Density, porosity, mineralogy, and internal structure of cosmic dust and alteration of its properties during high velocity atmospheric entry

TL;DR

This study uses advanced imaging and analytical techniques to examine the internal structure, mineralogy, and physical changes of micrometeorites during atmospheric entry, revealing how velocity influences melting, porosity, and mineral redistribution.

Contribution

It provides detailed insights into the mineralogical and physical transformations of micrometeorites at different entry velocities, highlighting the effects of melting and oxidation processes.

Findings

Porosity varies from 0-12% in unmelted to 16-27% in partially melted micrometeorites.

Complete melting reduces porosity to about 1% and causes metal oxidation and redistribution.

Melted cosmic spherules exhibit oriented crystallographic structures, unlike other types.

Abstract

X-ray microtomography (XMT), X-ray diffraction (XRD) and magnetic hysteresis measurements were used to determine micrometeorite internal structure, mineralogy, crystallography, and physical properties at ~{\mu}m resolution. The study samples include unmelted, partially melted (scoriaceous) and completely melted (cosmic spherules) micrometeorites. This variety not only allows comparison of the mineralogy and porosity of these three micrometeorite types, but also reveals changes in meteoroid properties during atmospheric entry at various velocities. At low entry velocities, meteoroids do not melt, and their physical properties do not change. The porosity of unmelted micrometeorites varies considerably (0-12%) with one friable example having porosity around 50%. At higher velocities, the range of meteoroid porosity narrows, but average porosity increases (to 16-27%) due to volatile evaporation and partial melting (scoriaceous phase). Metal distribution seems to be mostly unaffected at this stage. At even higher entry velocities, complete melting follows the scoriaceous phase. Complete melting is accompanied by metal oxidation and redistribution, loss of porosity (1 $\pm$ 1%), and narrowing of the bulk (3.2 $\pm$ 0.5 g/cm$^{3}$) and grain (3.3 $\pm$ 0.5 g/cm$^{3}$) density range. Melted cosmic spherules with a barred olivine structure show an oriented crystallographic structure, whereas other subtypes do not.