Characteristics and Origins of the Solar System
Lecture 27
November 20, 2000
Asteroids are intriguing little worlds. They are fascinating objects in their own right, and give us insight into the processes going on in the solar system since its formation.
There is a big gap in the solar system between the orbit of Mars (a=1.52 astronomical units) and Jupiter (a=5.20 au). This was noted centuries ago, and astronomers expected to find a planet there. What they found (beginning in 1801) was a lot of little ones. After the first discovery, lots more were found.
Table 12.1 of your textbook gives the largest 13 asteroids. This table contains a lot of interesting information. For starters, see that the biggest one (Ceres) is almost 1000 kilometers in diameter (think about how this stacks up relative to the other objects we have talked about). All of these 13 are larger than 250 kilometers in diameter.
At the present time, there are about 10000 asteroids which have had their orbits determined. An absolutely stunning picture of them is shown in Figure 12.2 of the book. All of these are probably at least several kilometers in diameter. Figure 12.2 illustrates the main features of the asteroid orbits.
>>>>>>>>>>> Transparency of Figure 12.2
Until 1995, we did not have any close-up pictures of asteroids. Since they are small, solid, airless worlds, we had an idea that these would show cratered surfaces, but it is nonetheless interesting to look and see if our ideas are right. Since then, there have been spacecraft flybys of the asteroids Gaspra, Ida, and Mathilde. In addition, the NEAR spacecraft has been in orbit around the asteroid Eros for several months (check out its website on my homepage).
We can check out the pictures of the following asteroids:
1. Gaspra (Type S) 16 km “diameter”
2. Ida (Type S) 56 km in length
3. Mathilde (Type C) 60 km in diameter.
4. Eros: 33km
5. Vesta (imaged by HST)
Given what we have talked about this semester, you might expect that the surfaces of these objects would be cratered at a density consistent with that from the age of bombardment. Interestingly enough, this is not really what was found. The surface of Gaspra has been exposed to space for about 200 million years, that of Ida for about one billion. This would seem to be very odd, since these objects do not seem to be big enough for geological processes, hydrology, etc. The relative youth of the surfaces of these objects is a good hint as to their geological history.
One would expect that with these asteroids, a “you see one, you’ve seen them all situation would apply, but this is not the case. Much information has been obtained from reflectance spectra of asteroids; measuring the spectrum of sunlight reflected from the surfaces of these objects. For example, such reflectance spectra in the cases of many asteroids show an absorption feature due to the mineral Pyroxine, which is Mg2(Si2O6).
>>>>>>>>>> Figure from Figure 7.3 of Hartmann
On the basis of these reflectance spectra and the albedo, or reflectivity, of asteroids, there have been at least 9 categories or classes established. Some of the major ones are as follows.
1. S: these have albedos of 7 to 23 %, and they appear to have reflectance spectra similar to chrondite or stony meteorites. They are the main class of asteroid in the inner and central asteroid belt.
2. C: carbonaceous, these have albedos of 2 – 7 percent (about like coal). They have reflectivity and spectra like the carbonaceous chrondite meteorites. They become the dominant type of asteroid in the outer belt.
3. M: metallic, like iron and iron-nickel meteorites. They are primarily found in the central belt.
4. V: for Vesta. Vesta has high reflectivity (albedo of 38%), and shows a reflectance signature characteristic of basalt, the type of mineral involved in lava flows.
From these characteristics, there are a couple of points worth making.
· It is interesting that there is a relation between the location in the asteroid belt and the type of asteroid. It is not a 100% correlation, but it is a strong one.
· The carbonaceous asteroids are interesting. They once again illustrate the trend in the solar system to coat things with a hydrocarbon substance, sometimes containing water incorporated in the mineral (water of hydration). It is speculated that some of these C asteroids might contain water bound in the form of frost under their surfaces. There is an even more extreme type of C asteroids called class D, which have been described as “D materials are probably en even lower temperature organic sludge that condensed just as the ices were beginning to condense” (It’s alive!!!).
These characteristics should be kept in mind when thinking about the different asteroids that have been photographed close up.
The characteristics of the asteroids seem much more varied than expected. There are several different mineralogical classes, and they are in different parts of the asteroid belt. Furthermore, they do not seem to have been unaltered in space since the age of bombardment. What is going on?
The answer to this is that, just as tides determined the geology of the Galilean satellites, collisions dominate the characteristics of the asteroids. Asteroids generally don’t have surfaces much older than a few hundred million years because this is the mean interval between collisions. At least some of the differences in types are due to the collisional disruption of a differentiated object, i.e. one where the metal had been able to settle to the core, leaving a silicate mantle. Finally, it has been found that there are certain families of asteroids that have similar reflectance spectra (thus composed of similar minerals) and are also on similar orbits.
Next time we will see that these processes, and other things
going on in the asteroid belt, have implications for us Earthlings (Men of Earth!)
