Vaca Muerta

A mesosiderite scattered across the Atacama Desert, named after a 'dead cow' and found by prospectors in the 19th century.

Key facts

Type

Stony-iron, mesosiderite

Fell or found

Found

Year

1861

Country

Chile

Mass

nearly 4 tonnes, in many fragments

Official name

Vaca Muerta

In 1861, prospectors searching for minerals in the Atacama Desert of northern Chile came across unusual heavy rocks, with metal glinting among their stony parts. They were meteorites. The area where they were found takes its name from the Sierra de Vaca Muerta, the 'Dead Cow' range, and the meteorite became known as Vaca Muerta. Over the following decades, and especially since the late 20th century, many more pieces were found, making it one of the largest known mesosiderites.

Desert conditions

The Atacama is one of the driest places on Earth. Some parts receive almost no rain for years at a time. That makes it an excellent place for meteorites to survive, because the lack of water slows the rusting of metal and the breakdown of minerals. Even so, many Vaca Muerta pieces have weathered surfaces, with rusty brown coatings on their metal.

The meteorite was scattered over a large strewn field, the area where fragments of a single meteoroid land. Pieces of all sizes, from small fragments to masses of tens of kilograms, have been recovered. Its total known mass is nearly 4 tonnes, making it one of the largest mesosiderites known.

What is a mesosiderite?

Vaca Muerta is a mesosiderite, a stony-iron meteorite made of roughly equal amounts of metal and rock. The name comes from Greek words meaning 'half iron'. In mesosiderites, metal and rock are jumbled together in an irregular breccia, a rock made of broken fragments cemented together. The rocky parts consist mainly of pyroxene and plagioclase feldspar, resembling the basaltic rocks of the HED meteorites from the asteroid Vesta, while the metal resembles material from an asteroid's core.

When a slice of Vaca Muerta is cut and polished, it shows a striking mixture of bright metal and dark grey to greenish stone. Some slices contain larger fragments of basaltic or pyroxene-rich rock, giving clues to the different rocks that were mixed together.

How did it form?

Scientists think that mesosiderites formed when molten or hot metal from the core of an asteroid was mixed with crustal rocks during a violent collision. One idea is that a large impact disrupted a melted asteroid and the debris then re-accumulated, so that core and crust material ended up mixed together. Afterwards, the mixture cooled extremely slowly, perhaps buried deep within a large body. Studies of the metal suggest cooling rates among the slowest known for any meteorite. Exactly how and where this happened is still debated.

Researchers also use the ages of minerals in mesosiderites to date these events. They suggest that the main mixing happened very early in the history of the Solar System.

Why it matters

Vaca Muerta is one of the largest and best-studied mesosiderites. Its abundance means there is plenty of material for research, and slices are held in collections around the world. By studying it, scientists can explore one of the most puzzling questions about asteroids: how material from deep cores and surface crusts could have been brought together.

It also illustrates the importance of hot deserts for meteorite discovery. The Atacama has become one of the world's most productive regions for finding meteorites, and systematic searches there continue. Chile has laws regulating meteorite collection and export, and anyone exploring the desert must follow them. The Atacama can also be dangerous, with extreme dryness, strong sun and remote terrain, so careful preparation is essential.

When you look at a slice of Vaca Muerta, think of it as a frozen moment from a giant collision, when metal and rock from different parts of an asteroid were thrown together and then left to cool for millions of years.

Did you know?

See more

Vaca Muerta in the catalogue