Rock of ages: pristine sample from beyond Neptune Michael Holly; Creative Services; University of Alberta.

A fireball that streaked through the sky over a decade ago may have brought the first meteorite from the outskirts of the solar system.

Most meteorites found on Earth are thought to start out in the asteroid belt, which lies between Mars and Jupiter, but the makeup of the Tagish Lake meteorite, which fell on an icy lake in Canada’s British Columbia in 2000, bears little resemblance to other space rocks.

That might be because it formed much further out in the Kuiper belt, the ring of icy bodies beyond Neptune that has Pluto as its most famous member. NASA recently approved an extension of the New Horizons mission to visit a Kuiper-belt object called 2014 MU69, so having a sample of similar material on Earth could help us understand how this region of the solar system formed.


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Bill Bottke of the Southwest Research Institute in Boulder, Colorado, and his colleagues suggest that in the early solar system the shifting of the giant planets – Jupiter, Saturn, Uranus and Neptune – may have hurled debris from the outskirts into the asteroid belt. And some rocks, including the Tagish Lake meteorite, might then have travelled onwards to Earth.

Giant planets

“For a short time in the solar system’s history, you have giant planets encountering one another while surrounded by this big sea of comets,” Bottke says. The team studied how this dance of giants may have hurled smaller objects inward, including asteroids born from comets. “Things were very dramatic for a short time.”

One theory of the early solar system suggests there was once a fifth giant planet, later ejected from our cosmic neighbourhood. The team suggest that the gravitational pull of this fifth giant could have helped seed the asteroid belt with primitive objects from the Kuiper belt.

That seeding could explain the origins of the Tagish Lake meteorite, which previous studies suggested might be a small class of objects known as D-type asteroids. Only a handful of the unusual class of rock lurks in the asteroid belt, though the dark samples are abundant around gas giants.

Samples from the Kuiper belt could provide clues to how planets formed, something that could help solve the mystery of where Earth’s water came from, Bottke says. Most researchers think it was deposited on the planet from elsewhere in the solar system, but they’re still not sure what brought it.

“[Their research is] pretty convincing and it makes a lot of sense that those objects are implanted from this distant region which was the initial Kuiper belt,” says Pierre Vernazza at the Marseille Observatory in France. He isn’t quite convinced that surface observations of D-type asteroids match them with Tagish Lake meteorite, though he tentatively suggests the object could have come from the heart of one of the rare rocks. “For now, we don’t have the observations to establish that it is possible.”

Bottke remains confident, pointing out that the five-planet model still works if the meteorite comes from either the surface or the heart of the unusual asteroids. Either way, the object that crashed on to Tagish Lake may provide the first direct sample from the outermost edges of the solar system to help us understand its early history.

Journal reference: The Astronomical Journal, DOI: 10.3847/0004-6256/152/2/39