METEORITES

A meteorite is a natural solid body of extraterrestrial origin, fallen onto the surface of the Earth. Meteorites can be of varying sizes. Whatever its origin, it has to traverse the atmosphere of our planet at high speed, which gives rise to a brief luminous phenomenon, called a meteor. The fall of a meteorite is a rare but impressive show. As the meteoroid (the initial arriving cosmic body) reaches the top of the atmosphere, friction against the air decelerates it and the conversion of kinetic energy to heat can raise the temperature of the bolide surface and of the surrounding air to several thousand. Once the meteor enters the earth’s atmosphere, it is termed ‘meteorite’.

The characteristics of a fall depend on the velocity and mass of the meteoroid, the orientation of its trajectory, and the density and strength of its material. The lower the mass, the more efficient the air drag. Small particles, in the millimeter size range, are stopped in the upper atmosphere and the corresponding meteors do not last more than a fraction of a second: these are the familiar ‘shooting stars’. Larger masses penetrate deeper into the atmosphere and make a longer and more spectacular show, in particular in the case of meteorite-producing events. The initial velocity in this case is generally about 15-20 km/s. A fireball appears at a height of about 100 km. The outermost part of the bolide matter is melted, vaporized and ejected, leaving a trail of vapor, dust and ionized atoms. The mass of the meteoroid thus decreases continuously, a process known as ablation.

Depending on their structure and composition, meteorites are classified into different classes and groups. The traditional classification sorted meteorites into stony, iron and stony-iron meteorites. Although this purely descriptive classification can still be convenient, it is now preferred to make the major separation between differentiated and non-differentiated meteorites. Non-differentiated meteorites—also called Chondrites - have not been melted in the tiny planets they come from, so that they keep memories of the materials of the solar nebula, the gas and dust cloud from which the solar system was born. They can thus be considered as witnesses of the birth of the solar system. Differentiated meteorites are very diverse, but they have all experienced extensive melting on their parent planets. They have lost to a large extent memories of nebular processes, but are useful to study planetary processes, in particular those which took place on small planets in the early solar system.

As of now, there are more than 1000 specimens of meteorites in various places in the earth. Of these, twenty-six meteorites are widely believed to be samples of the planet Mars. Around twenty-one others come from the Moon, from regions that have not been sampled by the Apollo nor the Luna missions, including probably the far side. The overwhelming majority of the meteorites are fragments of asteroids, the tiny planets that orbit the Sun between Mars and Jupiter. Meteorites have allowed us to study the geology and past evolution of these very diverse small bodies for many decades, while their exploration by space probes is only at its very beginning. They give us the opportunity to study worlds that we have not reached yet.

But the interest in meteorites is more general: because of their small sizes, many of their parent asteroids did not experience the geological processes which have reprocessed all the materials of the large planets like Earth, Mars, or even the Moon. Although some of them have also gone through significant geological reprocessing, most meteorites are thus samples of ‘primitive matter’ which keep memories of the sequence of physio-chemical processes which gave birth to the Solar System, 4.5 billion years ago. Meteorites have allowed us to study the composition and structure of the materials from which the planets were made, and to establish the age of the solar system.

In 1987, rare and tiny crystals were isolated from meteorites, which were made in the environment of other stars, before the birth of the Sun, giving us direct access to the nuclear processes which take place in stars and lead to the synthesis of the chemical elements.

Questions to Ponder

• What is the typical size of a visible shooting star?
• How can meteorites tell us about the origin and formation of the Solar System?