Scientists have discovered a tiny fragment of a comet in a meteorite. The bulk of the rock itself was once an asteroid, but when the team cracked it open and analyzed the inside, they found that the growing asteroid must have swept up the seed of a comet billions of years ago.

In a way, asteroids, comets and meteorites are different words for basically the same thing – smallish space rocks. The main distinction is where they form in the solar system. Asteroids are usually born in the inner solar system – from Jupiter inwards – while comets form further out. Thanks to the much colder conditions on the fringes of the solar system, comets contain more water ice and far more carbon than asteroids. Meteorites, meanwhile, are the fragments that survive the fall to Earth, and can be the remains of either comets or asteroids.

What makes this new discovery interesting is that the rock appears to be a bit of both. The meteorite is known as LaPax Icefield 02342 after the region of Antarctica where it was discovered in 2002. From the outside the rock appeared to be a primitive carbonaceous chondrite meteorite, which can stay relatively unchanged for billions of years until they crash-land on Earth. That makes them excellent time capsules.

"Primitive meteorites provide a snapshot of the early solar system that we can study in the lab," says Jemma Davidson, an author of the study. "The LaPaz meteorite is a nice example since it has experienced minimal terrestrial weathering."

For this study, the researchers examined LaPaz slice by slice, shining polarized light through from behind to see what materials make them up. To their surprise, the scientists spotted a small section of primitive material that was very carbon-rich. After examining that material further with chemical and isotopic analyses, the team found that it probably originated out towards the Kuiper belt, the icy realm where comets are born.

The team's theory for how it got there begins just 3.5 million years after the solar system started forming. This seed of a comet, measuring just one-tenth of a millimeter wide, was likely pulled in towards the inner solar system, where it collided with the larger asteroid that was forming just beyond the orbit of Jupiter.

"Because this sample of cometary building block material was swallowed by an asteroid and preserved inside this meteorite, it was protected from the ravages of entering Earth's atmosphere," says Larry Nittler, lead author of the study. "It gave us a peek at material that would not have survived to reach our planet's surface on its own, helping us to understand the early solar system's chemistry."

This discovery is just the latest in a long line of fascinating stories that have been pried out of meteorites. In recent years, space rocks have told scientists about the Sun's rowdy youth, volcanoes on Mars that erupted for 2 billion years straight, large long-lost planets, ancient ocean worlds, and even stones that may have interstellar origins.

The new research was published in the journal Nature Astronomy.

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