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What is a Meteorite?

| questions | August 15, 2011

Meteorites have many meanings and have many fascinating tales to tell, but they can be summed up simply as – rocks from space.

Meteorites are fragments of rock and iron that populate the solar system, and most of them originate in the asteroid belt between Mars and Jupiter.  Each one orbits in it’s own path around the Sun, much like our own planet does on a larger scale.

While they are still in space, meteorites are called “meteoroids” and these meteoroids can vary in size from that of a pea and up to massive chunks of rock the size of Texas,  There are millions of small bodies that reside between those two extremes in size, and these bodies will collide with others when their orbits intersect at the wrong time,  The resulting impact causes the wrecked bodies to throw out debris in all directions, and some of that debris will become meteoroids that may eventually strike Earth and become meteorites.

Meteorites are made up of a variety of minerals, but the most common is a heavy stone with the presence of nickel-iron in the form of flecks or veins of metal.  The stone meteorite is also made up of olivine, silicates, or (rarely) carbonaceous material.  There may also be tiny spherical formations of minerals present in the meteorite.  These curious formations are unique to meteorites, and they are called “chondrules”.

Iron meteorites are solid iron consisting of various nickel-iron alloys and occasional silicate inclusions.  Iron meteorites are very dense and heavy for their size – imagine holding a similar-sized piece of railroad track, and that is how heavy an iron meteorite would feel in hand.

Many meteorites have a distinct “meteorite” look to them, but others can closely resemble Earth rocks, and telling them apart can be difficult.  In some cases, a visual examination is not enough to determine if a rock is a meteorite or not.   For example, a simple streak test will rule out hematite, magnetite, and other common rocks that are often mistaken for meteorites.  A test for the presence of nickel will also help identify a suspected meteorite because almost all meteorites contain elevated levels of nickel in comparison to Earth rocks.  One can also use the specific gravity method because meteorites are more dense than terrestrial rocks.  Perhaps the simplest test is to grind or cut a window into the stone – this will get past the outer surface and reveal the interior of the rock.  If this window shows chondrules or metal flecks, then the stone in question may be a meteorite.

An experienced eye can usually tell a meteorite from an imposter (often called a “meteorwrong”), but only a scientific institution or university with a planetary sciences department has the proper equipment and expertise needed to positively identify a meteorite and determine what type it is.  A variety of tests are employed at this level, including : thin-section study, electron microprobe, isotope analysis, and scanning electron microscope.  These sophisticated methods can precisely measure the chemical compositions of meteorites and leave no doubt that rocks do fall from space.

If one is lucky enough to see a bright shooting star in the night sky, then one can easily imagine that those elegant streaks of light are really chunks of celestial rocks and iron that are hurtling through our upper atmosphere at cosmic velocities.  The meteoroids that cross paths with Earth will be captured and pulled into the atmosphere for a fiery violent descent that observers on the ground see as a shooting star.  Most of these meteoroid shooting-stars are surprisingly tiny and they quickly burn up in our atmosphere.

A rare few meteoroids are large enough or sufficiently robust to survive the blazing-hot atmospheric passage and these will produce meteorites which survive to strike the Earth.  These meteorites often fall unnoticed into the oceans or into unpopulated regions of the world, and these space rocks are lost.  A small handful of these meteorite falls will be seen by witnesses on the ground or they will be detected on radar or sky survey instruments.  In these “witnessed fall” events, it is often possible to track the projected path of the meteoroid and the impact area where meteorites might be waiting to be found.

Meteorites can also be found in remote areas where climate and geography favor the preservation and concentration of old meteorite falls – dry lakes, deserts, snow fields, cultivated land, etc.   While meteorites might seem less than rare in places like the Saharan desert or Antarctica, they are still the rarest material on Earth.  Meteorites, by weight, are many times more rare than diamonds, gold, or platinum.  But one does not have to be wealthy or undertake expeditions to the Sahara to find meteorites.  The meteorite collector can use the internet to find and acquire all of the known meteorite types, in sizes to fit every budget.

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