Where do meteorites come from?
Most meteorites are fragments of small planets shattered by collisions early in the history of the Solar System. These remnants now form the Asteroid Belt, where many thousands of small objects endlessly circle the Sun between the orbits of Mars and Jupiter.
Fragments of these asteroids may be knocked out of their orbits by collisions. If their path through space crosses the Earth’s orbit they may eventually hit the Earth and fall as meteorites. The best analogy is to compare the Earth to a car driving along the motorway, and the meteorites are the bugs that hit the windscreen.
A few rare meteorites have been identified as fragments of rock from the Moon or Mars. These were blasted into space by the impact of large meteorites or asteroids on these other celestial bodies. They are identified by their chemistry which is subtly different from the rocks that make up the Earth.
What is a chondrite?
The young Solar System was crowded with many small planets. Collisions were commonplace. In the first few million years, collisions between very hot planetismals (tiny planets) created showers of molten droplets. These droplets cooled quickly to form small stony beads, or chondrules: it is believed that the chondrules clumped together (accreted) to form larger extraterrestrial bodies with a tell-tale signature: if they fall to Earth they are classified as chondritic meteorites.
About 90% of meteorites that fall to Earth are chondrites. There are more than 15 types. Most belong to just three types of ordinary Chondrite.
Do meteorites show signs of life?
Carbonaceous chondrite meteorites contain some of the original material from which the Earth and everything on it, both living and non-living, ultimately is made. They contain a mixture of stony mineral dust, spherical chondrules, iron, nickel and even organic carbon compounds, including amino acids! Armagh Planetarium’s collection includes a fragment of the Allende Meteorite from Mexico which is a carbonaceous chondrite, and which is rich in organic material.
The Planetarium’s chondrite collection is best represented by the Bovedy meteorite which has excellent examples of chondrules. It also has a well preserved fusion crust and contains veins of nickel iron metal.
What are iron meteorites?
Iron meteorites come from the solidified core of small planets or planetismals, that were shattered by collisions. More than 90 different types of iron meteorite have been found. Each represents a different planetismal. Their crystal structure shows that the planetismal core cooled very slowly, perhaps at only a few °C per million years. This allowed large metal crystals to nucleate and grow.
What is a pallasite?
Between the iron core of a planet and the rocky crust is the mantle. It is made largely of dark green to black minerals including olivine, pyroxene, spinel and garnet. Pallasites are a rare and beautiful type of meteorite, made of a nickel iron alloy studded with fragments of olivine. Pallasites are thought to be derived from the core - mantle boundary of a shattered planetismal (a small planet). They are stony irons in the simple classification scheme.
What are Martian and Lunar meteorites?
In the Planetarium’s collection we have a small piece of the Zagami Meteorite. This is very like a basaltic rock from Earth, but it is classified as a shergottite. It fell in Nigeria on 3rd October 1962. It is believed to be a piece of the surface of Mars. Sometime in the past an asteroid impact on Mars was so energetic that surface fragments from the planet were blown into space. After millions of years some of these pieces fell to Earth as meteorites. They are identified by their unique chemistry.
Why are meteorites so destructive?
Small meteorites are slowed by friction with the Earth’s atmosphere from more than 11km per second to less than 100 metres per second. This means that they have enormous kinetic energy, based on the well known equation KE = 1/2 mass x Velocity2. The hole that they make when hitting soft ground is normally only a little bigger that the meteorite itself. Very large meteorites, of 100 tonnes or more are not slowed by atmospheric drag as much as their smaller cousins, they are still travelling so fast that they explode or vaporize on impact. They will explode with the effect of a nuclear bomb. This forms a crater far bigger than the meteorite. Immense volumes of rock, melted and blasted out by the explosion, eventually fall back to Earth hundreds or even thousands of kilometres from the impact crater.
Tektites are also a result of such a blast, and small spherical glass spheres are found preserved in Earth’s rocks as evidence of very large impact events in the past. As glass devitrifies over time, the original small glassy beads are often now converted to clays. The Planetarium collection includes a sample from 214 million years old rocks found near Bristol, England. It contains many small green “beads” which are microtektites, glassy beads formed from a spray of impact melted rock. They form a thin layer in the rock. These small spheres were once glassy beads that have been altered to green clay; they represent some of the fallout from a giant impact which took place at that time.
Where can I see an impact crater?
The best preserved meteorite impact crater on Earth is in the USA. The 30 metre diameter iron meteorite which formed the 1 km wide Meteor Crater in Arizona (also known as Canyon Diablo Crater) was vaporised on impact. However, many pieces broke away from the main mass shortly before the impact and lie strewn to the northwest of the crater. This crater is also known as the Barringer Crater the after the family who own the site.
What are tektites?
Tektites are pieces of natural glass formed by the melted material thrown out of the impact site by a large meteorite striking the Earth. The largest known tektite field covers a tenth of the Earth’s surface around Australasia. They are thought to have formed around 780 000 years ago when a giant meteorite exploded high in the atmosphere, blasting the ground beneath with a superheated ‘fireball’ that melted rock, sand and soil.
Tektites are usually very well rounded and appear to be black. If you cut them thinly enough so that light can pass through, they are actually a very dark green colour. When the first nuclear bombs were tested in the Arizona desert in 1945, the enormous temperatures generated by the atomic fireball fused the desert sand into a dark green radioactive glass.
How can I tell if I have found a meteorite? Top tips for identifying a meteorite.
So you think that you have found a meteorite? Use the following checklist to discover if you have really found a meteorite, or just an odd rock.
Is it heavy?
Most meteorites are heavy for their size as almost all contain metal.
Does it attract a magnet?
Most meteorites are magnetic because they contain nickel and iron which are both magnetic. But many Earth rocks are magnetic too, so this is not conclusive. Suspend a magnet on a string and it should be attracted to genuine meteorites.
Does it have a dark coloured fusion crust?
When they fall through the atmosphere a fusion crust is formed. This is a thin layer of rock material that coats the meteorite. Sometimes the edges or rounded corners are knocked off and the lighter coloured inside is quite different from the fusion crust.
Is the outer dark surface covered in what look like thumb prints, or are there oriented lines?
Remaglypts and flow lines are seen in meteorites that have flown through the atmosphere in a specific orientation. Meteorites almost never have holes in them. They are almost never angular with sharp jagged edges, nor are they spherical like cannon balls. They can be light coloured, but are mostly dark.