Meteorites

A meteorite is a rock that falls to the Earth, including particles like fine dust. The source material may be any solid, including comets, asteroids, meteoroids, fragments of the above, and even stardust. A meteor is a shooting star - the visible glow that shows the path of the meteorite. So, in space, the object may be a meteoroid, in the atmosphere a meteor, and once landed, it is a meteorite.

Most meteors burn up in the atmosphere. A typical speed is 15 to 30 kilometers per second, and the air friction at these speeds is enormous. A typical millimeter to centimeter grain burns up in the upper atmosphere (50-100 km in altitude). Boulder-size (1 to 10 meters) may descend further, but the minimum size needed to penetrate the atmosphere is likely 30 to 100 meters, depending upon composition. For meteors larger than perhaps 10 centimeters, the visible glow is mostly due to atmospheric ram pressure, as a shock wave is generated by the meteoroid's passage.

Larger meteors often fragment, splitting into multiple pieces, sometimes repeating the process. This can be easily understood: there is a lot of air blocking the meteor's passage. The minimum amount of air encountered is for a vertical approach. Since air weighs 15 pounds per square inch, a 1 square inch meteor (which might weigh an ounce or two) encounters 15 pounds of atmosphere - perhaps 100-200 times its own weight. Note that a square meter of air weighs 10 metric tons, and even a boulder-size meteor is blocked by a buildup of air several times its mass, which reaches enormous pressure, typically greatly exceeding the structural strength of the meteor which effectively explodes. Imagine a fist-size ball of dried mud, and how well it will survive being struck by a sledge hammer at 15 kilometers per second.

Small pieces of a car-size meteor might reach the surface, if the meteor was a relatively strong rock, and some pieces of it were lucky enough to be protected by other pieces as it blasted through the air. But for the most part, the fragments we see came from much larger meteoroids which broke into smaller pieces before slowing down to terminal velocity and falling to the ground.  It takes a really large chunk to leave a crater, as it would have to outmass the air it encounters by a significant ratio to keep from being slowed significantly.

A meteor's surface is heated to thousands of degrees, yet that is a surface effect, and the interior of the meteor will retain the cold of space, even if it has a fusion crust on the outside, formed as the minerals undergo extreme heat, melting, and oxidation. This is simply because of the time it takes for heat to diffuse into the meteorite (especially a rocky one), which is long compared to the few seconds of fiery passage experienced by the meteorite. The heat of ablation might only penetrate a stony meteor to a depth of 1 or 2 millimeters, and might penetrate an iron meteorite to a depth of 1 or 2 centimeters.

Approximately 10,000 tons of meteoric material strikes the Earth each year. Much of that is in the form of micrometeorites - dust - which is small enough that it does not burn up but simply drifts to the surface over a period of months. Try this experiment: take a strong magnet and clean it thoroughly such that no metal filings remain. Then drag it along some dusty window sills and examine it using a loupe or microscope. You are likely to find hundreds or thousands of tiny bits of iron, most of which are likely nickel-iron micrometeorites.

Until the 20th century, over 80% of all known meteorites were nickel-iron meteorites. Today, they comprise only 6%, with the rest being stony meteorites. There are two reasons for this. First, iron meteorites are strong and tend to hold together, and more importantly, they are easily recognized. After all, if you stumble across a rock in a field, the odds are good that it's an ordinary rock, so why would you take a second glance? But if it is magnetic, or extremely heavy, or covered by an obvious layer of thick rust, then it's likely a meteorite. Anyone can find an iron meteorite, nearly anywhere (with a bit of luck), using a metal detector. Several ancient civilizations have even used iron meteorites as a source of metal. But there are only a few places where a stony meteorite would stand out, and these were not recognized until fairly recently. These locations include the surface of glaciers or ice fields far from a mountain, on fields with no other rocks (such as parts of Kansas), desert sand dunes, or on dried up lakebeds or ancient ocean floors far from glacial deposits (places where thick layers of silt would have covered up any local rocks). Some ice fields in Antarctica have yielded thousands of stony meteorites.

Large meteorites, even iron ones, tend to fragment into hundreds or thousands of pieces as they blast through the atmosphere. Often the process is visible as a bright meteor will burst apart into fragments, each of which leaves its own glowing trail, much like fireworks. When the resulting pieces eventually fall to Earth, they result in a strewn field, which is generally a long, oval shaped area containing the fragments. The lightest meteorites fall out near the beginning of the strewn field, and the heaviest at the end.

Note that when a large enough meteorite strikes to create a large crater, the splashed-out materials are not considered meteorites, even if their appearance is similar. For example, tektites are thought to result when a meteorite strikes an area of silicate sands, melting and splashing the liquid silicates into a circularly-shaped strewn field. These materials cool so rapidly that they become glasses, and in some cases their purity is great enough that the resulting tektites are highly valued as jewelry. The best-known of these are called Moldavites, after Moldavia, Czechoslovakia where they are found.

There are a number of well-known meteorite falls, including these:

• Campo del Cielo meteorites are iron octahedrites found in a large strewn field straddling the border between the provinces of Chaco and Santiago del Estero in northern Argentina. The fall has been dated to approximately 4500 years ago. More than 2 dozen craters pockmark the area, and thousands of fragments have been recovered, massing in excess of 100 tons (qualifying this as the largest meteorite recovered).
• Canyon Diablo, Meteor Crater, Arizona, USA. About 30 tons of the iron octahedrite meteor recovered from the vicinity of a 49,000 year old, mile-wide crater.
• Allende, a CV3 Carbonaceous Chondrite (and the largest known carbonaceous chondrite fall), massing at least 2 tons and possibly as much as 5. Fell 08-Feb-1969 in Allende, Chihuahua, Mexico in a large strewn field (8x50km).
• Murchison, a CM2 Carbonaceous Chondrite which fell near Murchison, Victoria, Australia, on 28-Sep-1969. About 100 kilograms have been recovered.
• Sikhote-Alin was an iron octahedrite meteoroid that fell in the Sikhote-Alin Mountains, Primorye, Russia, on 12-Feb-1947. More than 28 tons have been recovered out of an estimated total mass of 100 to 900 tons. The daytime fall was witnessed by many witnesses who said it was brighter than the Sun. They observed the breakup and a major explosion; thunder was heard 300 kilometers away.
• Imilac is a stony-iron pallasite meteorite found in a valley southwest of Imilac in the Atacama Desert of Northern Chile in 1822. The total mass is estimated at about 1000 kg. The larger fragments of these meteorites are highly prized, as they contain transparent peridot (olivine) crystals in an iron matrix, a combination that makes for beautiful specimens in thin slices.
• Nantan iron octahedrite meteor that fell near Nantan, China, in May of 1516.
• Gibbeon meteors are fine-grained iron octahedrites found in the Gibbeon Desert of Namibia. The strewn field is large (100 km by 275 km), and in excess of 26 tons of fragments have been recovered. These meteorites often are well-formed, with fusion crusts and pockets. The Namaqua people used the iron to build arrows and other tools. Some estimates age the Gibbeon meteorites at approximately 50,000 years.
• Esquel is a pallasite meteorite found near Esquel, a patagonian town in the northwest part of the province of Chubut (Argentina). It is a beautiful stony-iron pallasite meteorite that was uncovered in 1951. The original mass was 1500 kilograms.
• Cape York is an iron meteorite whose fragments are found near Cape York in Greenland. A total of 58 tons (30.9 in largest fragment) fell at least 10,000 years ago and pieces were used by the Inuit Indians as a source of metal for tools including knives and harpoons.