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Iron meteorites reveal unexpected metal distribution in solar system

The mysteries of the cosmos are as numerous as its stars, with astronomers and scientists relentlessly pursuing answers through iron meteorites and other clues.

Among the many enigmas is the elusive nature of our solar system’s formation about four and a half billion years ago.

Originally a swirling cloud of gas and dust centered around the sun, its exact structure has remained a matter of conjecture.

Peering through the cosmic veil

What might our early solar system, a protoplanetary disk, have looked like? While astronomers can harness the power of telescopes to examine distant protoplanetary disks, our own solar system’s infancy remains beyond our visual reach.

But space, in its vastness, has gifted us with certain clues in the form of meteorites – fragments that formed early in the solar system’s history and subsequently plunged through Earth’s atmosphere.

Unearthing secrets of iron meteorites

These meteorites, specifically iron meteorites, are akin to cosmic historians, their compositions narrating tales of the solar system’s inception. However, decoding these tales often leads to additional queries.

A recent paper published in the Proceedings of the National Academy of Sciences, featuring insights from a team of planetary scientists from UCLA and Johns Hopkins University Applied Physics Laboratory, delves into this cosmic cryptic narrative.

The study found that iridium and platinum, refractory metals which are known to condense in high temperatures, were more abundant in meteorites formed in the outer (colder) solar disk at a distance from the sun. This discovery was puzzling. Logically, these metals should have formed closer to the sun, where temperatures were higher.

Could there have been a pathway that moved these metals from the inner disk to the outskirts?

The journey of meteorites

Most meteorites came into being within the first few million years of our solar system’s existence. Some, known as chondrites, are unmelted amalgamations of grains and dust left from the formation of planets.

Others experienced sufficient heat to melt while their parent asteroids were in process of creation. As these asteroids melted, the silicate and metallic parts separated due to their density difference, much like oil and water.

Presently, the majority of asteroids reside in a dense belt between Mars and Jupiter. Scientists theorize that Jupiter’s gravitational pull may have disrupted these asteroids, leading to collisions and subsequent fragmentation.

The remnants of these asteroids, when they fall to Earth and are recovered, are what we call meteorites.

Iron meteorites: Keys to our solar system’s past

Iron meteorites, specifically, come from the metallic cores of the earliest asteroids – making them older than any other rocks or celestial objects in our solar system.

The molybdenum isotopes contained within these iron siblings of ours, point to various locations across the protoplanetary disk where they were formed, subsequently giving scientists insights into the disk’s chemical composition.

Observations using the Atacama Large Millimeter/submillimeter Array in Chile has revealed many disks around other stars resembling concentric rings, akin to a dartboard.

These planetary disks, such as HL Tau, have physical gaps, rendering them incapable of creating a route that could transport refractory metals from inner to outer disk.

The study, therefore, suggests that our solar disk didn’t share a similar ring structure at the start. More likely, our planetary disk was akin to a doughnut, and the asteroids with metal grains rich in iridium and platinum metals migrated to the outer disk as it rapidly expanded.

Unveiling the doughnut theory

This doughnut theory led to another perplexing question. Following the disk’s expansion, why didn’t gravity pull these metals back into the sun?

The answer to that lies with Jupiter, according to UCLA planetary scientist Bidong Zhang. “Once Jupiter formed, it very likely opened a physical gap that trapped the iridium and platinum metals in the outer disk and prevented them from falling into the sun,” Zhang explained.

These metals were then incorporated into asteroids that formed in the outer disk, validating why meteorites formed there have much higher iridium and platinum contents than their inner-disk counterparts.

This is not the first time Zhang and his team have utilized iron meteorites to unlock cosmic secrets. They also used them to reconstruct how water was distributed in the protoplanetary disk.

“Iron meteorites are hidden gems. The more we learn about iron meteorites, the more they unravel the mystery of our solar system’s birth,” concluded Zhang.

The study is published in the journal Proceedings of the National Academy of Sciences.


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