Pure iron grains are rare in the universe

TL;DR

This study shows that pure metallic iron grains are extremely rare in space because their formation process is highly inefficient, implying most iron exists as compounds or impurities rather than pure metal grains.

Contribution

The paper demonstrates through microgravity experiments that homogeneous nucleation of metallic Fe grains is highly ineffective in space environments.

Findings

Homogeneous nucleation of Fe grains is extremely inefficient.

Most Fe in space is likely in compound or impurity form.

Pure Fe grains are rare in the universe.

Abstract

The abundant forms in which the major elements in the universe exist have been determined from numerous astronomical observations and meteoritic analyses. Iron (Fe) is an exception, in that only depletion of gaseous Fe has been detected in the interstellar medium, suggesting that Fe is condensed into a solid, possibly the astronomically invisible metal. To determine the primary form of Fe, we replicated the formation of Fe grains in gaseous ejecta of evolved stars by means of microgravity experiments. We found that the sticking probability for formation of Fe grains is extremely small; only several atoms will stick per hundred thousand collisions, so that homogeneous nucleation of metallic Fe grains is highly ineffective, even in the Fe-rich ejecta of Type Ia supernovae. This implies that most Fe is locked up as grains of Fe compounds or as impurities accreted onto other grains in the interstellar medium.