Acceleration of cometary dust near the nucleus: application to 67P/Churyumov–Gerasimenko

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 Grain Impact Analyser and Dust Accumulator and Cometary Secondary Ion Mass Analyser instruments on Rosetta show that the nucleus emits fluffy dust particles with porosities above 50 per cent and sizes up to at least mm (Fulle et al. 2015b;Rotundi et al. 2015;Schulz 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 crosssection 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 three to five 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.

Evolution of Cometary Dust Particles to the Orbit of the Earth: Particle Size, Shape, and Mutual Collisions

“…c total mass supply rate into the IDP cloud complex by particles with β > 0.00114 d average of absolute values of log of ratio between our model and observed model (Grün et al 1985) The next model is composed of both compact spherical particles and fluffy aggregates. We assumed that the fluffy aggregates have constant β values regardless of mass (Mukai et al 1992;Skorov et al 2016). The SFD of compact spherical particles was assumed to be dn ∝ m −α1 dm for m ≤ m c1 and dn ∝ m −α3 dm for m ≥ m c1 .…”

Cometary Comae-Surface Links

“…This affects the dynamics of the particles as shown by Skorov et al. ( 2016a , 2018 ) who have found that porous aggregates are accelerated to significantly higher speeds than their compact counterparts. Additionally, rotating particles (Fulle et al.…”

“… 2011 ; Skorov et al. 2016a , 2018 ). The dust can be treated either in a test particle approach within the gas flow or as a separate fluid within a gas-dust multi-fluid approach (e.g.…”

“…In the immediate vicinity the nucleus gravity is the main opposing force. For longer time scales solar radiation pressure needs to be included as well but can be neglected in the innermost coma (Tenishev et al 2011;Skorov et al 2016aSkorov et al , 2018. The dust can be treated either in a test particle approach within the gas flow or as a separate fluid within a gas-dust multi-fluid approach (e.g.…”

“…Although we often approximate the dust grains as test particles and consider them to be spherical, especially the larger dust grains are porous and fluffy aggregates (Kolokolova and Kimura 2010;Schulz et al 2015;Rotundi et al 2015;Levasseur-Regourd et al 2018). This affects the dynamics of the particles as shown by Skorov et al (2016aSkorov et al ( , 2018 who have found that porous aggregates are accelerated to significantly higher speeds than their compact counterparts. Additionally, rotating particles that have oblate or prolate shapes will also affect the dynamics as studied by Ivanovski et al (2017a,b).…”