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What is a Meteorite?
Some basic facts.
A meteorite is a rock from space. Most come from the asteroid belt, a region of rocky and metallic chunks orbiting the Sun between Mars and Jupiter, where no planet ever formed. A few come from the Moon or Mars, blasted off the surface of those worlds by gigantic impacts in the distant past. It may take millions of years or more for them to find their way to Earth.
How are they discovered?
Sometimes meteorites are seen falling, often after a bright fireball has exploded in the sky. They are called falls. More often they are discovered on the ground by chance and are called finds. Some climates are more friendly to meteorites than others, for example, dry areas with little vegetation. High humidity and heavy vegetation make finding them much less likely.
Do they come from meteor showers?
No. Meteor showers—like the Perseids in August or the Leonids in November—are annual events that occur when Earth travels through clouds of dust left behind by comets as they orbit the Sun. The dust particles are very tiny and always burn up high in the atmosphere. There are no known meteorites related to meteor showers.
There are three basic types of meteorites
But those types are then sub-divided into many different subtypes depending on their physical characteristics and chemical composition.
Irons
An iron meteorite may be covered with ‘thumbprints. Or it may be sharply bent and distorted like shrapnel.
The cosmic crystal structure of a meteorite is revealed by cutting and etching. Some minerals in meteorites are not found on Earth. All iron meteorites contain nickel. To determine if a sample is a meteorite, it will need to be tested.
Stony: Chondrites
A typical stony meteorite has a dark fusion crust and lighter interior. Chondrites contain iron. You may see silvery flecks in a cut chondrite. Chondrites contain chondrules, tiny spheres that are older than Earth. They are found in asteroids. No Earth rock has them.
Some of them are black because they contain carbon, and have been found to be scientifically very important.
You will need a microscope to really look inside a chondrule.
Stony: Achondrites
An achondrite has no chondrules, and if it doesn’t have a dark fusion crust, it may not be recognized as a meteorite. Meteorites from the Moon and Mars are achondrites.
Stony-irons
Stony-iron meteorites are a mix of iron and rocky material, formed deep inside a large asteroid or by a violent collision. Some contain crystals of olivine in a network of iron-nickel. They are the Pallasites. And some are a jumble of melted iron and broken rock. They are Mesosiderites.
Did you find a meteorite?
It may be difficult to know. Some are obvious, but many are not. Here are some typical characteristics of meteorites to help you tell them from ‘meteor wrongs.’
Generally, meteorites:
- Are heavier than regular Earth rocks of the same size.
- Contain iron and is attracted to a magnet.
- If stony, are covered by a black fusion crust and maybe lighter inside. The surface may show signs that it was flowing when the rock was heated in the atmosphere.
- May have indentations like ‘thumbprints’ on the surface.
- If irons may show signs of violent explosions like shrapnel from a bomb.
- Completely melted. They don’t land with a splash. They aren’t on fire and don’t cause fires when they hit something. They may be warm but are often very cold.
- Covered with holes. Holey rocks are usually volcanic rocks of very Earthly origin.
- Radioactive. You can thank Hollywood for all these misrepresentations.