In the latest Science Advances issue (1 Oct 2025, vol. 11, #40):
Grewal, D. S. Zhang, Z. Manilal, V. Protracted core formation and impact disruptions shaped the earliest outer Solar System planetesimals adw1668
Most asteroids never melted; they then preserve the materials and morphologies of the dawn of the Solar System… but some asteroids did melt. Some asteroids grew large enough to hold their internal heat, and melt. The heavier components could flow down to form a core, and the lighter stuff floated on top, to form a crust- “differentiation”. The heavier parts were the iron and other dense metals, plus elements that dissolve in iron (“siderophiles” or iron-lovers). Iron meteorites, then, are fragments from these large asteroids (“planetesimals”) that later got shattered by impacts, scattering the core material. Some fragments then made it to Earth.
Some of these, in turn, show that they formed in the outer Solar System. This zone of the young solar nebula was cooler and thinner, but formation, differentiation, and impacts still happened, just at slower and sparser rates. The surviving fragments then show a different blend of elements and isotopes, which are the signature of the outer Solar System (including infall of material from interstellar space), as opposed to stuff from the inner Solar System (which follow the composition of the Sun). Our classification of iron meteorites includes this difference.
Grewal et al. attempt to decipher the history and processes of these outer Solar System large asteroids. Looking at iron meteorites with outer-Solar-System blends, the authors use elements and isotopes as tracers. This is their interpretation of that history.