The origins of I-type spherules and the atmospheric entry of iron micrometeoroids

The Earth's extraterrestrial dust flux includes a wide variety of dust particles that include FeNi metallic grains. During their atmospheric entry iron micrometeoroids melt and oxidize to form cosmic spherules termed I-type spherules. These particles are chemically resistant and readily collected by magnetic separation and are thus the most likely micrometeorites to be recovered from modern and ancient sediments. Understanding their behavior during atmospheric entry is crucial in constraining their abundance relative to other particle types and the nature of the zodiacal dust population at 1 AU. This article presents numerical simulations of the atmospheric entry heating of iron meteoroids to investigate the abundance and nature of these materials. The results indicate that iron micrometeoroids experience peak temperatures 300-800 K higher than silicate particles explaining the rarity of unmelted iron particles which can only be present at sizes of <50 mu m. The lower evaporation rates of liquid iron oxide leads to greater survival of iron particles compared with silicates, which enhances their abundance among micrometeorites by a factor of 2. The abundance of I-types is shown to be broadly consistent with the abundance and size of metal in ordinary chondrites and the current day flux of ordinary chondrite-derived MMs arriving at Earth. Furthermore, carbonaceous asteroids and cometary dust are suggested to make negligible contributions to the I-type spherule flux. Events involving such objects, therefore, cannot be recognized from I-type spherule abundances in the geological record.