Comet Hale Bopp.
Comet Hale Bopp.
Comets are perhaps the most dramatic astronomical objects that humans can observe with their naked eyes. Yet, they are just the debris left over from forming the solar system - cosmic junk. But very valuable junk - comets retain vital clues about the early history of the solar system. We shall study the following aspects of comets:
Here we see the different parts of a comet - the nucleus, coma and tail. When we see a comet in the sky we are seeing the coma - gases and dust that have been vaporized from the body of the comet as it is heated up by the warmth of the Sun. We are seeing the gases and dust, that have expanded far away from the tiny nucleus, scatter sunlight. Large comets are ejecting as much as 30 tons of material every second when they pass closest to the Sun. This material is composed of volatile compounds - material that is a solid at the low temperatures of the outer solar system (about 50K) but turns into gas at temperatures in the inner solar system (about 300K) - materials like water, carbon dioxide, ammonia, etc.; Mixed in with these volatiles are dust particles that are carried away from the comet by the jets of gas. The best-studied comet, Comet Halley, is thought to have the following composition:
The tail of a comet has two components: a dust tail (which trails behind the comet in its orbit around the Sun) and an ion tail (which is pushed radially away from the Sun by the solar wind).
Comets orbit the Sun, just like all the other solar system objects, but their
orbits have very high eccentricity.
Here is a plot of the orbit of Comet Hyakutake. This and other comet orbits
are on the
Minor Planets website.
Figure 7-9 shows how the dust and ion tails grow and change their orientation
as the comet passes through perihelion. Unlike the planets, which all
orbit the Sun in the same direction (prograde), comets can have both
prograde and retrograde orbits.
Where do comets come from? Dutch astronomers seem to like comets. The Kuiper
Disk is the region between 50 and 150 A.U. where there are about
1 billion (CRUDE ESTIMATE) icy bodies - between the size of comets (10km)
and Pluto (2000 km). Another Dutch astronomer, Oort, proposed that there is
a cloud of 100 billion or so comets extending out to 100,000 A.U..
Or more--this is the spherical Oort cloud. Oort clouds are seen around
other stars.
Here we see the shapes and sizes of these two clouds of comets. Notice that comets from the Oort cloud can have any orbital inclination but comets from the Kuiper Belt will tend to have orbits that are closer to the eccliptic plane, with low inclinations.
Occasionally, these billions of comets bump into each other. extremely occasionally, one gets too close to a Pluto-sized object and is "kicked" into the inner solar system where it follows the stages described above. We will only be able to see the comet when it gets inside about 4-5 AU and is sufficiently warmed by the Sun that volatile materials evaporate from the comet's surface. The Oort Cloud is the source of long period comets while the Kuiper Belt is the source of the short period comets.
What happens to the surface of the comet after the volatiles are removed? We really do not know what the surface of a nucleus of a comet looks like. By following comets with large telescopes after they have passed the Sun and are retreating back out into the solar system, we can tell that the surface is very dark - in fact, as black as coal (albedo ~ 0.04). This suggests that after the volatiles are removed either a pile of "rubble" remains or the exposure of the outside of the nucleus to energetic particles and sunlight produces a dark "crust". After several passages past the Sun much of the volatiles near the surface gets removed and the comet becomes "dead" - sometimes looking more like an asteroid than a comet.
If a comet passes close to a planet (particularly Jupiter, of course) then the tidal forces - the difference in gravity across the object - can pull the comet apart. Remember Comet Shoemaker-Levy-9. When it first passed Jupiter SL9 was broken into many fragments. The second time it passed Jupiter - the fragments hit Jupiter in a dramatic display of explosions. We are learning that this was not perhaps such an unusual event - comets often break up. Evidence of such comet distruction is provided by meteor showers. Material from the break up of a comet continues along the orbit of the comet. If the Earth happens to cross the orbit of a comet that has broken up then material will bombard the Earth's atmosphere - and we see a shower of material vaporizing in the atmosphere.
Links:
In class we made up lists of 10 things we knew about each swarm and 5 things we would like to know.
Asteroids:
Know:
10,000s - mostly (but not all) tracked - see this plot for all of the known asteroid orbits
Some are inside 1 AU - plot of innermost
Range of compositions
Organized in families
Degrees of differentiation
Range of densities
Maximum size ~ 500km - small - only a couple are round
Satellites - several of them have moons
like to know
Are they solid or "rubble piles"?
Did they bring water to Earth?
When will next one hit Earth?!
Are they worth mining?
Kuiper Belt
Knowns:
487 detected as of last count (see the list and plot)
Between ~30 AU and 100 AU - is there an outer edge?
Many are in resonances with Neptune.
Largest, Quahua, is between Pluto and Charon.
Some are similar in size to Charon - BUT we really do not know size - just assume using Albedo~0.04
Formed when Uranus and Neptune migrated outwards (according to Hal Levison)
Composition varied - but poorly determined - mostly just some idea of color
Like to Know:
Composition?
Did they really form when U&N migrated?
Are they differentiated?
How do they supply the "Jupiter Family Comets" to the inner solar system?
Why did the amount of mass in the solar system drop off so dramatically at about 30 AU?
Oort Cloud of Comets:
Knowns:
Millions of them.
1000s of AU
Millions of years orbits
~10 km - VERY small
Ice and volatiles - dirty snow balls
Formed when kicked out by giant planets - scattered by galactic tides to form sphere.
Also scattered in to inner solar system - some burn up close to sun (hit?) - see the movies!
Most comets only last about 6 or so orbits once they get into the inner solar system
Broken up comets in the inner solar system cause the regular meteor shows - eg last week's Leonids.
Like to Know:
How many comets / how much mass in the Oort cloud?
How /when did the comets get there?
Did they bring water to Earth?
How do jets form on comets?
Are they single, solid lumps or loose rubble piles of ice and rock?
