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Space Rocks: What Are Meteors Made Of?

Last week, scientists hauled a 1255 pound space rock out of a lake in Siberia. This chunk of meteorite, measuring nearly five feet across, came from the famous fireball explosion over Russia in February of 2013. Many incredible videos captured the enormous fireball and the damage its shockwave created:

An event so dramatic raises questions: How many objects from space crash into us every year? How many do we ever find? What are they made of?

Each year, tens of thousands of tons of material from space falls into the earth's atmosphere. Most of this material is burnt up before it reaches the planet's surface by aerodynamic heating from colliding with air molecules at enormous velocities. How many surviving space rocks have been found? NASA says more than 50,000.

From single grains of dust to nearly a meter (39 inches) wide, a countless number of these meteors impact us constantly. Any fragment which impacts the ground intact becomes a meteorite. Objects larger than a meteor are termed asteroids and occur much less frequently. (Meteorites less than one meter in diameter can be called meteroids, just to cause confusion.) An object as large as four meters (13 ft) strikes roughly once per year, while a 25 meter (82 ft) asteroid is likely to impact earth only every century or so. Catastrophically large impacts, which can theoretically wipe out species and dramatically change the climate, appear to only occur on timescales of millions of years apart.

Meteorites fall into a few basic compositional types. The most common, chondrites, account for roughly 85-90% of finds. They are named for their most distinctive feature: chondrules, which are tiny spherical silicate structures. Chondrites are composed of a large number of chondrules, packed into a compressed mass of dust.

The tiny chondrules tell us something about the infancy of our solar system. They formed 4.57 billion years ago when the solar system was an enormous swirling disc of hot material, in the process of condensing into the sun, planets, and other objects such as comets and the far away dim orbiting bodies beyond Neptune.

Carbonaceous chondrites attract particular attention. These meteorites contain organic compounds and can carry amino acids. So, essential building blocks of earth's life are raining down from outer space! As cool as this is, the concentration of amino acids is low, probably much too low to have bearing on the origin of life on earth. Additionally, claims of fossils or remnants of life found in meteorites have, in every case, turned out to be either mistaken or falsified.

Iron meteorites, composed mostly of iron, with varying amounts of nickel and silicates, are relatively rare (roughly 5% of discoveries). The famously beautiful Willamette meteorite is an example of this type:

Willamette Meteorite.png

Iron meteorites are thought to originate from the inner cores of asteroids, shattered and freed to space from collisions. For millennia, native peoples have found these stones and utilized their metal for tools and weapons. This "easy iron", far simpler to access than iron locked in buried ore, is probably some of the first ever used by humans.

A few iron meteorites contain crystals embedded in the metal. Jewelry and other ornaments are made from polished fragments of these pallasites:

Thousands of meteors of different types fall on us from space every year. Thankfully, they have caused no human deaths in recorded history, and truly devastating impacts are incredibly rare. Want to find a meteorite yourself? Book a trip to Antarctica.

WIllamette Meteorite Image: WIkimedia commons
Pallasite Image: Wikimedia commons

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Tom Hartsfield
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