1794

Meteorites Come from Space

Ernst Chladni (1756–1827), Jean-Baptiste Biot (1774–1862)

We take it for granted that rocks sometimes fall from the sky, but for much of human history such an idea was regarded as simply crazy. Many ancient and indigenous cultures were aware of special stones with unique magnetic properties or high concentrations of iron metal, but the fact that these were extraterrestrial samples of Main Belt and near-Earth asteroids was not deduced until the late eighteenth and early nineteenth centuries.

The German physicist Ernst Chladni proposed in 1794 that some of these special rocks, including a large metal-rich sample that he studied called the Pallas Iron, found in 1772 near the Russian city of Krasnoyarsk, were debris from outer space. The idea was dubbed preposterous, as many scientists believed these rocks to be volcanic or produced by lightning strikes. By the early nineteenth century it was possible to make detailed laboratory measurements of these rocks. In 1803, the French physicist and mathematician Jean-Baptiste Biot proved Chladni’s hypothesis by showing that the chemical compositions of rocks found by the thousands near the town of L’Aigle shortly after a spectacular shower of shooting stars were unlike any known Earth rocks. Such rocks became known as meteorites, and the scientific field of meteoritics was born.

Scientists have now collected more than 30,000 meteorites from around the world—many from desolate deserts or Antarctic snow fields, where it is relatively easy to notice an odd rocky interloper that fell from above. The vast majority of rocks that fall to Earth (86 percent) are made of simple silicate minerals and tiny spherical grains called chondrules, thought to be some of the first materials condensed from the Solar Nebula and the building blocks of asteroids and eventually planets. About 8 percent more are silicates without chondrules—samples of igneous rocks from formerly geologically active crusts of large asteroids, the Moon, and Mars—and only about 5 percent are made of iron and iron-nickel (like the rocks originally studied by Chladni and Biot) and are pieces of the cores of ancient, now shattered asteroids and planetesimals that had grown large enough to differentiate into core, mantle, and crust before being destroyed by impacts in the solar system’s violent early history.

A small (14-ounce [408-gram]) ordinary chondrite meteorite that was found in 2008 on the hard pebbly surface of the Rub ‘al-Khali desert near Ash-Sharqīyah, Saudi Arabia. The black surface is a thin fusion crust formed during the rock’s short, fiery passage through Earth’s atmosphere.