, which was performed aiming at detection of chiral signatures on its surface. We also compare our new polarization measurements with those of the trailing hemisphere of Iapetus which were obtained in our previous work. Methods. Using the FORS2 instrument of the ESO VLT, we performed spectro-polarimetric observations of Iapetus' leading side. Results. While the linear polarization measurements of Iapetus' leading side show an opposite trend in phase angle dependence to that of its trailing hemisphere, the polarization values measured for the two hemispheres around similar phase angles (in the range ∼3.0−6.0• ) differ by a factor of three. Besides this, the degree of negative linear polarization of Iapetus' leading hemisphere shows a slight dependence on the wavelengths of observations. Furthermore, like that of its trailing hemisphere, the circular polarization measurement of Iapetus' leading side indicates no evidence of a significant amount of optically active (chiral) molecules on its surface.