Acceleration of cometary dust near the nucleus: Application to 67P/Churyumov-Gerasimenko

TL;DR

This paper introduces a detailed model for simulating the dynamics of large, porous, non-spherical cometary dust particles near 67P's nucleus, revealing higher velocities than previous spherical models and matching early observed dust speeds.

Contribution

The paper presents a novel hierarchical aggregate model for large, porous dust particles, improving the accuracy of force calculations and dust velocity predictions near comet nuclei.

Findings

Large porous dust particles have 3-5 times higher velocities than spherical models.

The model successfully reproduces early dust speed observations of 67P.

Enhanced dust structure modeling impacts understanding of cometary dust dynamics.

Abstract

We present a model of cometary dust capable of simulating the dynamics within the first few tens of km of the comet surface. Recent measurements by the GIADA and COSIMA instruments on Rosetta show that the nucleus emits fluffy dust particles with porosities above 50% and sizes up to at least mm (Schulz et al. 2015, Rotundi et al. 2015, Fulle et al. 2015). Retrieval of the physical properties of these particles requires a model of the effective forces governing their dynamics. Here, we present a model capable of simulating realistic, large and porous particles using hierarchical aggregates, which shows previous extrapolations to be inadequate. The main strengths of our approach are that we can simulate very large (mm-scale) non-spherical agglomerates and can accurately determine their 1) effective cross-section and ratio of cross-section to mass, 2) gas drag coefficient, and 3) light scattering properties. In practical terms, we find that a more detailed treatment of the dust structure results in 3-5 times higher velocities for large dust particles in the inner coma than previously estimated using spherical particles of the same mass. We apply our model to the dynamics of dust in the vicinity of the nucleus of comet 67P and successfully reproduce the dust speeds reported early on when the comet was roughly 3.5 AU from the Sun. At this stage, we employ a simple spherical comet nucleus, we model activity as constant velocity gas expansion from a uniformly active surface, and use Mie scattering. We discuss pathways to improve on these simplifications in the future.