MeteoritesProperties & Origin

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.

Fusion crust

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.