The Meteorite

Meteorites

     When you see a falling star, it's not really a star.  It's a meteor burning up on entry into our atmosphere.  

     What's the difference between a meteor, meteorite and meteoroid?  A meteoroid is a solid celestial body floating in space. A meteor is what we see as a streak of light or falling star.  It's a phenomenon caused by a tiny fleck of cosmic dust that burns up entering Earth's atmosphere.  A meteorite is a solid celestial body that survived it's fall to Earth.  We can hold these in our hands.

     What is a fall and a find?  A fall is a meteorite that was seen streaking through the air and falling to the ground.  Many people only collect falls.  A find is a meteorite that was found and was not seen falling.  

     Meteorite Classification

Stones - Chondrites - with chondrules - carbonaceous, enstatite, olivine-bronzite, olivine-hypersthene and amphoterites.

Stones - Achondrites - without chondrules - angrites, aubrites, ureilites, howardites, eucrites, diogenites, shergottites, nakhlites and chassignite

Stony Irons - lodranites, mesosiderites, pallasites and siderophyre

Irons - hexahedrites, octahedrites and ataxites.

Irons

Yearly Meteor Showers

Hourly counts vary from year to year.

       Comet Giacobini-Zinner
Comets are small bodies made of rock, dust and ice, formed in the distant reaches of the solar system. Some comet orbits take them near the Sun. Then solar radiation heats the comet's surface, causing part of the ice to boil off into space and carry some of the dust with it. When this happens the comet can develop a "tail" millions of kilometers long, and leave a dust trail behind in solar orbit. (NASA Jet Propulsion Laboratory)

When the Earth crosses a comet's trail some of the dust enters our atmosphere and burns, making bright streaks called meteors in the sky. Some of the particles do not burn, but stay in the upper atmosphere, where we can collect them with high-flying aircraft. Scientists call these "interplanetary dust particles," or more simply "cosmic dust."

 Meteor
As the asteroid fragment approaches the Earth, it passes through our atmosphere where friction heats it white hot. We see this moving flame as a meteor crossing the sky. If the asteroid fragment is big enough, fist-sized or so, it won't burn up completely and will fall to the Earth as a meteorite. These small meteorites do not explode or make craters when they hit. They just hit like an extra large hailstone or a rock thrown from across the street. Newly fallen meteorites are usually found sitting on the ground, a rock that wasn't there the day before. (Artist's conception, NASA JSC photo S79-29471)

Stony meteorite
Stony meteorites are commonly made of familiar minerals like plagioclase, pyroxene, and olivine. Scientists believe that they were formed in the outer parts of asteroids. Stony meteorites look a lot like Earth rocks, and are often not recognized as meteorites. Their outer surfaces are usually melted as they pass through our atmosphere, giving them dark "fusion crusts." That is why the outside of the Noblesville meteorite looked so dark. (Photo by Cecilia Satterwhite, NASA  JSC photo S94-44343)

Iron meteorite
Iron meteorites are made almost entirely of iron metal with some nickel. As with stony meteorites, iron meteorites also have fusion crusts and show distinct molten metal shapes and flow markings, like those on the fragment of Sikhote-Alin.

Inside, many iron meteorites are made of criss-crossing intergrown crystals of two different iron-nickel minerals. The sample of the iron meteorite in the Meteorite Sample Disk has an etched surface that shows these patterns. (Photo by Carl Allen, NASA JSC photo S94-43470)

Stony-iron meteorite
Some meteorites are mixtures of iron and fragments of rock; they are called stony-iron meteorites. This sample, like the one in the Meteorite Sample Disk, formed at the boundary between the metal core and the rocky mantle of an asteroid. (Photo by James Holder, NASA JSC photo S94-44546)

Solar nebula
The story of meteorites begins 4.6 billion years ago. The solar system began as a spinning cloud of gas and dust, called the solar nebula, which collapsed under its own weight to form a new star, our Sun. As the solar nebula spun and churned, dust grains stuck together to form dust balls, and huge bolts of lightning melted them into small spheres. These solidified into rocky balls called chon rules. (Painting by Don Dixon, NASA JSC photo S76-25001)

Differentiation
Some very primitive meteorites are just masses of chon rules stuck together. In most meteorites, though, the chon rules have been partially or totally destroyed. To destroy chon rules takes a lot of pressure and cooking time, so much that these meteorites could not have formed as small rocks floating in the solar nebula. They must have come from inside larger objects, the asteroids, where the weight of overlying rock created enough pressure to obliterate the chon rules.

But more happened inside asteroids than merely squeezing chon rules. In the largest asteroids, heat and pressure got so great that rock could melt and crystals move under the force of gravity. Heavy iron crystals sank toward the center and lighter silicates floated toward the surface. The results were differentiated asteroids with metal cores and rocky mantles and crusts. (Artist's conception, NASA JSC photo S94-43469

Death of the dinosaurs

 But meteorites have also had devastating effects on life. The dinosaurs were killed 65 million years ago after a huge meteorite hit the Earth. The explosion caused great storms and waves, and the sky was dark for months with dust and ash. The dinosaurs, along with many other animals and plants, were probably killed by the climate changes that followed the explosion. But their deaths permitted other animals to flourish and spread, including primitive mammals. We may owe our very existence as a species to a long-ago meteorite. (Painting by Don Davis, used with permission)

The Moon
The Moon is covered with craters in a wide range of sizes. You can see a few of the largest with your naked eyes, and many more with binoculars. (NASA JSC photo AS17-152-23311)

Chondrules                  
Chon rules are the primitive building blocks of the solar system. In the early solar nebula they came together to form larger and larger masses - the forerunners of asteroids and planets. The largest chon rule in this picture is less than 1 cm across. Most chon rules are so small that it is difficult to learn much about them without a microscope. (Photo by Allan Treiman, NASA JSC photo S93-33279)

Accretion and metamorphism
In the early solar nebula chondrules came together to form larger and larger masses. The process is called accretion. Some very primitive meteorites are just masses of chondrules stuck together. In most meteorites, though, the chondrules have been partially or totally destroyed by metamorphism. To destroy chondrules takes a lot of pressure and cooking time, so much that these meteorites could not have been formed as small rocks floating in the solar nebula. They must have come from inside larger objects, the asteroids, where the weight of overlying rock created enough pressure to obliterate the chondrules. (Artist's conception)

Volcanism
Action on an asteroid may not end with differentiation. Some asteroids got so hot that they melted inside and spewed lava onto their surfaces, just like this lava flow on Earth. The lava hardens to a rock called basalt. Some basalts from asteroids fall to Earth as meteorites. (Photo by Carl Allen, NASA JSC photo S94-44543)

Meteorite from the Moon
This is a meteorite which was found a few years ago in Antarctica. It is made mostly of broken pieces of feldspar. Scientific studies have proven that this meteorite and a few others like it are from the Moon, not from asteroids. They were blasted off the Moon by other meteorite impacts there, and quickly traveled the short distance from the Moon to the Earth. (Photo by Roberta Score, NASA JSC photo S82-35865)

Antarctica
The frozen continent of Antarctica has proven to be the best place on Earth to find meteorites. The meteorites fall onto glacial ice and are carried along until the glacier encounters a mountain range or other barrier. The ice then stops and eventually evaporates, leaving the meteorites behind. (Photomosaic, National Oceanic and Atmospheric Administration)