A meteorite is an extraterrestrial rock that survives its fiery passage through Earth’s atmosphere and lands on the surface. Distinguishing these space visitors from the vast array of terrestrial rocks, often called “meteor-wrongs,” can be challenging due to Earth’s diverse geology. Identification relies on a combination of external markings left by atmospheric entry, unique internal composition, and physical properties uncommon in Earth-surface rocks. These three categories of evidence confirm the rock’s non-terrestrial origin.
External Indicators: Fusion Crust and Regmaglypts
The most immediate indicators of a meteorite are the surface features generated by intense heat during entry. The friction and compression of air as the rock plummets through the atmosphere cause its outer layer to melt, a process known as ablation. This results in the formation of a thin, dark, glassy coating called a fusion crust.
A freshly fallen meteorite displays a rich black or dark brown fusion crust, often likened to the appearance of an eggshell. This crust is usually less than a millimeter thick on stony meteorites, representing the rapid cooling of the molten surface material. Over time, exposure to weather causes the iron within the rock to rust, turning the fusion crust a reddish-brown color.
Another unique external feature is the presence of regmaglypts, which appear as shallow, thumbprint-like indentations on the surface. These marks are sculpted into the rock as the molten material is stripped away by atmospheric turbulence and pressure during the rapid descent. Regmaglypts are a strong indicator of an extraterrestrial origin, as Earth rocks do not possess this feature.
Compositional Signatures: Iron, Nickel, and Magnetism
The chemical makeup of a meteorite provides a definitive signature rarely duplicated in naturally occurring terrestrial rocks. The vast majority of meteorites contain concentrations of native iron and nickel metal. This metal exists in its unoxidized, or free, state, unlike the iron in most Earth-surface rocks which is typically bound up in minerals like hematite or magnetite.
The nickel content is particularly telling, as meteoritic iron-nickel alloys contain 5 to 30 percent nickel, a concentration significantly higher than that found in most terrestrial iron. The presence of this metal alloy, often dispersed as tiny metallic flecks throughout the rock matrix, allows for a simple magnetic test. The high iron and nickel content causes most meteorites, including the common stony varieties, to be strongly attracted to a magnet.
While some Earth rocks, such as those rich in magnetite, are also magnetic, they lack the characteristic metal flecks and the high nickel composition of a meteorite. The native metal in meteorites exists as the alloys kamacite and taenite, which are not naturally found in Earth rocks, further confirming the extraterrestrial nature of the sample.
Internal Structure and Density
Beyond the surface and bulk composition, the internal structure and weight-to-volume ratio of a rock offer further means of identification. Meteorites often feel significantly heavier than a similarly sized Earth rock due to their concentration of dense metal. For instance, iron meteorites have a high density (typically 7 to 8 grams per cubic centimeter), and even the most common stony meteorites (chondrites) are usually denser than common terrestrial rocks like granite or limestone.
A distinctive internal feature, visible when the rock is broken or cut open, is the presence of chondrules. These are minuscule, grain-like spheroids, typically ranging from 0.1 to 2 millimeters in diameter, that are packed within the matrix of chondrites, the most abundant type of meteorite. Chondrules are solidified droplets of silicate minerals that formed in the early solar system, making them approximately 4.55 billion years old.
Because chondrules represent a primitive material that predates the formation of planets, they are completely absent from any igneous, metamorphic, or sedimentary rock found on Earth. Conversely, a true meteorite will typically lack common terrestrial features such as quartz crystals or vesicles, which are small holes formed by gas bubbles in volcanic rocks.