Allende

The meteorite that fell on a Mexican village months before Apollo 11 – and turned out to hold the oldest solids in the Solar System.

Key facts

Type

Carbonaceous chondrite (CV3)

Fell or found

Fell (witnessed)

Year

1969 (8 February)

Country

Mexico

Mass

more than 2 tonnes recovered

Official name

Allende

Shortly after one o'clock in the morning on 8 February 1969, a brilliant fireball lit up the sky over the state of Chihuahua in northern Mexico. It broke apart with a series of booms and showered stones across a long stretch of desert near the village of Pueblito de Allende. By morning, people were picking up black-crusted rocks from fields, roads and rooftops. Over the following years more than two tonnes were collected from a strewn field more than 50 kilometres long, making Allende the largest carbonaceous chondrite ever found.

Perfect timing

Allende could hardly have arrived at a better moment. In 1969 laboratories around the world were preparing to receive the first samples of Moon rock from the Apollo missions, and many scientists had new instruments and clean rooms ready and waiting. Scientists from the Smithsonian Institution in Washington were among the first to reach the area, working with local people to gather stones; the largest single stone weighed about 110 kilograms. Allende gave laboratories a large supply of fresh, primitive extraterrestrial material to practise on. Some of the most important discoveries in planetary science came from that coincidence.

What is a carbonaceous chondrite?

A chondrite is a stony meteorite that has never been melted since it formed. Most contain chondrules, small rounded grains, often about a millimetre across, that were once droplets of molten rock floating in the cloud of gas and dust from which the planets formed. Carbonaceous chondrites are a family of chondrites that are relatively rich in carbon and other volatile substances. Allende belongs to the CV group, and the number 3 in CV3 means it has been only slightly altered by heat or water since it formed.

Break open a piece of Allende and you see a dark grey matrix crowded with chondrules and, most strikingly, irregular white or pinkish patches. These are calcium-aluminium-rich inclusions, usually shortened to CAIs.

The oldest solids

CAIs are made of minerals that would be among the first to condense from a cooling cloud of hot gas, and they turned out to be extraordinarily old. Careful measurements of radioactive uranium and lead in CAIs from Allende and similar meteorites give an age of about 4.567 billion years. They are the oldest solid objects ever dated, and their age is used as the starting point, or 'time zero', of the Solar System.

In the 1970s, scientists working on Allende's CAIs found evidence that they had once contained aluminium-26, a radioactive form of aluminium that decays away within a few million years. Its presence meant that freshly made radioactive elements had been mixed into the young Solar System. One explanation is that a nearby exploding star, a supernova, or a massive star's winds seeded our birth cloud. Aluminium-26 also provided a powerful source of heat that helped melt early asteroids.

A treasure chest of minerals

Allende has been studied more than almost any other meteorite. It has yielded several minerals entirely new to science, including panguite, a titanium-rich oxide described in 2012 and named after Pangu, a figure from Chinese mythology who separated the sky from the Earth. Tiny grains in Allende record chemical conditions in the early solar nebula that no rock on Earth has preserved.

Why it matters

Allende made the early Solar System something scientists could hold in their hands. Thanks to its abundance, specimens are found in museums, universities and private collections all over the world, and it remains one of the most widely used meteorites for teaching. Every time you hear that the Solar System is about 4.57 billion years old, a large share of the credit belongs to the white flecks inside a stone that fell on a Mexican village one February night.

Did you know?

See more

Allende in the catalogue