Models of Rosetta/OSIRIS 67P dust coma phase function

TL;DR

This study models the dust coma phase function of comet 67P using oriented elongated particles, successfully reproducing observed features like backscattering enhancement and a deep minimum at 100° phase angle.

Contribution

It introduces a new model of cometary dust particles as oriented, elongated, absorbing, and porous, explaining the observed phase function features.

Findings

Oriented elongated particles reproduce the phase function features.

Particles need to be absorbing with specific refractive index.

Porosity of 60-70% is consistent with observations.

Abstract

The phase function of the dust coma of comet 67P has been determined from Rosetta/OSIRIS images \citep{Bertini17}. This function show a deep minimum at phase angles near 100$^\circ$, and a strong backscattering enhancement. These two properties cannot be reproduced by regular models of cometary dust, most of them based on wavelength-sized and randomly-oriented aggregate particles. We show, however, that an ensamble of oriented elongated particles of a wide variety of aspect ratios, with radii $r \gtrsim$10 $\mu$m, and whose long axes are perpendicular to the direction of the solar radiation, are capable of reproducing the observed phase function. These particles must be absorbing, with an imaginary part of the refractive index of about 0.1 to match the expected geometric albedo, and with porosity in the 60-70\% range.