How and When Did Meteorites Form
How and when meteorites formed is a subject of scientific theorizing. Scientists look at what they know--the form and composition of meteorites as the see it in the laboratory and the form and composition of the universe as they see it through their telescopes. They then make a guess--pose a hypothesis--as to how meteorites formed. They test the hypothesis by trying to prove it false. Following are the current theories of how meteorites formed:
Chondrules--the First Rocks
Scientists have hypothesized that the first solid matter was dust--tiny mineral grains that solidified from the gases that proceeded them at the beginning of the universe. The dust clumped together. Perhaps electrical charges or gravitational attraction made this happen.
The clumps melted. We are uncertain of the reasons for the melting. Heat from star systems may have caused it--or radioactivity in the clumps themselves. (Billions of years ago, matter was far more radioactive. Because of their age, meteorites are among the least radioactive rocks known.) When the clumps melted, small spheres were formed. These are chondrules. Here is what they look like in chondrite meteorites. Here is another view.
Chondrules floating in space eventually clumped together again, this time with other chondrules and the cosmic dust. The result was chondrite parent bodies--we know them as asteroids or planets. Below is a table that shows how chondrite parent bodies might have formed. It is redrawn from a more complete one in Rocks From Space by O. Richard Norton. This book includes considerable explanation as well.
Iron and Stony Iron Meteorite Parents
The clumping together of the larger material, like the initial clumping, had the effect of concentrating the radio activity and resulting heat. Parts of the rock melted. The heavier material--metallic iron and nickel settled by gravity to the center of the forming mini-planet. This is the parent for iron meteorites. Outside of the iron, a mixture of iron and other minerals formed the parents of stony-iron meteorites. The process is called differentiation.
By the theory, the melting and differentiation would also result in lighter rocks. These would be like the lava we see emerging from volcanoes on Earth. They would form the parents for achondrites.
How Old Are Meteorites
If meteorites formed from dust from the early solar system, then we would expect them to be very old. Indeed, this is what we find. Scientists have used radiometric dating to measure the ages of meteorites. The results show ages of around 4,500,000,000 years--about seven hundred million years older than the oldest rocks on Earth.
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