Characteristics and Origins of the Solar System
Lecture
24
November
5, 2001
Preliminary Announcements: Events of interest in the coming weeks.
Now on to the subject of minor solar system objects.
Chapter 12 begin with a nice discussion of why small objects in the solar system, comets, asteroids, meteorites, are of very great interest. These objects contain primitive material, unlike highly processed rocks on Earth.
I will begin with a discussion of comets. This is because it will add to the anticipation of the Leonid meteor shower, when we will see pieces of cometary material entering the Earth’s atmosphere. The first page of Chapter 12 also gives a nice description of the difference between comets and asteroids.
Comets are striking astronomical objects, unique in appearance, that appear in the sky at (typically) unpredicted times. Let’s look at some pictures of some of them
http://www.solarviews.com/eng/comet.htm
>>>>>>>>>>>>> Picture of comet Ikeya-Seki
Until the 17th century, they were totally unpredicted, and seemed to lack the regularity and clockwork of the other astronomical objects. As such, they were viewed as (usually bad) omens. There are many examples of famous comets in history that were viewed as portents.
(1) A comet that appeared at the death of Julius Caesar, as recorded by Plutarch in the biography of Caesar: “The most signal preternatural appearances were the great comet, which shone very bright for seven nights after Caesar’s death, and then disappeared, and the dimness of the Sun, whose orb continued pale and dull for the whole of that year, never showing its ordinary radiance at its rising, and giving but a weak and feeble heat”.
(2) The comet (now known to be Halley’s comet) which appeared in 66AD at the time of the Jewish uprising against the Roman empire.
(3) The comet (also Halley’s ) which appeared just before the Norman conquest of England, and which is recorded in the Bayeaux tapestry.
There are three aspects of comets I want to deal with: (1) their orbits, (2) their structure, and (3) their chemical composition.
I will begin with the second of these, and defer (1) until later. However, at the outset it should be said that comets travel on highly eccentric elliptical orbits, with a great difference between their perihelia and aphelia.
The Structure of Comets. The layout of a comet is illustrated in Figure 12.13. What we see as the head of a comet is called the coma. Stretching out from the coma are two tails, the dust tail and the ion tail. Spectroscopy reveals the nature of these tails. The dust tail shows the spectrum of reflected sunlight. It is due to small pieces of ice and dust “flaked off “ by the comet, with each following its own orbit around the Sun.
The ion tail glows like an aurora or lightning discharge. It shines with the light of “molecular ions”, i.e. molecules which have lost an electron and are positively charged. The fact that the ion tail and dust tail are separated means different forces act on electrically charged particles than electrically neutral particles. (See cover picture for this chapter to see the appearance of these two types of tails).
It was realized years ago that the coma is really a sort of outflowing atmosphere. It can be thought of as a fog which reflects sunlight and then flows out into interplanetary space. There must be a source of the material which forms the atmosphere. This is the nucleus, which is a small piece of solid material, mainly ice, that sublimates and forms the coma. This nucleus ranges in size from a few hundred meters for the case of a small comet, to the case of Halley, a very prominent comet, with a nucleus which is 6 X 10 kilometers in size. In the case of a giant comet, like Hale-Bopp, the nucleus may get as big as 50 kilometers in its maximum dimension.
All of this was deduced from astronomical observations decades ago. It was confirmed in 1986 by the European spacecraft Giotto, which flew close enough to Halley’s comet to photograph the nucleus. The famous picture return is shown in Figure 12.16 of your textbook. More recently, the NASA spacecraft Deep Space 1 passed close to the nucleus of comet Borrelly and returned pictures of it. The nucleus of this comet was 8 kilometers long, and it was photographed from 2000 miles.
http://nmp.jpl.nasa.gov/ds1/img/borrelly_1.jpg
One of the amazing facts about comets is that an object which visibly extends over distances comparable to an astronomical unit would have their source in a “dirty iceball” only a few kilometers in size.
Orbits of Comets. In the time of Isaac Newton, enough was learned about orbital mechanics that people were able to figure out orbits of celestial objects from a few observations. It was found that comets are moving on elliptical orbits like all other objects we have talked about, but the eccentricities are high.
For the short period comets, eccentricities typically range from 0.40 to 0.95, and semimajor axes are in the range 3 au or larger. In the case of the long period comets, the eccentricities are in excess of 0.990 and semimajor axes of 150 au or greater.
We can use what we have learned so far to reach some interesting conclusions about comets.
1. The visual appearance of comets is determined by sublimation of ices, primarily water ice but also carbon dioxide ice, ammonia ice, etc.
2. Water ice only begins to sublimate at distances from the Sun slightly outside the orbit of Mars. A comet further than 1.5 au would be essentially invisible. The only signal it would display would be reflected light from the nucleus.
3. Kepler’s Second Law would then indicate that most of the time, comets will be dormant, almost invisible objects. (Question: explain why that follows)
The Oort Cloud. In the 1920’s, the Dutch astronomer Jan Oort put together some of the observed characteristics of long period comets to reach an astonishing conclusion.
1. Comets coming into the inner solar system are a common phenomenon. One is typically discovered every few months.
2. The fraction of comet’s lifetime that it spends near the Sun is, via Kepler’s Second Law, extremely tiny. The total number of comets out in space must be larger by the ratio of orbital period to the time it spends close to the Sun.
3. The properties of these orbits (very long periods, eccentricities extremely close to unity) means that these comets spend almost all of their time way out in space. The numbers one comes up with are thousands to tens of thousands of astronomical units from the Sun.
4. We only see those comets whose orbits carry them close enough to the Sun to cause gases like water to sublimate. We cannot see the ones that cruise by 10 or 20 astronomical units from the Sun. The net conclusion is once again that the total number of comets in the solar system must be huge.
We thus deduce the existence of Oort’s Cloud, a giant cloud of comets in the extreme distances of thousands to tens of thousands of astronomical units from the Sun. The total number of comets in the Oort Cloud is deduced to be 1012 .
An interesting calculation carried out in the book is that the total mass of material in these comets is comparable to that in the major planets, and larger if you make educated guesses about comets between the outer solar system and the Oort Cloud. Thus, the dominant form of solid material in the solar system may not be planets at all, but the ice of these comets.
Yet another “belt” or “cloud” is the Kuiper Belt, composed of a population of objects out beyond the orbit of Neptune. These have orbits which are pretty close to the plane of the ecliptic, and probably have compositions similar to that of comet nuclei. The only point is that, at least the biggest ones, have diameters of order 200 kilometers or larger. As I mentioned before, these objects seem generically related to Pluto. It would be great if one of these got knocked into the inner solar system to produce a super-comet!
Comets almost certainly comprise pieces of matter left over from the formation of the solar system. They cannot have undergone any significant processing. There is therefore interest in getting samples of this material to study the distribution of elements and isotopes, see what chemicals (including organic chemicals) are there, and to check the nature of small particulate matter that probably antedates the formation of the solar system.
In January, 2004, the Stardust spacecraft will pass close to a comet, and collect samples to bring back to a laboratory here on Earth. This will provide unique samples of this primitive solar system material.
http://stardust.jpl.nasa.gov/mission/details.html
