Chassigny

Seen to fall in France in 1815, this olivine-rich stone was the first known Martian meteorite to be witnessed falling.

Key facts

Type

Martian meteorite (chassignite, dunite)

Fell or found

Fell (witnessed)

Year

1815 (3 October)

Country

France

Mass

about 4 kg

Official name

Chassigny

On the morning of 3 October 1815, people near the village of Chassigny, in the Haute-Marne region of north-eastern France, heard explosions and saw a stone fall from the sky. It was collected and found to weigh about 4 kilograms. At the time it seemed simply another of the many meteorites being recorded across Europe in the years after scientists had accepted that stones really do fall from the sky. Nobody could have guessed that it was a piece of Mars, the first Martian meteorite ever seen to fall.

An unusual rock

Chassigny is a dunite, a rock made almost entirely of the mineral olivine, a green magnesium-iron silicate. On Earth, dunites are found deep in the mantle or in layers at the base of large bodies of magma, where olivine crystals that form early sink and pile up. Chassigny formed in a similar way: olivine crystals settled out of a magma on Mars and accumulated into a dense layer. Smaller amounts of other minerals, including pyroxene, feldspar and chromite, fill the spaces between them. Some olivine crystals contain tiny pockets of trapped melt that preserve samples of the magma from which they grew. A few of these pockets contain water-bearing minerals, which scientists have used to estimate how much water the original Martian magma held.

Because Chassigny is so different from other meteorites, it became the type example of its own group: the chassignites. For a very long time it was the only one known. A small number of similar meteorites have since been found in deserts, but chassignites remain among the rarest of all Martian meteorites.

Recognising a Martian origin

For more than 150 years Chassigny was regarded as an unusual achondrite, a stony meteorite formed from melted rock. Like the nakhlites, it crystallised only about 1.3 billion years ago, far too recently for a small asteroid. In the 1970s and 1980s, scientists grouped Chassigny with the Shergotty and Nakhla meteorites as the 'SNC meteorites' and concluded that they must come from a large planet that had remained volcanically active. When gases trapped in a related Antarctic meteorite were found to match the Martian atmosphere measured by the Viking landers, the case for Mars became compelling.

Chassigny has another special feature. The noble gases trapped inside it, especially xenon, differ from those in the Martian atmosphere. Many scientists think they represent gas from the Martian mantle, the deep interior of the planet, giving a rare window into material that never reached the surface.

A family connection

Chassigny and the nakhlites have similar ages, and measurements of the effects of cosmic rays suggest that they were blasted off Mars at about the same time, roughly 11 million years ago. Many scientists think that they came from the same region of Mars, perhaps different layers of the same volcanic system, launched by a single impact.

Why it matters

Chassigny is one of only a handful of Martian meteorites seen to fall, and the earliest. It provides a sample of the Martian interior unlike any other, allowing scientists to study how magmas formed and crystallised on Mars and to measure gases from deep inside the planet. The largest surviving piece is held in Paris, at the National Museum of Natural History, with smaller samples distributed to researchers, who handle them sparingly because so little exists.

It is also a reminder of how long it can take to understand what we have. Chassigny sat in museum drawers for more than a century and a half before anyone realised that it had come from another planet. Who knows what other secrets are waiting in today's collections?

Did you know?

See more

Chassigny in the catalogue