West LA Times

Iron meteorites are remnants of the metallic cores of the earliest asteroids in our solar system. These meteorites contain refractory metals, such as iridium and platinum, which formed near the sun but were transported to the outer solar system. New research indicates that for this transportation to have occurred, the protoplanetary disk of our solar system had to have been doughnut-shaped because refractory metals could not have crossed large gaps in a target-shaped disk of concentric rings.

The paper suggests that refractory metals moved outward as the protoplanetary disk rapidly expanded and were trapped in the outer solar system by Jupiter.

Four and a half billion years ago, our solar system was a cloud of gas and dust swirling around the sun until gas began to condense and accrete along with dust to form asteroids and planets. This cosmic nursery, known as a protoplanetary disk, has been studied by astronomers using telescopes to observe distant disks far from our mature solar system. However, it is impossible to directly observe what ours might have looked like in its infancy.

Fortunately, space has provided clues through fragments of objects that formed early in solar system history and plunged through Earth’s atmosphere as meteorites. The composition of these meteorites offers insights into the birth of our solar system but often raises more questions than answers.

In a paper published in Proceedings of the National Academy of Sciences, a team of planetary scientists from UCLA and Johns Hopkins University Applied Physics Laboratory reports that refractory metals such as iridium and platinum were more abundant in meteorites formed in the outer disk, which was cold and far from the sun. These metals should have formed close to the sun due to higher temperatures there. The study explores whether there was a pathway moving these metals from the inner disk to the outer regions.

Most meteorites formed within the first few million years of solar system history. Some meteorites, called chondrites, are unmelted conglomerations of grains and dust left over from planet formation. Other meteorites experienced enough heat to melt while their parent asteroids were forming. When these asteroids melted, silicate parts separated from metallic parts due to differences in density.

Today, most asteroids are located in a thick belt between Mars and Jupiter. Scientists believe Jupiter’s gravity disrupted these asteroids' courses, causing many collisions and breakups. When pieces fall to Earth and are recovered, they are termed meteorites.

Iron meteorites originate from the metallic cores of early asteroids, older than any other rocks or celestial objects in our solar system. They contain molybdenum isotopes indicating various locations across the protoplanetary disk where these meteorites formed. This allows scientists to infer chemical compositions during its infancy.

Previous research using the Atacama Large Millimeter/submillimeter Array (ALMA) found many disks around other stars resembling concentric rings like dartboards. These rings are separated by physical gaps; hence such disks could not transport refractory metals from inner regions outward.

The new paper proposes that our early planetary disk lacked ring structures initially; instead, it resembled a doughnut shape where metal grains rich in iridium and platinum migrated outward during rapid expansion.

This posed another puzzle: after expansion ceased gravitational forces should have pulled these metals back toward their origin point—the Sun—but this did not occur.

“Once Jupiter formed,” said first author Bidong Zhang—a UCLA planetary scientist—“it very likely opened up physical gaps trapping iridium & platinum preventing them falling back into Sun.” He continued “These later incorporated into forming outer-disk asteroids explaining why carbonaceous chondrites/iron-meteorite types possess higher iridium/platinum content versus inner-disk counterparts.”

Zhang alongside collaborators previously utilized iron-meteorite data reconstructing water distribution patterns within proto-planetary-disks.

“Iron-meteorite studies unravel mysteries surrounding Solar-System's genesis,” Zhang concluded.