A meteoroid is a small piece of rock or iron traveling through space, typically ranging in size from a grain of dust to a few meters in diameter. When this space rock enters Earth’s atmosphere at high speed, friction causes it to burn up, creating a bright streak of light known as a meteor, or “shooting star.” A meteorite is the solid fragment of the original meteoroid that survives atmospheric passage and lands on the planet’s surface. Identifying a true meteorite among Earth rocks requires a systematic process, moving from initial visual cues to physical tests and finally to examining internal structures.
External Clues for Identification
The intense heat generated during atmospheric entry leaves specific, observable features on a meteorite’s exterior. The most distinct is the fusion crust, a thin, dark, and often glassy layer resulting from the rock’s outer surface melting and rapidly solidifying. This crust is typically black or dark charcoal gray on a freshly fallen specimen, sometimes exhibiting fine flow lines. While the fusion crust will weather to a rusty brown color over time due to iron oxidation, its initial appearance is unlike any naturally occurring terrestrial rock.
Many meteorites also display shallow, thumbprint-like indentations known as regmaglypts. These depressions are formed by the ablation process, where molten surface material is unevenly scoured away by turbulent air currents during descent. Terrestrial rocks, shaped primarily by erosion, do not develop this characteristic sculpted appearance. A meteorite’s overall shape is usually irregular, often with rounded or blunted edges, contrasting with the sharper, fragmented look of many common Earth rocks.
Testing Density and Magnetic Properties
A simple test involves assessing the specimen’s density. Most meteorites contain significant amounts of iron-nickel metal, making them noticeably denser than the majority of common terrestrial rocks like granite or quartz. Iron meteorites, for example, typically have a density of 7 to 8 grams per cubic centimeter, feeling about three times heavier than a similarly sized Earth rock. Stony meteorites, the most common type, are less dense than iron types but still fall in the range of 3.0 to 3.7 grams per cubic centimeter, which is heavier than many silicates found on Earth.
The presence of iron-nickel metal also provides a reliable magnetic test. Over 95% of all meteorites contain enough of this alloy to be attracted to a common ceramic magnet. Iron meteorites exhibit a strong magnetic pull, while stony meteorites may show a weak attraction, testable by suspending a lightweight magnet near the rock. While a few Earth rocks, such as those containing magnetite, are also magnetic, the combination of high density and magnetic attraction strongly suggests an extraterrestrial origin.
Examining Internal Composition
Identification often requires examining the rock’s interior, typically revealed by a fresh break or cut surface. The vast majority of meteorites, including common stony chondrites, show small, shiny flecks of iron-nickel metal scattered throughout the rock matrix. This metallic component has a much higher concentration of nickel (5–30%) than industrial or naturally occurring Earth iron, providing a unique chemical signature.
The internal structure of chondrites, the most common type of space rock, is defined by the presence of chondrules. These tiny, spherical, grain-like particles are composed of high-temperature silicate minerals, making their presence indicative of a meteorite. Another internal clue is the general absence of vesicles, which are small holes or bubbles common in volcanic terrestrial rocks like basalt or scoria but rare in meteorites. Unlike terrestrial look-alikes such as hematite, a meteorite will generally not leave a colored streak when rubbed across an unglazed ceramic surface.