A meteorite is a piece of rock or metal from space that survives its fiery journey through Earth’s atmosphere and lands on the surface. These extraterrestrial objects offer unique insights into the solar system’s formation and composition. Identifying a true meteorite can be challenging, but understanding its distinct visual characteristics, both external and internal, provides essential clues for differentiation from common terrestrial rocks.
External Visual Hallmarks
When a meteorite plunges through Earth’s atmosphere, it experiences extreme heat, causing its surface to melt. This process forms an outer layer known as a fusion crust. A fresh fusion crust typically appears black or dark brown, often with a dull or slightly glassy texture, contrasting with the lighter interior. This thin rind can be smooth, bubbly, or show tiny flow lines where molten material streamed across the surface.
Meteorites often exhibit irregular, angular, or rounded shapes, though rarely perfectly symmetrical. Aerodynamic forces during atmospheric entry can sometimes sculpt them into cone-like or shield-like forms. Regmaglypts are another external feature: shallow, thumbprint-like depressions on the surface. These indentations form as hot gases ablate the meteorite’s surface during its rapid descent.
Some meteorites have a smooth exterior, while others are rough or pitted where the fusion crust has flaked away. A tactile clue is their weight. Due to their metallic content, meteorites often feel denser and heavier than ordinary Earth rocks of similar size.
Internal Appearance and Composition
Breaking open or cutting a meteorite reveals its internal structure and composition, important for confirming its identity. Many stony meteorites, called chondrites, contain tiny, spherical mineral grains known as chondrules. These chondrules resemble small beads or pebbles embedded within the rock’s matrix and are considered primordial material from the early solar system.
Most stony meteorites, particularly chondrites, also contain shiny, silvery flecks of nickel-iron metal. The presence of these metallic grains is an important indicator, as metallic iron is rarely found in terrestrial rocks. When cut and polished, iron meteorites, largely composed of nickel-iron metal, typically display a solid, silvery-grey interior. Etching their surface with acid can reveal criss-cross patterns called Widmanstätten patterns, a feature of these metallic meteorites.
Stony-iron meteorites represent a blend of metallic nickel-iron and silicate minerals. Pallasites, a type of stony-iron meteorite, are striking when cut, featuring colorful, often translucent olivine crystals embedded within a continuous network of metallic iron. This combination of rock and metal creates a visually distinct internal appearance.
Differentiating from Earth Rocks
Distinguishing a meteorite from common Earth rocks, often termed “meteor-wrongs,” requires careful observation. Many terrestrial rocks, such as industrial slag, magnetite, or dark volcanic rocks, are frequently mistaken for meteorites. Magnetism is a key test; most meteorites, particularly iron meteorites and stony meteorites with metallic flecks, are magnetic due to their iron-nickel content. Testing with a strong magnet can provide an initial indication, though some Earth rocks are also magnetic.
Density is another test. Meteorites are generally denser than typical Earth rocks of comparable size. Lifting a suspected meteorite and comparing its weight to a common rock of similar dimensions can highlight this difference. An important distinguishing characteristic is the absence of vesicles, which are gas bubbles or holes. Unlike many volcanic rocks that appear porous or sponge-like due to trapped gases, meteorites rarely contain such internal cavities.
Performing a streak test can also be helpful. When rubbed on unglazed porcelain, meteorites typically leave no streak or only a very faint grey mark. This contrasts with many iron-rich Earth rocks, which often produce a reddish-brown streak. Common Earth minerals like quartz are seldom found in meteorites. Observing these combined features helps in the identification process.