Sutter's Mill

In 2012 a rare carbon-rich meteorite fell on the very spot where the California Gold Rush began – and weather radar helped find it.

Key facts

Type

Carbonaceous chondrite (CM2, breccia)

Fell or found

Fell (witnessed)

Year

2012 (22 April)

Country

United States (California)

Mass

about 1 kg recovered

Official name

Sutter's Mill

Just before eight o'clock on the morning of 22 April 2012, a brilliant fireball streaked across the sky over California and Nevada. A loud sonic boom rattled windows and startled people across a wide area. The object had broken apart high above the foothills of the Sierra Nevada, and its fragments came down around the small town of Coloma. That location gave the meteorite a remarkable name, because Coloma is the site of Sutter's Mill, where the discovery of gold in January 1848 sparked the California Gold Rush.

Radar to the rescue

Finding small meteorites after a fall is difficult, especially in wooded, hilly country. This time, scientists had an unusual helper. Marc Fries, a planetary scientist, examined data from the network of Doppler weather radars in the United States. These radars, designed to watch rain and storms, had detected clouds of fragments falling through the air after the fireball. The radar echoes pointed searchers to where the pieces were likely to land.

Within days, local people and visiting scientists began finding small black stones on roads, in fields and in a car park. Some were found just before heavy rain, which was lucky, because this type of meteorite can be damaged quickly by water. Altogether, around a kilogram of meteorite was recovered in several dozen pieces.

A fast and fragile visitor

Scientists, including the meteor astronomer Peter Jenniskens, who coordinated a large research consortium, used videos and eyewitness reports to reconstruct the fireball's path and orbit. The meteoroid had hit the atmosphere at about 28 kilometres per second, one of the fastest entry speeds measured for a recovered meteorite. It was estimated to have been a few metres across and to have weighed tens of tonnes before it entered the atmosphere. Only a tiny fraction survived to reach the ground.

What it is

Sutter's Mill is a CM carbonaceous chondrite, the same group as the famous Murchison meteorite. CM chondrites are rich in clays and other minerals formed by water inside their parent asteroid, and they contain a few per cent of carbon, including organic compounds such as amino acids. Sutter's Mill turned out to be a breccia, a rock made of many fragments of different kinds stuck together, including pieces that had been heated after their alteration by water. It also contains oldhamite, a calcium sulphide mineral that is very unusual in this type of meteorite. Its mix of materials shows that its parent body was battered and mixed by collisions.

Because the first pieces were collected so quickly, they allowed scientists to study delicate organic compounds and minerals before Earth's environment had much chance to change them. Later finds, made after rain, showed how quickly such material can be altered.

Why it matters

Sutter's Mill showed that weather radar can be a powerful tool for finding fresh meteorites, a technique that has since helped with other falls. It also gave scientists rare, fresh samples of a carbonaceous chondrite at a time when spacecraft were being prepared to visit carbon-rich asteroids and bring samples home.

It also shows how much the public can contribute. Many fragments were found by local residents and amateur hunters who reported their finds. If you ever search after a fall, always ask permission from landowners, avoid protected land, and handle any finds as little as possible, ideally with clean foil or a bag rather than bare hands.

There is a pleasing symmetry in the story: a place famous for a rush to find gold became the site of a rush to find something even rarer, pieces of an asteroid that had travelled for millions of years to reach the Californian hills.

Did you know?

See more

Sutter's Mill in the catalogue