A “space rock” refers to a meteorite, which is a fragment of material that originated in outer space and survived its fiery passage through Earth’s atmosphere to land on the ground. Before reaching the surface, the object was a meteoroid traveling through space, and the brilliant streak of light it produced while burning up was a meteor. Understanding the visual characteristics of the surviving rock is the most effective way to identify a true meteorite.
External Features: The Tell-Tale Signs of Entry
The most immediate feature on a newly fallen space rock is the fusion crust, a thin, dark, glassy layer resulting from extreme heating during atmospheric entry. This crust forms as the meteoroid’s outer surface melts and rapidly cools upon descent, creating a distinct coating that is typically black or dark brown. Freshly fallen meteorites may have a shiny, glass-like appearance, sometimes exhibiting fine lines where the molten material flowed across the surface before solidifying.
The intense heat and abrasion during high-velocity flight also sculpt the rock’s exterior into unique shapes. Meteorites have an irregular, blocky form with edges often rounded off due to ablation. This process can create shallow depressions resembling thumbprints pressed into wet clay, which scientists call regmaglypts.
These distinctive, pitted markings are particularly noticeable on iron meteorites, but they can be present on all types of space rocks. Over time, exposure causes the fusion crust to dull, wear away, and often turn a rusty brown or orange color as the iron content begins to oxidize. A terrestrial rock will not display this specific combination of a dark, thin, non-porous crust and thumbprint-like surface features.
What the Interior Reveals
Breaking open a suspected meteorite exposes the interior, revealing its unique extraterrestrial composition and structure. Most space rocks contain flecks of nickel-iron metal, which appear as bright, silvery spots scattered throughout the rock matrix when the surface is cut or polished. This metallic content is a strong indicator of cosmic origin, as metallic iron is not typically found in terrestrial rocks due to weathering and oxidation.
A significant number of stony meteorites, known as chondrites, contain small, spherical grains called chondrules. These tiny, bead-like structures, often about a millimeter in diameter, are mineral droplets that cooled rapidly in the early solar system and are embedded within the rock. Chondrules may appear in different colors and are not found in rocks originating on Earth.
Another characteristic revealed by the interior composition is the rock’s unusual density, which is noticeably greater than most common Earth rocks of similar size. This increased density is directly attributable to the presence of metallic iron and nickel within the structure. While a stony meteorite may be about one and a half times heavier than a terrestrial rock, the pure metal composition of an iron meteorite can make it feel three to four times heavier.
Categorizing Meteorite Appearance
The visual characteristics of space rocks fall broadly into three main categories based on their metal and silicate content. Stony meteorites, which make up the vast majority of recovered specimens, generally resemble ordinary Earth rocks on the outside, often with a weathered, rust-colored exterior. Their identification relies heavily on features like the remnant fusion crust and the internal presence of chondrules and minute metal flakes.
Iron meteorites, conversely, are almost entirely composed of nickel-iron alloy, giving them an exceptionally high density and a pronounced metallic appearance when a surface is exposed. They are frequently characterized by deep regmaglypts on their surface. A cut, polished, and acid-etched surface often displays a distinct, intersecting crystal pattern known as the Widmanstätten structure.
Stony-iron meteorites represent a unique combination, consisting of roughly equal parts metal and silicate minerals. The pallasite subgroup is particularly striking, featuring bright, often olive-green olivine crystals embedded in a continuous network of nickel-iron metal. These specimens are formed at the boundary between the metal core and the silicate mantle of their parent asteroid.
Simple Tests for Identification
A few simple, non-destructive steps can help determine if a suspected rock is a genuine meteorite. The first involves a magnetism check, as the vast majority of space rocks contain enough nickel-iron to be strongly attracted to a common magnet. Iron meteorites will hold a magnet firmly, and even many stony meteorites will show a distinct magnetic pull.
Another helpful step is the streak test, which involves rubbing the specimen across an unglazed ceramic surface, like the back of a tile. Common terrestrial iron ores often mistaken for meteorites, such as hematite or magnetite, will leave a distinct reddish-brown or black streak. A stone meteorite, unless heavily weathered, will typically leave no colored streak or only a very faint gray mark, distinguishing it from these common “meteor-wrongs.”
Finally, the density check is an important clue, as meteorites feel disproportionately heavy for their size due to their high metal content. Comparing the weight of the rock to a similarly sized common terrestrial rock can confirm this unusual density. If a rock passes all three preliminary checks—magnetism, no colored streak, and unusual heaviness—it warrants further professional examination.