Lesson Objectives:- An overview of asteroids
- Types of meteorites
- The asteroid belt
Asteroids are important because many of them remain essentially the same as when they formed, early in the life of our solar system four and a half billion years ago.
All are rocky in composition, but those that are farther from the Sun contain more carbon compounds and water.
The largest asteroid in our solar system is also considered a dwarf planet -- Ceres. It is just under 1000 kilometers in diameter and its mass is nearly equal to all of the other asteroids put together.
That gives you an idea of how small most asteroids are. If you were to add up the mass of all of the asteroids, it would still be less than the mass of our Moon.
In recent years, spacecraft have been sent with the purpose of collecting samples from asteroids. Even without those samples, however, scientists have had access to tens of thousands of asteroid samples that have fallen to the Earth as meteorites.
Meteorites are categorized into primitive meteorites and processed meteorites.
Primitive meteorites are those that are essentially unchanged since the birth of the solar system. They are from the time when solid material first condensed from the solar nebula and are the oldest rocks in the solar system. In addition to telling us how old the solar system is, they provide us with information about the solidified material that accreted to form planets and asteroids.
Processed meteorites are different from primitive meteorites in that they are not original, primitive material. They have undergone change, coming from asteroids that were large enough to have undergone differentiation into a core-mantle-crust structure. They tell us that some asteroids were large enough to have had substantial internal heat and volcanic activity, and when the meteorite is a fragment of a core or mantle of a shattered asteroid, they serve as proof of the internal structure of large worlds such as Earth.
Virtually all planetesimals that formed inside of the orbit of Mars accreted onto one of the terrestrial planets, but those that formed beyond Mars were heavily influenced by Jupiter's gravity. Orbital resonances with Jupiter caused many of them to be kicked out of their orbits -- either to crash into a planet or moon, or even the Sun, or flung out of our solar system altogether.
This interference from Jupiter is also what kept those planetesimals from accreting to form another planet. The asteroid belt originally contained a lot more rocky material than it does now, but orbital resonances with Jupiter disrupted many of their orbits and the asteroid belt lost most of its original mass during billions of years.
Basically, the asteroid belt represents the asteroids in the solar system that survived - they did not get accreted onto an existing planet and they did not get kicked out of their orbits by Jupiter.
Change is always happening, however. Orbital resonances continue to shape asteroid orbits and still lead to asteroid collisions today.