From the outside, the little pebble looks like an ordinary rock — yellowish-brown, a little lumpy, and only aboutthree3 grams in mass. But beneath that brownish veneer, it’s studded with microscopic diamonds and another super-hard form of carbon, both formed in the heat and pressure of a meteorite’s impact with Earth.
A recent study, published in the journal icarussuggests that part of the unassuming little pebble may have come from beyond our Solar System, long before our Sun was even born.
What’s new — Chemical tests on the meteorite, nicknamed Hypatia, suggest that some of the material it’s made of came from outside our Solar System. In fact, the team behind the study says it was forged in the death throes of a massive star long before our Solar System began to form.
University of Johannesburg geochemist Jan Kramers and his colleagues measured the chemical composition of the stone, and they found that it’s actually made of two types of material. Most of the stone is almost pure carbon, but small patches contain a mix of heavier elements, like aluminum, silicon, zinc, and iron. In all, the authors measured 15 chemical elements in the rocky matrix. They were especially interested in how much of each element was mixed into the rock compared to each of the other elements. Those ratios act like a chemical fingerprint that can reveal information about how and where the mysterious stone formed.
But Hypatia’s chemical fingerprint was a surprise — it didn’t look like any scientists had seen before.
“That’s why it was very important to at it for additional evidence, because it did not look like any meteorite. Then we tried to compare it to comets, and it didn’t look like a comet,” University of Johannesburg geochemist Georgy Belyanin, a coauthor of the recent paper, tells inverse. It also didn’t quite match the ratios of elements that astronomers had seen in interstellar dust and gas. The closest match Kramers and his colleagues could find was in the cosmic debris from a powerful stellar explosion called a Type Ia supernova.
When massive stars die, the tremendous heat and pressure of the cataclysm fuse atoms together into new elements. Without supernovae, we’d have nothing in the universe heavier than carbon. And Kramers and his colleagues’ simulations of Type Ia supernovae (a specific kind of supernova that happens in binary star systems when one star is a white dwarf) produced a mix of elements very similar to the chemical makeup of Hypatia.
Why it matters — According to Kramers and his colleagues, sometime before 4.5 billion years ago, gravity drew a tiny bit of supernova-forged dust into a much larger nebula. That nebula would eventually coalesce into our Solar System. In the process, the tiny bit of interstellar dust got stuck to a larger mass of mostly carbon, with a bit of oxygen mixed in.
Somehow, the chunk of rock that contained our interstellar dust sample stayed far out on the fringes, away from the blazing heat of the newly-formed Sun. It became — like many other asteroids and comets out in the Kuiper Belt and the Oort Cloud — a sample of material that originally formed the Solar System.
That’s probably not an unusual origin story — many of the carbon-rich asteroids known as chondrites in our Solar System contain grains of material that once drifted through interstellar space before winding up here. And each of them bears the chemical fingerprint of its own formation, whether in the heart of a star or the in the cataclysm of a supernova. As Kramers and his colleagues put it in their paper, those grains “bear witness to matter originating from many different stellar processes having contributed to the solar nebula.”
What’s unusual is that this particular sample ended up on Earth, and that scientists found it and analyzed it.
“That’s the first actual piece found on Earth which, we assume, was formed before the Solar System was formed,” says Belyanin. And if he and his colleagues are correct, it’s also a piece of a long-ago supernova that scientists here on Earth can now hold in their hands. Eventually, studying Hypatia could help us better understand not only the formation of the Solar System, but the makeup of the interstellar medium and the physics of stellar explosions.
Here’s the background — A geologist picked up the stone in 1996, from an area of the Sahara Desert littered with shards of yellowish glass. The glass, called Libyan Desert glass or impactite, is made of pure silica. Its edges are sharp, and Paleolithic people in the region shaped blades and other tools from it. Egyptian pharaoh Tutankhamen even had a breastplate featuring a scarab carved from the yellowish glass.
But melting and fusing silica into glass takes temperatures much hotter than any volcano on Earth — and there’s also not a volcano anywhere near the Libyan Desert glass fields. The glass that once adorned a pharaoh’s chest was produced by an otherworldly impact: a meteor or comet that struck the Sahara Desert, fusing some of its sand into impactite glass. Geologists have dated the glass itself to around 30,000 years ago, but they haven’t found any definitive trace of the space rock that created it. Unless, of course, that’s what Hypatia is.
Kramers and his colleagues measured the ratios of isotopes of the element argon in Hypatia and found that its signature didn’t match rocks that formed here on Earth. Instead, Hypatia appeared to have come from somewhere in space. That’s when Belyanin took a closer look under a microscope and realized that Hypatia was actually made of two different types of material stuck together. If he and the rest of Kramer’s team are correct, some of that material came from far, far away.
And there may be more interstellar samples waiting to be found – or already in collections, waiting to be analyzed. One of the most widely-accepted theories about the impact that formed the Libyan Desert glass involves a meteor that exploded in midair, creating a thermal shock wave that baked some of the sand beneath into glass. If that’s true, and if Hypatia is a fragment of that exploding meteor (called a bolide), then other fragments may be mingled with the scattered glass across eastern Libya and western Egypt.
Belyanin suggests there may be other meteorites with similar backstories elsewhere on Earth, too.
“The chances of that happening are obviously very, very minimal,” he says, “But considering that the age of the Solar System is about 5 billion years, it might have happened, and there might have been another landing somewhere, much earlier than 30 million years ago. Obviously, if they found something like in the future, that will obviously prove our assumptions, especially if they get something similar in terms of that chemical signature.”