Letter to the Editor

An update of the correlation between polarimetric and thermal properties of cometary dust

¹ Institut für Geophysik und Extraterrestrische Physik, Technische Universität Braunschweig, Mendelssohnstr. 3, 38106 Braunschweig, Germany
e-mail: y.kwon@tu-braunschweig.de
² Department of Astronomy, University of Maryland, College Park 20742, MD, USA
³ Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany

Received: 28 April 2021
Accepted: 26 May 2021

Abstract

Context. Comets are conglomerates of ice and dust particles, the latter of which encode information on changes in the radiative and thermal environments. Dust displays distinctive scattered and thermal radiation in the visible and mid-infrared (MIR) wavelengths, respectively, based on its inherent characteristics.

Aims. We aim to identify a possible correlation between the properties of scattered and thermal radiation from dust and the principal dust characteristics responsible for this relationship, and therefrom gain insights into comet evolution.

Methods. We use the NASA/PDS archival polarimetric data on cometary dust in the red (0.62−0.73 μm) and K (2.00−2.39 μm) domains to leverage the relative excess of the polarisation degree of a comet to the average trend at the given phase angle (P_excess) as a metric of the dust’s scattered light characteristics. The flux excess of silicate emissions to the continuum around 10 μm (F_Si/F_cont) is adopted from previous studies as a metric of the dust’s MIR feature.

Results. The two observables – P_excess and F_Si/F_cont – show a positive correlation when P_excess is measured in the K domain (Spearman’s rank correlation coefficient ρ = 0.71_−0.19^+0.10). No significant correlation was identified in the red domain (ρ = 0.13_−0.15^+0.16). The gas-rich comets have systematically weaker F_Si/F_cont than the dust-rich ones, and yet both groups retain the same overall tendency with different slope values.

Conclusions. The observed positive correlation between the two metrics indicates that composition is a peripheral factor in characterising the dust’s polarimetric and silicate emission properties. The systematic difference in F_Si/F_cont for gas-rich versus dust-rich comets would instead correspond to the difference in their dust size distribution. Hence, our results suggest that the current MIR spectral models of cometary dust, which search for a minimum χ² fit by considering various dust properties simultaneously, should prioritise the dust size and porosity over the composition. With light scattering being sensitive to different size scales in two wavebands, we expect the K-domain polarimetry to be sensitive to the properties of dust aggregates, such as size and porosity, which might have been influenced by evolutionary processes. On the other hand, the red-domain polarimetry reflects the characteristics of sub-micrometre constituents in the aggregate.