Meteors and Meteorites and Their Origin in Asteroids

In the last lecture, I mentioned that the geology of asteroids is determined by collisions. They knock into each other, and this breaks them into smaller pieces and knocks pieces off them.

There are a number of pieces of evidence for this.

We can see cracks in asteroids that are plausibly caused by collisions. If you look at the picture of Gaspra below, you can see some faults and lines which are cracks in the asteroid caused by past collisions.

The ages of the surfaces of asteroids are considerably less than the age of the solar system. They have cratered surfaces, but the number of craters per unit area is not as great as the lunar terrae, and usually indicates “exposure times” of only 200 million to a billion years.
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. This suggest pieces broken off in a collision.

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.

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.

The gaps correspond to 2:1, 3:1, etc resonances between asteroids that would be in the gaps with Jupiter. The asteroids (and bits of asteroids) that were in these orbits were transferred to other orbits. These gaps arise in exactly the same way that the gaps in the ring of Saturn. In the case of the gaps in the rings of Saturn, ring particles fall into resonance with moons orbiting further out. In the case of the asteroid belt, asteroids in Kirkwood’s gaps fall into orbital resonance with Jupiter.

So what happens to the asteroids that were in these resonance orbits? See below

Calculations show that resonant transfer from the asteroid belt to Earth-orbit crossing orbits can occur.

There are two manifestations of these transfers.

Earth-orbit-crossing asteroids
Meteors

As we see above, there are asteroids close to the orbit of Earth, and indeed inside the orbit of Earth. We can calculate the orbits of some of these, which are shown in the figure below.

Notice that the orbits often go out beyond the orbit of Mars (into the asteroid belt), but then swoop inside the orbit of the Earth. It looks like they have been ejected from the asteroid belt.

At the end of September of this year, the asteroid Toutatis passed very close to the Earth, being only about 4 times the distance to the Moon at its closest approach. At this time, an observer with a small telescope could have seen Toutatis moving against the background stars before his or her very eyes.

Meteors

Meteors is the technical term for shooting stars. Meteors are due to small pieces of solid matter which come from outer space, enter the Earth’s atmosphere at speeds of tens of kilometers per second, and glow from the frictional heat. We see the glowing object and the hot gases around it as the shooting star.

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 may see around November 17. The second type is the meteors that leave meteorites.

Meteorites

Meteorites are rocks from meteors that make it all the way to the ground and can be recovered. Some pictures are shown below.

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.

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 15.6 – 15.8, that show 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 term that has arisen for these tiny pieces of unbelievably old matter is stardust.

Be sure and take a look at the video clip about meteorites that is on the WebCT site for this course. In this clip, I take some of the collection of the University of Iowa meteorites and show them to you. If you live in the vicinity of the Quad Cities, go to the John Deere planetarium on the campus of Augustana College in Rock Island, where there is a very nice collection on display.

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. Another textbook on this subject has a very interesting discussion of these objects. 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 in January, 2000 when the Tagish Lake Meteorite fell in Canada and was quickly recovered. A picture of this meteorite is shown below.

Just think that this rock was out in space until January of 2000!

There were a number of neat features about this meteorite.

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 (see diagram above).
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.
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!