The Curie-Line in Protoplanetary Disks

With its dominant iron core Mercury is an exception in the solar system. Considering extrasolar planets, however, it is not an isolated case. Therefore in the protoplanetary disk, iron and silicates must be separated in one way or the other. If Mercury’s structure is rooted in its formation, the separation might take place early on before planetesimals and planets form from an iron enriched reservoir. We introduce the idea of a Curie-line within the inner AU, which could provide a preferred spot for planetesimal formation with a potential bias for iron rich planetesimals [in prep].

In a hydrogen atmosphere we heated chondritic material at increasing temperatures up to 1400 K. Mössbauer spectroscopy and magnetometric measurements reveal that above 1200 K iron is reduced to metallic iron which results in a strong increase in magnetization. Since solids drift inwards in the protoplanetary disk, the amount of metallic iron is increased in a spatially well confined region of high temperature. Magnetic fields are estimated to reach up to the mT range in the inner disk, decreasing with distance from the star. If large enough magnetic fields and a significant fraction of metallic iron come together, magnetic dust aggregates might grow into iron rich bodies by magnetic clustering [Kruss & Wurm 2018 and 2020].

The iron’s Curie temperature at 1043 K, however, sharply distinguishes between strong ferromagnetic and weak paramagnetic properties. Therefore, material beyond the Curie temperature cannot react to the disks magnetic field. Material that drifted far enough inwards to form metallic iron and is diffused back outwards through turbulence, however, will start forming larger clusters containing iron rich matter. The aggregates will drift inwards again, passing the Curie-line, creating a zone with increased iron clusters that might readily react to streaming instabilities.

References: Kruss, M., & Wurm, G. (2018). Seeding the formation of Mercurys: an iron-sensitive bouncing barrier in disk magnetic fields. The Astrophysical Journal, 869(1), 45.

Kruss, M., & Wurm, G. (2020). Composition and size dependent sorting in preplanetary growth: seeding the formation of Mercury-like planets. The Planetary Science Journal, 1(1), 23.