Meteorites may come from planetary embryo collisions

Chuck Bednar for – Your Universe Online

Rather than being planetary building blocks, asteroids and meteorites may actually be the byproducts of planetary collisions occurring billions of years ago, according to researchers from MIT and Purdue University.

The study, which was published online Wednesday in the journal Nature, explains that collisions taking place between planetary embryos (the seeds to the planets in our solar system) could be the true origin of the tiny grains of solidified melted rock that formed asteroids.

These bead-like grains, known as “chondrules,” have been found in meteorites – the name given to asteroids when they fall onto the Earth’s surface – for over 100 years. Their origin has remained a mystery, but the new study has linked them to collisions taking place between these planetary embryos roughly four billion years ago.

“Understanding the origin of chondrules is like looking through the keyhole of a door,” said Jay Melosh, a professor of Earth, atmospheric and planetary sciences at Purdue who was involved in the research. “While we can’t see all that is happening behind the door, it gives us a clear view of one part of the room and a glimpse into the very beginnings of our solar system.”

“We’ve found that an impact model fits extremely well with what we know about this unique material and the early solar system, and this suggests that, contrary to the current opinion among meteorite experts, asteroids are not leftover planet-building material and clumps of chondrules are not prerequisite to a planet,” he added.

Melosh, who is also a professor of physics and aerospace engineering, explained that the impact model for chondrules also resolves another issue – the fact that chondrules are identical in shape, size and texture to materials created by impacts on the Earth and the moon. The only difference is their chemical compositions, and that is because they are made from different starting material from impacts on different bodies, he explained.

Those Earth and lunar impact materials, which are known as spherules, are small droplets of solidified molten rock found embedded in rocks on the surface of the moon or planet. It has been widely accepted that impacts create the spherules, which formed from droplets of molten rock in the plume of debris ejected when larger asteroids made impact with the Earth, the authors said.

The droplets condensed and solidified to form the spherules, which then fell back to the surface and formed a distinct layer on the Earth, added Melosh. The processes which form chondrules are somewhat different, he and his colleagues propose, focusing instead on a small portion of debris that is ejected during the earliest moments of impact through a process known as jetting.

Jetting, the study authors explain, takes place at the beginning of impact as the surfaces of the two objects meet. The rock caught in the pinch between the two colliding objects becomes compressed to high pressure and intensely heated, which causes the initial bright flash observed during impacts conduced in the laboratory. The heat created by jetting is capable of melting rock and creating droplets in the ejected debris, and those droplets could become chondrules.

“Impact origin theories proposed in the past had been dismissed because they could not explain the melted material found in chondrules,” the university said. Collisions taking place in the early solar system occurred at much slower speeds than they do now, and were somewhat gentle due to the relatively small size of the planetary embryos. Typical collisions would have been able to cause rock to be broken into fragments, but not to melt it, Melosh said.

“Jetting allows a low-velocity impact to melt a small quantity of the target rock,” he added. “The melted material, but not the broken rock, is then ejected at high speed, such that the molten droplets can escape their parent bodies and depart into space, to later loosely bunch together. Millions of years of additional impacts and other compression mechanisms then created the asteroids and meteorites we know today.”

Some of the ejected debris would have been travelling fast enough to escape the planetary embryo’s gravitational pull, while the rest would have fallen back to the surface. The dust and molten droplets would have slowed to relatively low velocities due to the nebular gas found in the early solar system, which would have allowed them to accumulate and form asteroids.

David Minton, an assistant professor of earth, atmospheric and planetary science at Purdue who also was involved in the research, said that not everyone in the field will welcome their findings.

“Chondrule-bearing meteorites have long been thought to be similar to the building blocks of planets,” he said. “This study suggests that instead chondrules might actually be byproducts of impacts between objects of an earlier generation, and meteorites may not be representative of the material that made planets.”

The researchers explained that the next step could be to investigate how this chondrule formation mechanism fits into the new “pebble accretion” model of early planetary formation, in which the effects the effect of gas drag from the protoplanetary nebula is a key part of the process.

Editor’s note: The UCLA Meteorite Collection also contains a collections of rocks that were thought to be meteorites, but are not. They’re kept in a case titled “Meteorwrongs”.

ucla meteorite gallery


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