Characteristics and Origins of the Solar System

Lecture 28

November 14, 2001

Meteors and Meteorites and Their Origin in Asteroids

 

Announcements

  1. Look for Leonids Sunday morning (if clear).  Anytime after midnight (when the constellation of Leo is up).  The best prospect for a meteor storm will be about 4:30 AM.
  2. Remember the test on Friday. 
  3. Out of my concern for you, and my desire that you not be tormented by boredom during Thanksgiving recess, I will put another homework problem set on the web over the weekend.  It will be due the Friday after return from Thanksgiving. 
  4. Course Advertisement #1:  I will teach a first year seminar next semester (if anyone signs up!) entitled The Origins of Natural Science in Antiquity.  It will meet on Tuesday once a week, in a small class setting. Freshmen only.
  5. Course Advertisement #2: If you found this course interesting and have not taken 29:50, Modern Astronomy, think about learning about the rest of the universe.  Two sections are offered next semester, and I will be teaching it next Fall.  I will guarantee there is virtually no overlap with what you have studied so far.

 

            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.

  1. We can see cracks in asteroids that are plausibly caused by collisions.
  2. The ages of the surfaces of asteroids are considerably less than the age of the solar system.
  3. Double asteroids are quite common.  See for example Ida and Dactyl.  This is also indicated by radar images, as well as evidence of double craters on Earth, which indicates we were hit by two objects.

 

 

Kirkwood’s Gaps

 

            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.

  1. Earth-orbit-crossing asteroids
  2. Meteors

 

There are two kinds of meteors.  Those that are associated with comets, and can be called

Meteor shower meteors.  An example is the Leonids  that we might see this weekend.  The second type is the meteors that leave meteorites. 

 

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. 

 

Carbonaceous Meteorites and Very Primitive Rocks

 

            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…”

 

The whole subject of carbonaceous chrondites meteorites and their association with the C and D type meteorites got a big break less than two years ago when the Tagish Lake Meteorite fell in Canada and was quickly recovered.  There were a number of neat features about this meteorite.

  1. There were enough observations to allow its orbit to be determined, and sure enough, it came from the asteroid belt, and in fact the outer belt where the C asteroids are numerous. 
  2. The sample was well preserved and provided many samples for analysis. A picture and some facts about this meteorite are given at the following site. http://www.geocities.com/~dweir/TAGISH.HTM
  3. Recent scientific papers have shown that its reflectance spectrum closely matches the D asteroids.  Tagish Lake was the first meteorite associated with a D asteroid.  In fact, the reflectance spectrum is virtually identical to a D asteroid named 368 Haidea, which is near the 2:1 resonance with Jupiter. 

 

The Last Word on Asteroids and Meteors: BIG IMPACTS again

 

I mentioned that asteroid impacts on the Earth have had a big effect on the Earth in the past.  Let’s look again at the map with the known asteroid impacts: http://gdcinfo.agg.emr.ca/crater/world_craters_e.html

 

In particular, look at the evidence for double asteroids in the impacts at Steinheim and Ries in Germany, where two impact craters were formed 15 million years ago.  The double nature of the impact was a vestige of the collisional processes that helped send them our way in the first place!