Iron meteorites are pieces of extraterrestrial metal that have survived the journey to Earth’s surface. They are predominantly composed of an iron-nickel alloy, with nickel content typically ranging from 5% to 25% of the metallic mass. These meteorites represent only about 5% of all witnessed falls, making them relatively rare compared to their stony counterparts. However, they are often over-represented in collections because their metallic composition makes them highly resistant to weathering and easy to distinguish from terrestrial rocks.
Parent Bodies: Cores of Differentiated Planetesimals
Iron meteorites originate from the metallic cores of ancient, small planetary bodies known as planetesimals. These were the building blocks of planets that formed in the inner Solar System within the first few million years of its history. Most of these parent bodies were relatively small, likely ranging from a few tens to a few hundred kilometers in diameter.
These parent bodies were primarily located within the main asteroid belt. These planetesimals underwent differentiation, a process that created the distinct layers necessary to form a metallic core. The iron meteorites we find today are samples of that ancient core material. Scientists have identified evidence for at least 50 distinct parent bodies based on the chemical signatures found in the recovered meteorites.
The Mechanism of Core Formation
The process that created these metallic cores is known as planetary differentiation, driven by intense internal heating shortly after the planetesimals accreted. The primary heat source was the radioactive decay of short-lived isotopes, most notably Aluminum-26 (Al-26) and Iron-60 (Fe-60). These radioactive elements were abundant in the early Solar System and released significant thermal energy during their rapid decay.
This heat caused the planetesimals to melt substantially, creating a molten interior. The materials within the molten body separated based on their density. The heavy, denser elements, primarily iron and nickel, sank toward the center, forming the liquid metal core. Lighter silicate materials floated upward to form a surrounding mantle and crust. This entire process of core formation was remarkably fast, occurring within the first 1 to 4 million years after the formation of the Solar System’s earliest solids.
The Journey to Earth
For the core material to become an iron meteorite, the parent planetesimal had to be violently dismantled. This occurred through catastrophic impacts, where the differentiated body collided with another large asteroid or planetesimal. These massive collisions shattered the parent body, exposing the metallic core material to space.
The resulting fragments continued their journey until their paths were altered. Gravitational perturbations, often caused by Jupiter or by passing through specific orbital resonances, nudged the fragments. This orbital shifting gradually altered their trajectories until they crossed Earth’s orbital path. Iron fragments are particularly successful at surviving atmospheric entry because the pure metal is much stronger and more durable than stony material.
Reading the Evidence: Classification and Structure
The internal structure of iron meteorites provides the most compelling evidence of their origin deep inside a differentiated body. When a slice of an iron meteorite is polished and etched with acid, it reveals a crystalline pattern known as the Widmanstätten pattern. This striking geometric arrangement is an intergrowth of two distinct iron-nickel minerals: the nickel-poor kamacite and the nickel-rich taenite.
This pattern is a direct result of extremely slow cooling, on the order of one degree Celsius per million years. Such slow cooling could only happen if the metal was insulated by a large, rocky mantle. This process allowed the two metal phases to crystallize and separate into distinct, interlocking bands over tens of millions of years. Scientists classify iron meteorites into chemical groups, such as IIIAB or IVA, by analyzing trace elements like gallium, germanium, and iridium. These chemical distinctions allow researchers to link different meteorite finds back to a common parent body.