Characteristics and Origins of the Solar System
Lecture 28
November 27, 2000
Last time, I mentioned that the geology of asteroids is determined by collisions. They knock into each other, this breaks them into smaller pieces and knocks pieces off them.
There are a number of pieces of evidence for this.
One final topic that is not presented in your book, but is crucial to understanding an important point about asteroids and meteorites.
>>>>>>> Sketch of asteroid belt, orbits of Earth and Jupiter.
As mentioned last time, most of the asteroids have semimajor axes between 2.2 and 3.3 au. You can calculate that there are resonances with Jupiter in this range. As mentioned earlier, in a resonance, the orbit is perturbed, and changes into a different orbit.
An interesting facet of the orbits of asteroids was discovered in the 1860s by an American astronomer, Daniel Kirkwood. He found that the distribution of asteroid orbits was not uniform.
>>>>>>> Asteroid orbit with Kirkwood gaps http://ssd.jpl.nasa.gov/a_histo.html
The gaps correspond 2:1, 3:1, etc resonances. The asteroids (and bits of asteroids) that were in these orbits were transferred to other orbits.
Calculations show that resonant transfer from the asteroid belt to Earth-orbit crossing orbits can occur.
There are two manifestations of these transfers.
There are two kinds of meteors. Those that are associated with comets, and can be called
Meteor shower meteors. An example is the Leonids we saw the other night. The second type is the meteors that leave meteorites.
We can analyse meteorites and identify a number of interesting properties. Meteorites are categorized according to classes like chrondites, metal, carbonaceous chrondites. These are reminiscent of asteroid types.
The reflectance spectra of meteorites also are very similar to that of the corresponding classes of asteroids.
Finally, in a many cases, orbits have been determined for meteors that produced meteorites. They extend out to the asteroid belt.
>>>>>> Figure 6-15 of Hartmann, also Sky and Telescope article.
The amazing conclusion we reach is that the meteors that produce meteorites come from the asteroid belt. These objects are pieces off asteroids.
One could go on and on about the interesting properties of meteorites. First look at Figure 13.7, that shows some pictures of them. Most of the meteorites are primitive rocks. Their ages are almost always in the range 4.48 – 4.56 billion years. We interpret this as the age of the solar system. By analyzing them chemically and physically, we get some hints about physical processes in the early solar system.
In many meteorites, there are very small particles of matter that have different isotopic abundances than other solid objects in the solar system. These small particles are believed to antedate the formation of the solar system, and are particles of matter formed and thrown out by now-dead stars, perhaps billions of years before the formation of the solar system.
The carbonaceous chrondite meteorites are perhaps the most interesting type. They are composed largely of minerals that would have been destroyed by heating. We therefore believe that they are some of the most pristine examples of minerals that have changed little since the formation of the solar system. The book has a very interesting discussion of these objects on p286 of the book. One passage is particularly worth citing.
“Most of the carbon compounds in carbonaceous meteorites are complex, tar-like substances that defy exact analysis. Murchison (a meteorite) also contains 16 amino acids, 11 of which are rare on Earth. The most remarkable thing about them is that they include equal numbers with right-handed and and left-handed molecular symmetry…”