Meteorites
Here is a brief definition, as well as history, on what meteorites are and how they are formed.
- Meteorites come from the stony, glassy, and metallic debris left over by the formation of our solar system. This debris collected in the asteroid and Kuiper belts, as well as in the Oort cloud. The larger objects are called “asteroids” and “planetesimals,” and the smaller objects become “meteoroids.”
- While these objects generally maintain stable orbits, interactions with planetesimals, asteroids, and in-system planets sometimes send meteoroids into the Earth’s path. Comets also sometimes carry meteoroids, trailing them into Earth’s orbit as the comets melt away in the glare of the sun.
- When a meteoroid falls into the Earth’s atmosphere, friction from falling through air heats the meteoroid, causing it to glow. When the meteoroid glows, it is called a “meteor.”
- Meteors create visual effects that range from the commonly seen short, white streak in the sky to the mesmerizing orange fire balls that traverse the skies with jet-like speed but merely seem to float. Meteors in the atmosphere can burn away to dust, explosively fragment into many pieces, or glow until they strike the ground.
- Meteors that hit the ground are called “meteorites.” Some meteorites fall to the ground without leaving any traces of their landing, but some create holes and “craters.” Craters are bowl-shaped depressions in the ground ringed by walls of earth thrown up by the shock. The floors of the craters sometimes contain “shatter cones,” which are pieces of stone that the impact of the meteorite solidified into cone shapes.
- Sometimes people find meteorites on the ground. They sell these meteorites to collectors, museums, and research institutions.
- Research institutions cut meteorites open to find out what each has inside. While each meteorite is as different as a person’s fingerprint, researchers have discovered that they fall into basic types, each with its own information to tell about the formation of the solar system.
- All meteorites have one characteristic in common: the fusion crust. When the meteorites were meteors falling through Earth’s atmosphere, they all began to melt on the outside. This melted and burned-looking layer is usually one or two millimeters thick, and gives the meteorite a uniform brown or black look. Because all materials in the meteorite’s surface slagged together into this layer, it is called the “fusion crust.”
- Inside the fusion crust, meteorites divide into two types. These are the “differentiated” types and the “undifferentiated” types. These two types tell us about the history of the meteorite as a meteoroid.
- Undifferentiated meteorites have all the stone, glass, and metal jumbled together in no particular way. This shows that undifferentiated meteorites formed by accreting dust and small crumbles, but never grew very large as meteoroids.
- Differentiated meteorites have distinctly separated stone, metal, and glass. This shows that differentiated meteorites accreted dust, crumbles, and boulders until they, as meteoroids, were large enough to start melting in the middle. Because the meteoroids started melting, the denser metals sank to the center and the stones and glasses floated to the top of the meteoroid. If a differentiated meteorite is made completely of metal, we know that the meteoroid it was in was broken apart, and this piece came from the center.
- If a differentiated meteorite is all stone and glass, we know that this piece came from the surface layers of the meteoroid. If a differentiated meteorite is made mostly of metal with bubbles of stone and glass floating in it, we know that the meteoroid it was in was broken apart, and this piece came from the layer in between the surface and the center where stone and metal still mixed.
- Researchers create many subsets of meteorites within the basic types. These subsets focus on the types of stones, glasses, and metals within the meteorites, whether the stones form particular shapes, how much water the meteorite contains, whether it has organic or inorganic materials, and other factors. The Meteorite Market offers a basic chart for meteorite classification.
- Meteorites offer answers about how our solar system formed. How much water was there, and where? Was there supposed to be another planet somewhere in our system? Was Earth created by the violent collision of large rocks, or by the slow accretion of small boulders and dust? If we see metal and stone flowing in a meteorite, does that mean they flow similarly in the core of the Earth?
- Meteorites offer answers about how we got here. Did we develop here on Earth, or did comets and meteorites carry our organic beginnings? Are there viruses and bacteria outside of our planet that can hurt us? Could there be life in other places?
- Meteorites puzzle us. Inside stony and stony iron meteorites, we find the glassy droplets we call “chondrules.” Conventional wisdom tells us that chondrules only form between 1200 and 1500 degrees Celsius, yet readings of the carbon surrounding the chondrules tell us that the meteoroid never rose above twenty degrees Celsius. Why are the chondrules there? And why do some meteors fall most of the way to the ground only to explode and vaporize before hitting it, while others land and shift earth? Why doesn’t the photophoresis effect thin the density of asteroid belts.
Department of Physics
MSC 4502
James Madison University
Harrisonburg, VA 22807-7702
PHONE : (540) 568-6109
FAX : (540) 568-2800
