Norton County

A shower of pale, almost iron-free stones over the Kansas–Nebraska border in 1948, including the largest aubrite ever found.

Key facts

Type

Achondrite (aubrite, enstatite-rich)

Fell or found

Fell (witnessed)

Year

1948 (18 February)

Country

United States (Kansas and Nebraska)

Mass

about 1.1 tonnes in total; main mass about 1,070 kg

Official name

Norton County

On the afternoon of 18 February 1948, a huge fireball blazed over the border between Kansas and Nebraska in the central United States. Its brilliance and booms were reported by people across a wide area of the Great Plains. It broke apart, and a shower of stones fell across farmland around Norton County in Kansas and Furnas County in Nebraska. Over the following months many stones were found. One of them was enormous: a mass of about 1,070 kilograms, which had buried itself several metres deep in a field. Altogether around 1.1 tonnes were recovered.

The hunt for the stones

The American meteorite scientist Lincoln LaPaz, director of the Institute of Meteoritics at the University of New Mexico, led the search in cooperation with the University of Nebraska. He collected witness reports to work out the fireball's path and the likely area of the fall, a technique he championed. Local people helped greatly, and many stones were found in fields and pastures. The main mass was dug out of the pit it had made and is now held by the University of New Mexico, where it is one of the highlights of the meteorite collection. Many Norton County stones look more like pale Earth rocks than typical meteorites, a reminder that not every space rock is dark and heavy.

A strange white rock

Norton County stones look very different from most meteorites. Inside, they are pale and largely white, made mainly of large crystals of the mineral enstatite, a magnesium silicate with almost no iron in it. Norton County is an aubrite, a rare type of achondrite, which is a stony meteorite that has melted and crystallised. Aubrites are named after the Aubres meteorite, which fell in France in 1836. They are breccias, rocks made of broken pieces cemented together, and they often contain large, clear crystals.

Aubrites formed in exceptionally reducing conditions, which means there was very little oxygen available. Iron that would normally combine with oxygen and end up in silicate minerals instead stayed as metal or combined with sulphur. As a result, aubrites contain unusual sulphide minerals not normally seen in Earth rocks, including oldhamite, a calcium sulphide that dissolves in water.

Where did it come from?

Aubrites are chemically related to enstatite chondrites, primitive meteorites formed in similar low-oxygen conditions, but aubrites have been melted. Scientists think they come from a parent body that formed in a part of the early Solar System poor in oxygen, perhaps close to the Sun, then heated, melted and separated. Some researchers have linked aubrites with bright asteroids of the E-type, whose surfaces are rich in enstatite, although this connection is still being tested.

Why it matters

Norton County is the largest aubrite known, and by far the best-known example of this rare meteorite type. Its stones have been used to study how rocks behave when they form with almost no oxygen. That research is also relevant to the planet Mercury, whose surface appears to be poor in iron and rich in sulphur, suggesting that it too formed under strongly reducing conditions.

The Norton County fall also shows the value of systematic searching. By collecting eyewitness accounts carefully and plotting the fireball's path, LaPaz and his colleagues were able to recover a large proportion of the fallen material, including the giant main mass, decades before cameras and radar could help with such searches.

Today, specimens of Norton County are held in many collections around the world. Its pale, crystalline interior surprises people who expect meteorites to be dark and metallic, and is a reminder that rocks from space come in a wide variety of forms.

Did you know?

See more

Norton County in the catalogue