Meteorites Properties & 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.