Characteristics and Origins of the Solar System
Lecture
26, Topic 17
April
8, 2004
Comets and Cometary Material
Last time, we saw that the major planets are not the only objects (other than the Sun) in the solar system. There are thousands of known asteroids, rocky little worlds which get as large as 910 kilometers in diameter. This isn’t that much smaller than Pluto.
Today, I will begin talking about another major class of
non-major-planet objects in the solar system.
Unlike asteroids, they often become some of the most striking objects in
the sky. With luck, we will have a chance to see one at the end
of the semester.
Comets
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
The comet below is Comet Ikeya-Seki, which appeared just like this in Iowa in late 1965.
Another example (which also looked just like this picture) was Comet West in 1975.
There are pictures of other comets in your textbook.
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
15.20.
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.
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 blackness of the nucleus is a realistic portrayal. Comet nuclei appear to be covered with the famous “black gunk” that covers the surfaces of satellites in the outer solar system and makes up the Carbonaceous Chrondite meteorites.
More recently, the NASA spacecraft Deep Space 2 passed close to the nucleus of Comet Borrelly and returned pictures of it. The length of this object is about 8 kilometers.
Once again, keep in mind that 20 years ago, there were no pictures like this.
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.
This diagram will make you a believer in ellipses! Believe it or not, Comet Halley is not particularly eccentric as comets go.
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)
Check out the 2nd Law here…
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 illustration of the Oort Cloud is shown in Figure 15.25 of the book.
Be sure and notice the incredible size of the Oort Cloud system. But the aforementioned reasoning steps of Oort show that it must exist.
An interesting calculation presented in another textbook 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.
In 2002, a very large Kuiper Belt object with a semimajor axis of 42 au, and a diameter of 1300 km was discovered. It was named Quaoar by its discovers. An artist’s conception of this distant, incredibly cold, dark object is shown below.
This object is clearly in the same class as Pluto, and both of them are in a different class than the major planets. It seems completely obvious that there are many, many more of these 1000+ kilometer objects waiting out there to be discovered. Estimates are that there are perhaps 70,000 Kuiper Belt objects with diameters greater than 100 km. 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.
This year, just as the semester was starting, there was another big development in solar system astronomy. The Stardust spacecraft passed close to the comet Wild 2. A picture of the spacecraft is below.
It took pictures of the cometary nucleus at closest approach (see below)
More importantly, the Stardust spacecraft had special panels to trap pieces of the comet as they were blown out by the nucleus. These pieces were trapped, brought back inside the spacecraft, and are now on their way back to Earth for analysis. They will be retrieved when the spacecraft returns in January, 2006. Stay tuned!
Stardust also returned a great movie of the closest approach to the nucleus of the comet. S
This is available at the Stardust website at
This shows pictures of the nucleus as the relative motion of the cometary nucleus and spacecraft brought them past each other. Again keep in mind that you are seeing the tiny, solid “heart” of the comet deep inside a huge coma and tail that we see from Earth.