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Thousands of meteorites have been found and some have even been seen to fall to the earth by eye witnesses. This enormous collection of meteorites has allowed to learn a great deal about the formation of our solar. We have found that meteorites can be broken into several distinct categories.
A few definitions
• Meteoroid – A small fragment of a comet or asteroid that orbits the Sun.
• Meteor – a small fragment burning up in our atmosphere.
• Meteorite – A small fragment of a meteor that survived until it reached the ground.
Iron Meteorites• These are very dense
and made almost entirely of iron
• They show a pattern of crystallization that indicates a VERY SLOW cooling rate
• These are called Widmanstatten pattern and they are identified by the very large size of the iron crystals
Stony-Iron Meteorites
• These are rare and contain both metal and stone.
• They probably formed in a differentiated planetesimal that was mostly or completely shattered.
Chondrites
• These are composed of stone and can vary quite a bit.
• They often contain “chondrules”, which are small bits of rounded glass that indicates a very rapid cooling period.
• This also means that the meteor was not reheated after formation
Chodrites
• Some chondrites do show mild reheating while others clearly were never reheated.
• We can tell that some have never been reheated because they contains “volatiles”.
• Volatiles are gases that would escape very easily if the meteor had been reheated.
• These chondrites might have originated before the formation of the planets.
Carbonaceous Chondrites
• These are regular chondrites but they contain significant amounts of carbon.
• This indicates that they have NOT been reheated because the carbon would have escaped.
• These also seem to be the original planetesimals from the formation of our solar system and may give us the best clues regarding how our solar system formed.
Achondrites
• These are also stony but unlike most chondrites, they have been severely reheated.
• These meteors resemble rocks that form on the planets from lava flows.
• It is very likely that these are pieces of the planets that have ejected during massive impacts.
SNC’s
• These meteorites formed their own group, different from all other meteors.
• In the 1980’s we found tiny air bubbles in these meteorites and when we examine the composition and isotopes they matched perfectly with the atmosphere of Mars!!
• These are the only rocks that we have from any other planet!
The Fall
• The speed of meteoroids when they enter the atmosphere is around 10 – 30 miles/sec.
• At this speed, meteors melt their surfaces due to friction with the air creating what is called a “fusion crust”.
• However, at their high speed, they only spend a few seconds in the atmosphere before they hit the ground.
• This means that the inside of the meteorite is undisturbed.
Meteor Showers
• There about one dozen meteor showers that occur on regular dates throughout the year.
• These showers are caused by debris left behind by a comet that passes our close to the orbit of our planet.
• Since the showers are caused by our planet running into the debris, the meteors seem to radiate from a point called the “radiant”.
Meteor Showers
• The most intense showers have been the Leonids
• About every 33 years the comet that created the debris trail passes our orbit again.
• In the years following we see spectacular showers with rates of up to 10,000 meteors per hour or more.