Meteorite

A meteorite is a fragment of rock or metal originating in outer space that survives its fiery passage through a planetary atmosphere and lands on the surface of a planet or natural satellite. When the parent object enters the atmosphere, it becomes luminous due to intense heating caused primarily by atmospheric compression. In this stage, it is observed as a meteor, often referred to as a ‘shooting star’. If the object is unusually bright, it may be termed a fireball, while the most intense examples are known as bolides. Once the object reaches the ground, it is classified as a meteorite. Meteorites vary enormously in size, from microscopic particles to massive bodies such as the Hoba meteorite in Namibia, the largest known intact specimen.

Fall Phenomena

Most meteoroids disintegrate in the atmosphere before reaching the surface. Only a small number—typically around five to ten each year—are observed to fall and subsequently recovered; these are known as meteorite falls. All other recovered meteorites are termed finds, as their fall was not witnessed.
Meteorite falls exhibit several well-documented features. As meteoroids enter the atmosphere, heating causes ablation of their surfaces. This ablation may sculpt the exterior into characteristic forms, including shallow depressions known as regmaglypts, resembling thumbprints. If the meteoroid maintains a steady orientation without tumbling, it may acquire a nose-cone shape. As deceleration continues, the molten surface layer cools rapidly to form a thin fusion crust, typically black on stony meteorites and lighter in colour on some achondrites.
The thermal alteration caused by atmospheric entry is generally confined to a thin outer layer, only a few millimetres deep on stony meteorites. In iron meteorites, which possess higher thermal conductivity, heat effects may penetrate deeper but remain limited. Reports from observed falls vary: some meteorites are described as warm or hot on landing, while others appear cold enough to condense moisture and form frost.
When a meteoroid fragments during entry, its pieces may fall as a meteorite shower, producing many individual stones over a wide area called a strewn field. Strewn fields are typically elliptical, with the largest fragments found furthest downrange.
Large meteoroids behave differently. Iron-rich meteoroids are disproportionately responsible for the majority of hypervelocity impact craters, as they are more resistant to disruption. Famous examples include Barringer Crater in Arizona, the Odessa Crater, and Wolfe Creek Crater. By contrast, stony or icy bodies—even those weighing millions of tonnes—often fragment entirely in the atmosphere, producing airburst events such as the Tunguska explosion of 1908. Very large stony impactors, however, can reach the ground and generate extensive craters, although such events are extremely rare.

Classification

Meteorites are traditionally divided into three broad types: stony meteorites, iron meteorites, and stony–iron meteorites. Modern classification systems refine these groups using chemical composition, isotopic signatures, structure, and mineralogy.

• Stony meteorites form the majority of meteorite finds and are subdivided into chondrites and achondrites.
  • Chondrites, comprising about 86% of meteorites, contain small spherical particles known as chondrules. These silicate-rich structures appear to have been melted while free-floating in the early Solar System. Many chondrites also contain organic compounds and presolar grains. They are some of the oldest known materials, with ages around 4.55 billion years, and represent primitive building blocks of the planets.
  • Achondrites, forming about 8% of meteorites, lack chondrules and often resemble terrestrial igneous rocks. Most come from differentiated parent bodies in the asteroid belt, although two notable groups originate from the Moon and Mars. Martian meteorites remain the only physical samples of another planet naturally delivered to Earth.
• Iron meteorites, making up roughly 5% of witnessed falls, consist predominantly of iron–nickel alloys such as kamacite and taenite. They are thought to derive from the metallic cores of once-molten planetesimals. After these bodies cooled and later collided with other objects, fragments were ejected and eventually reached Earth. Iron meteorites are over-represented among finds—especially in regions like Antarctica—due to their distinctive appearance and resistance to weathering.
• Stony–iron meteorites, about 1% of meteorites, are mixtures of silicate minerals and metallic alloys.
  • Pallasites likely originate from the boundary layer between the core and mantle of differentiated parent bodies.
  • Mesosiderites are brecciated mixtures produced by high-energy impacts involving differentiated asteroids.

Meteorites smaller than 1 mm are classified as micrometeorites, which usually melt completely and reach Earth as quenched droplets rather than solid stones.

Impact Events and Crater Formation

Most meteorites reach the ground at their terminal velocity, too slow to form substantial craters. Large meteoroids that retain a significant fraction of their extraterrestrial velocity create hypervelocity craters, the form and size of which depend on the impactor’s mass, composition, fragmentation behaviour, and angle of entry. Such events can release tremendous energy and pose significant hazards.
Iron meteoroids are most likely to survive atmospheric passage intact and create craters, whereas stony bodies often fragment before impact. The Morokweng impact structure in South Africa, identified in 2006, provided the first example of stony meteorite fragments found associated with a large impact crater.

Tektites

Tektites are natural glass objects formed not by direct meteoritic arrival but by the melting and rapid cooling of Earth materials following large meteorite impacts. Although early theories proposed a lunar volcanic origin, most scientists now interpret tektites as terrestrial material ejected and molten during major impacts.

Frequency of Meteorite Impacts

The frequency at which Earth encounters impactors follows a power-law distribution across a range of sizes. Small objects strike daily, while larger impactors are progressively rarer. The largest impactor likely to strike Earth on any given day would measure only a few centimetres; on an annual timescale, objects of several tens of centimetres may be expected, and on a century scale, impactors a few metres across are plausible. These statistical estimates derive from observed meteor frequencies and their size distributions.
Most meteorites originate from a small number of asteroid fragmentation events. Studies indicate that many can be traced to specific parent bodies or collisional families within the asteroid belt.
Meteorites provide unique insights into the earliest conditions of the Solar System. As ancient, relatively unaltered fragments of planetary formation, they offer direct evidence of the composition, structure, and evolution of primordial bodies, making them invaluable to planetary science and cosmochemistry.