CORE: IRON FROM ASTEROIDS

IRON METEORITES ARE SHATTERED FRAGMENTS OF THE METAL CORES OF DIFFERENTIATED ASTEROIDS.

METAL METEORITES

Most "iron" meteorites are iron-nickel alloy with a few scattered inclusions of sulfide minerals. The alloys are 5 to 12 percent nickel, with traces of cobalt, chromium, gold, platinum, iridium, tungsten and other elements that dissolved in the molten iron and traveled with it to the parent body's core.

On Earth's surface, gold and platinum are rare elements because they sank to the core here just as they did in other differentiated bodies. The relatively rich concentrations of these elements in irons from asteroids raises the possibility of someday mining asteroids for precious metals. Asteroids could also provide a virtually unlimited supply of steel for building spacecraft and space stations.

The asteroid Kleopatra is nearly the size of New Jersey-and it is almost entirely metal. Iron asteroids are the former cores of minor planets that shattered in collisions. Kleopatra's distinctive "dogbone" shape may be the result of two iron asteroids colliding and merging.

LOOK CLOSELY

All iron meteorites contain grains or nodules of other minerals. When the molten metal cooled and crystallized, elements that do not fit into the mineral structure of iron crystals, such as sulfur, became concentrated in droplets between the iron crystals.

SEPARATED AT BIRTH?

Though Costilla Peak (above) and Bella Roca (below) landed in different places on Earth, they have nearly identical compositions, indicating that they probably came from the same asteroid.

UNEARTHLY IRONS

Iron meteorites are much easier to spot than stony meteorites. If you saw Cruz del Aire lying on the ground, it would immediately catch your eye, whereas a stony meteorite might blend in with the other rocks. Irons stand out because chunks of iron are very unusual on Earth's surface. Most iron on Earth also rusts away very quickly. But the iron-nickel alloy in meteorites is a naturally rust-resistant steel, so iron meteorites can last for thousands of years before weathering away.

AN ODDBALL IRON

Unlike most iron meteorites, Gladstone probably did not come from the core of a differentiated asteroid. Its unusual iron includes silicate minerals, which in a fully differentiated asteroid would have floated to the mantle and crust. Gladstone may have formed in an unmelted chondrite when an explosive impact melted a pool of lava, and molten iron sank to the bottom and hardened.

GUARDING LIFE ON EARTH

Without Earth's molten outer core, life as we know it would not exist. The movement of liquid iron in the outer core, caused by convection and Earth's rotation, generates a magnetic field around our planet. This magnetosphere deflects the charged particles streaming toward us from the Sun. Without this invisible shield, the barrage of ions from the solar wind would make Earth's surface uninhabitable.

The iron core of Mars, in contrast, has cooled and almost entirely solidified-so Mars has only a tiny magnetic field. With little protection, Mars is bombarded by deadly ions, making life on its surface virtually impossible.

A magnetic field around Earth-generated by the movement of liquid iron in its core-diverts charged particles shooting out from the Sun. Most stream around Earth, but some are guided toward the North and South Poles.

Charged particles from the Sun are diverted to the Earth's magnetic poles. When these ions collide with atoms in the atmosphere, they cause the sky to glow, creating auroras, or northern and southern lights.