A visitor from interstellar space
In 1907, a meteorite fell near the town of Camocim in the state of Ceará, Brazil. Unlike the much larger Tunguska event which occurred in Siberia the following year, the Camocim event was a witnessed fall that left behind physical fragments. This space rock is classified as a chondrite, a type of stony meteorite that contains small, spherical mineral grains called chondrules. Chondrites are considered primitive materials, leftovers from the formation of the solar system approximately 4.6 billion years ago. They are built from the dust and grains that existed in the early solar nebula. The Camocim meteorite contains something even older.
Scientific analysis of the fragments revealed the presence of presolar grains. These are tiny, solid particles of minerals like silicon carbide, graphite, and even nanodiamonds that formed before our own Solar System existed. These grains are incredibly rare, making up only a tiny fraction of a meteorite's mass. Their isotopic compositions are very different from materials that formed in our solar system, which allows scientists to identify their ancient, extrasolar origin. These specks of dust condensed in the cooling outflows of long-dead stars, such as red giants or supernovae. They then drifted through interstellar space for millions or billions of years before being caught up in the molecular cloud that would eventually collapse to form our Sun and its planets.
Evidence of the galaxy's past
The presolar grains within the Camocim meteorite record ancient history. Some silicon carbide grains found in other meteorites have been dated to be as old as seven billion years, predating the Sun by over two billion years. Studying these ancient materials provides direct evidence of the types of stars that existed in our galactic neighborhood before the Sun was born. The specific isotopic signatures of elements within each grain can tell researchers about the nuclear processes that occurred inside its parent star.
The Camocim meteorite is classified as an ordinary chondrite. Chondrites are the most common type of meteorite that falls to Earth, accounting for the vast majority of all witnessed falls. They are distinguished from achondrites, which are stony meteorites that lack chondrules and often originate from larger parent bodies like asteroids that underwent melting and differentiation. The main mass of the Camocim meteorite is held in the collection of the National Museum of Brazil in Rio de Janeiro, which also houses the famous Bendegó meteorite, the largest ever found in the country.