Iron meteorites reveal unexpected metal distribution


The mysteries of the universe are as numerous as its stars, and astronomers and scientists continue to seek answers through iron meteorites and other clues.

One of these many mysteries is that our solar system was formed about four and a half billion years ago.

It was essentially a rotating cloud of gas and dust centred around the Sun, and its exact composition remains a matter of conjecture.

Peering beyond the cosmic curtain

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, the early stages of our own solar system remain beyond our visual reach.

But space, in its vastness, has given us some clues in the form of meteorites – fragments that formed early in the solar system's history and later fell into Earth's atmosphere.

Unravelling the mysteries of iron meteorites

These meteorites, especially iron meteorites, are like cosmic historians, their compositions telling stories about the solar system's beginnings. However, understanding these stories often raises additional questions.

This has been said in a research paper published recently. Proceedings of the National Academy of ScienceThis cosmic mystery delves deep into the mystery, featuring insights from a team of planetary scientists from UCLA and the Johns Hopkins University Applied Physics Laboratory.

The study found that iridium and platinum, refractory metals known to condense at high temperatures, were more abundant in meteorites that formed in the outer (cool) solar disk, some distance from the Sun. This finding was surprising. Logically, these metals should have formed closer to the Sun, where temperatures were higher.

Could there have been a pathway that carried these metals from the inner disk to the outer disk?

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 mixtures of particles and dust left over from the formation of the planets.

Others got hot enough to melt while their parent asteroids were still in the process of forming. As these asteroids melted, differences in their densities caused the silicate and metal parts to separate, much like oil and water.

Currently, most of the asteroids are located in a dense belt between Mars and Jupiter. Scientists believe that Jupiter's gravitational force may have disrupted these asteroids, leading to collisions and subsequent fragmentation.

When these asteroids fall to Earth and are recovered, their remains are called meteorites.

Iron meteorites: the key to our solar system's past

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

The molybdenum isotopes present in these iron siblings of ours point to different locations in the protoplanetary disk where they formed, giving scientists information about the chemical composition of the disk.

Observations using the Atacama Large Millimeter/submillimeter Array in Chile have revealed numerous disks around other stars that look like concentric rings, resembling a dartboard.

These planetary disks, like HL Tau, contain physical gaps that make them unable to form pathways that could transport refractory metals from the inner disk to the outer disk.

Therefore, the study suggests that our solar disk did not have a uniform ring structure in the beginning. More likely, our planetary disk was more like a doughnut, and asteroids containing metal particles rich in iridium and platinum metals moved into the outer disk as it expanded rapidly.

Unveiling the Doughnut Theory

This doughnut theory gives rise to another intriguing question: Why didn't gravity pull these metals back toward the Sun after the disk expanded?

According to UCLA planetary scientist Bidong Zhang, the answer lies in Jupiter. “Once Jupiter formed, it probably created 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, explaining why meteorites that formed there contain higher amounts of iridium and platinum than their inner disk counterparts.

This is not the first time Zhang and his team have used iron meteorites to uncover cosmic mysteries. They also used them to figure out how water was distributed in the protoplanetary disk.

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

This study has been published in the journal Proceedings of the National Academy of Science,

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