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A microscopic look at asteroid crashes might aid in our understanding of planet creation

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A novel method for timing collisions between asteroids and planetary bodies throughout our Solar System’s history might aid scientists in determining how and when planets were formed.

A team of academics headed by the University of Cambridge combined dating and microscopic examination of the Chelyabinsk meteorite, which crashed to Earth and made headlines in 2013, to get more precise limitations on the date of past impact events.

Their research, which was published in Communications Earth & Environment, looked at how minerals inside the meteorite were destroyed by various impacts throughout time, allowing them to pinpoint the largest and oldest events that may have been involved in planetary creation.

“Meteorite impact ages are frequently disputed: our work shows that we need to draw on multiple lines of evidence to be more certain about impact histories — almost like investigating an ancient crime scene,” said Craig Walton, who led the research and is based at Cambridge’s Department of Earth Sciences.

Planets, including the Earth, evolved early in our Solar System’s history as a result of enormous collisions between asteroids and even larger things known as proto-planets.

“Evidence of these impacts is so ancient that it has been lost on the planets — Earth, in particular, has a short memory since surface rocks are regularly recycled by plate tectonics,” said co-author Dr. Oli Shorttle, who works at Cambridge’s Department of Earth Sciences and Institute of Astronomy.

In contrast, asteroids and their bits that fall to Earth as meteorites are inert, cold, and considerably older, making them accurate timekeepers of impacts.

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The new study, conducted in partnership with the Chinese Academy of Sciences and the Open University, documented how phosphate crystals within the Chelyabinsk meteorite were fractured to varied degrees in order to put together a collision history.

Their goal was to confirm the meteorite’s uranium-lead dating, which looks at the time it takes for one isotope to degrade to another.

“The phosphates in most primordial meteorites are wonderful targets for timing the shock events experienced by the meteorites on their parent bodies,” said Dr Sen Hu, who conducted the uranium-lead dating at the Chinese Academy of Sciences’ Institute of Geology and Geophysics in Beijing.

Previous dating of this meteorite indicated two impact ages: an older, nearly 4.5-billion-year-old collision and a more recent, 50-million-year collision.

However, these ages are not so clear-cut. Similarly to how a painting fades with time, subsequent collisions may distort a once-clear image, causing scientific ambiguity about the age and even the number of hits recorded.

The new research reordered the collisions recorded by the Chelyabinsk meteorite by correlating new uranium-lead ages on the meteorite to microscopic evidence for collision-induced heating detected within their crystal structures. These minuscule signals accumulate in the minerals with each consecutive hit, allowing the collisions to be differentiated, placed in chronological order, and dated.

Their results demonstrate that minerals with the earliest collision imprint were either broken into numerous smaller crystals at high temperatures or forcefully distorted at high pressures.

The researchers also reported certain mineral grains in the meteorite that were broken by a smaller impact, at lower pressures and temperatures, and which had a significantly younger age of less than 50 million years. They believe the collision ripped the Chelyabinsk meteorite from its host asteroid and sent it flying toward Earth.

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Taken as a whole, this suggests a two-stage collision history. “The concern for us was whether these dates could be believed, if we could relate these hits to evidence of superheating from an impact,” Walton said. “We’ve shown that the mineralogical background for dating is really significant.”

Scientists are especially interested in the 4.5-billion-year-old impact date because it corresponds to the period when scientists believe the Earth-Moon system formed, most likely as a consequence of two planetary bodies colliding.

The Chelyabinsk meteorite is part of a series of stony meteorites, which all include heavily broken and remelted material that nearly coincides with this massive impact.

The latest dates back up earlier claims that multiple asteroids collided with tremendous energy between 4.48 and 4.44 billion years ago. “The fact that all of these asteroids record extensive melting at this period may suggest Solar System re-organisation, either as a consequence of Earth-Moon creation or maybe as a result of massive planet orbital motions.”

Walton now intends to refine dates over the Moon-forming impact window, which might reveal how our own planet came to be.

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