Study of Cometary Dust Particle Fragmentation from <i>In situ</i> Measurements of Cometary Dust

Abstract

Cometary nuclei, primitive remnants from the solar system’s birth, have compositions reflecting their time, and place, of formation. Cometary dust, a large fraction of cometary nuclei mass, is released by volatile vaporization as comets approach perihelion. Fragmentation of composite dust grains affects particle size distribution and gas-drag efficiency, affecting production rate calculations. Dust grains’ structures, implied by fragmentation process modelling, profoundly constrain comet formation theories. The causes, nature, and relative importance of fragmentation processes are investigated, supported by in situ cometary dust fragmentation observations. Results from spacecraft that have sampled cometary dust in situ, several providing evidence of fragmentation, are summarised, and two in situ datasets re-analyzed, to determine if fragmentation is ubiquitous, and if so, why it isn’t observed in every comet. Re-analysis of the International_Cometary_Explorer_(ICE) Plasma_Wave_Instrument dataset, which identified dust impacts at 21P/Giacobini-Zinner, shows evidence of grain fragmentation, clustering, and non-isotropic outflow, unidentified at the time. The Rosetta mission to 67P/Churyumov-Gerasimenko carried several dust detection and characterization instruments, including the Grain_Impact_Analyser_and Dust_Accumulator_(GIADA), data from which suggested a spacecraft-induced electrostatic deceleration and fragmentation mechanism for ultra-low-density (<1 kg m−3) aggregate grains. This work is critically reviewed, identifying several analytical deficiencies, dataset limitations, and problems with the eventual conclusions. The GIADA team analyzed Grain_Detection_System+Impact_Sensor (GDS+IS) data which fully characterized dust events. For this work GDS-only data, in which clusters were identified, was analyzed showing these data to be more reliable than initially believed, containing useful speed, and scattered light data for many of events. Re-analysis of GIADA data confirmed particles were fragmenting near the spacecraft forming detectable particle clusters, with different particle size, density, and velocity distributions to non-cluster particles. The grain fragmentation mechanism suggested isn’t proved or disproved, but associated mechanisms, including stress-induced particle break-up in the maneuvering spacecraft’s vicinity, were identified. Mean particle density, estimated using an alternative method, was between ρ− = 880 - 1180 kg m-