IIE iron meteorites are fragments of a planetesimal with a complex structure

IIE iron meteorite sample (Photo courtesy Carl Agee, Institute of Meteoritics, University of New Mexico)
IIE iron meteorite sample (Photo courtesy Carl Agee, Institute of Meteoritics, University of New Mexico)

An article published in the journal “Science Advances” reports the results of sophisticated analyzes indicating that IIE iron meteorites are fragments of a planetesimal that had a differentiated structure. A team of researchers conducted various types of tests that gave these results about meteorites called achondrites, composed of materials that were subjected to melting, differentiation, and crystallization. The difference compared to chondrites, meteorites composed of undifferentiated materials, shows that the objects they come from formed and evolved in different ways in space and time.

Different planetesimals contained different amounts of elements, including radioactive ones that generated heat with their decay. If the radioactive elements were in sufficient amount when a planetesimal was formed, the heat generated was enough to melt the materials that composed it. This led to a differentiation into layers, and the metals may have formed a core sufficient to trigger a magnetic field.

In the early solar system, there were probably a lot of planetesimals, but over time some grew to become dwarf planets and planets while many others got destroyed in collisions. The meteorites that fell on Earth are mainly made up of planetesimal fragments that dispersed in space. This allowed to examine their contents and to detect the differences between the unmelted undifferentiated materials of the chondrites and the melted and differentiated materials of the achondrites.

However, the division is not so neat. Partially differentiated internal structures of the planetesimals that generated the achondrites included individual bodies containing iron nuclei, achondritic silicate mantles, and chondritic crusts. To find these three layers, the researchers combined paleomagnetic synchrotron analyzes with thermal evolution, impact and collision models to show that the progenitor body of IIE iron meteorites was a partially differentiated planetesimal.

The results of the analysis imply that some chondrites and achondrites coexisted simultaneously on the same planetesimal. This indicates that its growth went on and that apparently undifferentiated asteroids could contain molten parts within them. In short, the primordial objects of the solar system may have had a more complex diversity than previously thought. Clara Maurel, a graduate student at MIT and lead author of the article, commented that finding an example of a planetesimal with melted and unmelted layers encourages the search for other evidence of composite planetary structures. Covering the full range of structures is the key to deciphering how planetesimals formed in the primordial solar system.

Several questions about planetesimals remain. For example, what generated the IIE iron meteorites probably originated in the asteroid belt between Mars and Jupiter, but where did the collision that destroyed it occur? In that area or closer to Earth’s orbit? And why did the fragments generated in that collision start crossing Earth’s orbit until they hit it many times? Was a planetesimal with a composite structure a rarity or was it a common type we know very little about just because few fragments fell to Earth?

Benjamin Weiss of MIT, one of the authors of the article, pointed out that most of the bodies in the asteroid belt appear unmelted on the surface. If we could see inside them, we might check if they have a composite structure. Help will come from NASA’s Psyche mission, which in a few years will allow to study 16 Psyche, the largest of the M-type asteroids, the ones composed almost entirely of metals.

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