A new study published in the journal Nature Astronomy is the first to look at meteorites from before giant space collision 466 million years ago.
This is an artist’s rendering of the space collision 466 million years ago that gave rise to many of the meteorites falling today. Image credit: Don Davis, Southwest Research Institute.
Around 466 million years ago (Ordovician period), there was a giant collision in our Solar System. Something hit an asteroid and broke it apart, sending chunks of rock falling to Earth as meteorites.
But what kinds of meteorites were making their way to our planet before that collision?
A research team led by Field Museum scientist Philipp Heck has tackled that question by creating the first reconstruction of the distribution of meteorite types before the collision.
The scientists discovered that most of the meteorites we see today are rare, while many meteorites that are rare today were common before the collision.
The discovery confirms a hypothesis presented in 2016 when Lund University Professor Birger Schmitz revealed that he had found a so-called ‘fossil’ meteorite.
The meteorite was given the name Osterplana 065 and was discovered in a quarry outside Lidköping in Sweden. The term ‘fossil’ was used because of its unusual composition, different from all known groups of meteorites, and because it originated from a celestial body that was destroyed in ancient times.
The discovery led to the hypothesis that the flow of meteorites may have been completely different 466 million years ago.
“The new results confirm this hypothesis,” said Prof. Schmitz, co-author on the current study.
“Based on 43 micrometeorites, which are as old as Osterplana 065, our new study shows that back then the flow was actually dramatically different.”
“The conventional view is that the Solar System has been very stable over the past 500 million years. So it is quite surprising that the meteorite flow at 466 million years ago was so different from the present,” he added.
Dr. Heck, Prof. Schmitz and their colleagues retrieved samples of rock from an ancient seafloor that contained micrometeorites, and then dissolved the rocks in acid so that only microscopic chromite crystals remained.
“Chrome-spinels, crystals that contain the mineral chromite, remain unchanged even after hundreds of millions of years,” Dr. Heck said.
“Since they were unaltered by time, we could use these spinels to see what the original parent body that produced the micrometeorites was made of.”
Analysis of the chemical makeup of the spinels showed that the meteorites and micrometeorites that fell earlier than 466 million years ago are different from the ones that have fallen since.
A full 34% of the pre-collision meteorites belong to a meteorite type called primitive achondrites; today, only 0.45% of the meteorites that land on Earth are this type.
Other ancient micrometeorites sampled turned out to be relics from the asteroid Vesta, which underwent its own collision event over a billion years ago.
“Meteorite delivery from the asteroid belt to the Earth is a little like observing landslides started at different times on a mountainside,” said co-author Dr. William Bottke, of the Southwest Research Institute.
“Today, the rocks reaching the bottom of the mountain might be dominated by a few recent landslides. Going back in time, however, older landslides should be much more important.”
“The same is true for asteroid breakup events; some younger ones dominate the current meteorite flux, while in the past older ones dominated.”
“Knowing more about the different kinds of meteorites that have fallen over time gives us a better understanding of how the main asteroid belt evolved and how different collisions happened,” Dr. Heck added.
“Ultimately, we want to study more windows in time, not just the area before and after this collision during the Ordovician period, to deepen our knowledge of how different bodies in Solar System formed and interact with each other.”
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P.R. Heck et al. 2017. Rare meteorites common in the Ordovician period. Nat. Astron 1: 0035; doi: 10.1038/s41550-016-0035
