Finding fossil meteorites represents a whole new level of difficulty because these rocks do not spread out across the surface but, like dinosaur fossils, burrow into ancient rocks. But geology professor Birger Schmitz at Lund University in Sweden has discovered a way to find them – and not just rare rocks, but considerable amounts of fossilized space dust.
It turns out that this can draw a unique picture of how the course of ancient life on Earth was influenced by events in space, and has already revealed how space dust from an asteroid collision 466 million years ago could have triggered a ice Age.
Rocks
Meteorites are rocks from space that have fallen to Earth, mostly fragments from asteroids that pushed between Mars and Jupiter.
We know that a huge rock crashed into Earth 66 million years ago, killing dinosaurs and most life. This was first theorized by Nobel Prize winner Luis Álvarez and his son Walter, both based at universities in Berkeley, USA. They led the team that discovered a thin layer of iridium-rich ash in sediments globally that It was formed when the dinosaurs became extinct . Iridium is extremely rare in the crust of our planet, so the experts concluded that this element had been brought in by a large asteroid. The implication was that this cataclysm wiped out the dinosaurs, along with three-quarters of all other living things.
Professor Schmitz did his post-doctorate with Alvarez’s team and it is fair to say that it shaped his career. “Until its discovery, the evolution of life and the history of the Earth was almost always viewed as a closed system,” said Professor Schmitz. “I was fascinated by trying to connect what happens in space with what happens on Earth.”
Returning to Sweden in 1990, Schmitz read some newspaper reports that an amateur geologist named Mario Tassinari had found some fossil meteorites in the Thorsberg quarry on the southern shore of Lake Vänern. At this time, there were only a handful of fossil meteorites known to science. Was this an opportunity to study what meteorites had looked like millions of years ago and reconstruct the effects they had on Earth? Schmitz called Tassinari and they agreed to collaborate on a systematic study of the quarry.
Quarry
Quarry workers carve sheets of limestone to use as tiles. When they find a piece that might appear to contain a space rock, they call Schmitz. Every year, they get four or five fossil meteorites. For the most part, they look like little more than black spots embedded in the rock, a few inches wide.
Professor Schmitz wondered if other quarries might have similar delicacies. He also went looking for meteorites in places with the right kind of tiles, like London’s Paddington station.
But it turns out that Thorsberg is, as far as we know, unique. Special conditions are required to preserve considerable fossil meteorites in reasonable quantities. You need sediment at the bottom of a body of water that turns into rock very gradually. Slowness is crucial, because that allows more meteorites to accumulate in a small area.
Around the year 2000, Professor Schmitz began to think that he was finding a lot of stones, by this time he had almost 50 in total. He calculated the amount of rock the quarry workers had excavated each year and divided it by the number of meteorites they found. That told him that when the rock formed, the flow of meteorites, the number that fell in a certain area at any given time, was about 100 times greater than it is today. An area the size of Wales would have been receiving not two meteorites a year, but 200.
“I still remember the day I thought about this,” the researcher commented. ‘I immediately went to the quarry. May I see your log books? Are you sure you have found such a large number of meteorites in such a small area? “.
There was no mistake. And Schmitz quickly gave an explanation. “There is a very likely scenario: if something explodes in space and breaks down into billions and billions of little pieces, well, what we saw in the quarry, that’s exactly what would happen.”
In 2004, Professor Schmitz and researchers from ETH-Zürich, Switzerland, published a paper detailing the analyzes of the fossil meteorites found. This showed that meteorites had been in space for relatively short periods of time, around a million years, by observing the effect of cosmic rays on their mineralogy. He concluded that a violent collision had exploded them on a ground trajectory . Still, these stones were little fry in the grand scheme of things; by no means large enough to cause an impact that would significantly affect Earth’s history.
You might think that Schmitz would have wanted to find something bigger. But it doesn’t work like that. It turns out that large meteorites are rare; smaller ones are more common.
Space dust
Micrometeorites, also known as space dust, are the most common of all: it is estimated that today we bathe with 100 tons of these objects every day.
One mineral, chromite, in micrometeorites is incredibly tough : “It’s extremely tough, it survives everything,” Schmitz said.
That gave him an idea, which he investigated through a project called the Astrogeobiosphere . “I said to my poor student, ‘Niklas, take 5 kilograms of rock and dissolve it in hydrochloric acid.”
In doing so, 10 small extraterrestrial chromite fragments were obtained, each one tenth of a millimeter long. And repeating this at intervals in the limestone revealed that there was a large increase in space dust in the 466-million-year-old rock, results that Professor Schmitz published in 2019.
The arrival of space dust coincides with a cold period known as the mid-Ordovician ice age. Schmitz concluded that a massive collision in the asteroid belt threw out large meteorites and a gigantic cloud of dust, which prevented much sunlight from reaching Earth, leading to an ice age . After the asteroid that killed the dinosaurs, it would be the second example of an event in the larger cosmos that profoundly influences Earth’s history.
“I think this is a very exciting story,” said Dr. Katie Joy, a planetary scientist at the University of Manchester, UK. “It’s a lot of work and I don’t envy you, having to crush tons and tons and rocks and run it through the acid.”
The expert adds that there is still work to be done to understand the biases in the sample. The minerals that Professor Schmitz is studying are not found in all types of asteroids and comets, which means that they do not represent all types of rocks that have fallen from space to Earth. “This record is a partial record,” he commented.
Through the Astrogeobiosphere, Schmitz has taken this work even further by dissolving 20 tons of rock from different quarries, with samples from each representing different periods in Earth’s deep history. The idea was to provide the first sketch of how incoming space dust has varied over time.
He states that this work is done and that he and his team have a collection of different meteorites that fell on Earth in the distant past, but will not be published for a few months. However , fossil meteorites continue to reveal new information about Earth’s past.
“(Many of) these meteorites are as different from the meteorites that fall today as some of the animals that lived at that time compared to the animals today,” concludes the researcher.
Original article
This article was originally published in Horizon, the EU Research and Innovation Magazine
