The Curie line in protoplanetary disks and the formation of Mercury-like planets

TL;DR

This paper investigates how the Curie line in protoplanetary disks influences the formation of iron-rich planetary bodies by affecting magnetic aggregation processes.

Contribution

It introduces the concept of the Curie line as a key factor in planetesimal formation, linking laboratory iron transition experiments to planetary formation regions.

Findings

The Curie temperature of iron creates a distinct boundary in protoplanetary disks.

Magnetic aggregation varies sharply across the Curie line, affecting planetesimal growth.

The Curie line may serve as a preferred zone for forming Mercury-like planets.

Abstract

In laboratory experiments, we heated chondritic material up to 1400K in a hydrogen atmosphere. Moessbauer spectroscopy and magnetometry reveal that, at high temperatures, metallic iron forms from silicates. The transition temperature is about 1200K after 1 h of tempering, likely decreasing to about 1000K for longer tempering. This implies that in a region of high temperatures within protoplanetary disks, inward drifting solids will generally be a reservoir of metallic iron. Magnetic aggregation of iron-rich matter then occurs within the magnetic field of the disk. However, the Curie temperature of iron, 1041 K, is a rather sharp discriminator that separates the disk into a region of strong magnetic interactions of ferromagnetic particles and a region of weak paramagnetic properties. We call this position in the disk the Curie line. Magnetic aggregation will be turned on and off here. On the outer, ferromagnetic side of the Curie line, large clusters of iron-rich particles grow and might be prone to streaming instabilities. To the inside of the Curie line, these clusters dissolve, but that generates a large number density that might also be beneficial for planetesimal formation by gravitational instability. One way or the other, the Curie line may define a preferred region for the formation of iron-rich bodies.