Rocks that journey through space and survive a fiery descent through Earth’s atmosphere to land on our planet are known as meteorites. These extraterrestrial objects offer unique insights into the formation and evolution of our solar system. Identifying a meteorite requires careful observation of several distinct features that set them apart from common terrestrial rocks.
External Characteristics
A prominent feature of many meteorites is their fusion crust, a thin, dark, often dull or glassy outer layer. This crust forms as the meteorite’s surface melts and ablates during its high-speed passage through Earth’s atmosphere, then rapidly solidifies upon cooling. The color of the fusion crust can range from black to dark brown, resembling burnt toast or an eggshell. Some meteorites exhibit regmaglypts, shallow, thumbprint-like depressions created by uneven melting during atmospheric travel.
While many meteorites are irregularly shaped, some may display a more aerodynamic or conical form, a result of their atmospheric entry. Despite their varied shapes, meteorites often possess a noticeable heft for their size. This high density is due to their metallic content, making them feel significantly heavier than most ordinary Earth rocks of similar dimensions.
Internal Structure and Composition
When a meteorite is broken or cut open, its internal structure reveals further distinguishing characteristics. Stony meteorites, the most common type, often contain small, spherical mineral grains called chondrules. These tiny beads, typically less than a millimeter, can appear in various colors and are a hallmark of chondritic meteorites, among the solar system’s oldest materials. The presence of chondrules provides a unique texture not found in terrestrial rocks.
Many meteorites, including stony and stony-iron types, also contain visible flecks of shiny metal. These metallic inclusions are composed primarily of iron and nickel, appearing as bright, silvery-gray specks embedded within the rock matrix. In contrast, iron meteorites are almost entirely composed of iron and nickel alloys, and when cut and polished, they often reveal intricate, intersecting crystalline patterns known as Widmanstätten figures. These patterns are unique to iron-nickel alloys that cool very slowly over millions of years in space.
Key Indicators for Identification
Several practical indicators can help distinguish a meteorite from a terrestrial rock, with magnetism being a primary test. Most meteorites contain significant amounts of iron and nickel, causing them to be attracted to a strong magnet. Even stony meteorites, which may not appear metallic externally, often contain enough iron-nickel to be picked up by a magnet.
Another indicator is the object’s density; meteorites typically feel much heavier than common Earth rocks of the same size. Furthermore, the absence of vesicles, which are small holes or bubbles caused by trapped gas, is an important negative indicator. Terrestrial volcanic rocks often contain these gas bubbles, but meteorites, formed in the vacuum of space, rarely do.
Performing a streak test can also provide valuable information. When a meteorite is rubbed across an unglazed ceramic surface, it generally leaves no streak or only a very faint gray or brownish mark. This contrasts with many iron-rich terrestrial rocks, such as hematite or magnetite, which often produce distinct red-brown or black streaks. While these indicators are helpful, definitive identification requires professional analysis by a meteorite expert.