This science article explains comets and what they are made of.
We learn about their nuclei, ‘comas’, hydrogen envelopes, gas and vapour tails. We also admire the exploits of Bart Simpson in his role as comet hunter.
Seymour Skinner, the Acting Principal of Springfield Elementary School, dreams of becoming a famous astronomer. His fame will be guaranteed if only he can discover a new object in the heavens unknown to astronomy. Skinner enlists Bart’s help in scanning the skies.
After only a few minutes it is Bart, and not Skinner, who strikes it lucky! Bart discovers a new comet …
… and immediately reports his finding to the authorities.
In honor of this amazing discovery the comet is named ‘Comet Bart Simpson’.
to be continued….!!
In view of all the publicity surrounding the discovery of Comet Bart Simpson John Frink, college professor at Springfield Heights Institute of Technology, feels that it is his duty to explain what comets are. Less intelligent people (with an IQ less than his IQ of 197) need to pay attention!
The nucleus is the solid body of the comet. Nuclei of comets are generally under 10 miles (16 kms) in diameter. This image shows the nucleus of comet Tempel 1. It is 4.7 miles long and 3.0 miles wide.
The nucleus of Tempel 1 is covered in a dusty powder that is finer than sand. Features of its ‘landscape’ include impact craters from colliding meteorites, steep slopes and cliffs. The interior of the comet has been described as ‘fluffy’ because it is lightweight and full of holes- just like a sponge. It has been calculated that Tempel 1 is 75% porous and has no solid core.
Water ice is clearly visible on the surface of the comet.
Tempel 1 is formed of large amounts of dust and loosely packed grains of ice and includes some frozen liquified gases. The comet is nothing but a giant, dirty snowball!
It is widely believed that the numerous giant dirty snowballs (aka ‘comets’) that fell to earth billions of years ago supplied essential raw materials, such as carbon and water, that were needed to create life on earth. This image shows comets falling on a young earth against the back drop of the (much closer) moon.
As a comet’s orbit takes it nearer the sun its nucleus starts heating up.
As the nucleus heats up, carbon dioxide ice locked inside interior cavities, also starts to warm. This carbon dioxide ice now changes from a solid state directly to a gaseous state through a process called ‘sublimation’.
The diagram below shows how ‘sublimated’ carbon dioxide is forced out of cracks in the nucleus and spurts into space. These columns of ‘sublimated’ carbon dioxide gases are called ‘jets’. Jets are rapid streams of gas forced out of cracks in the comet.
‘Jets’ occur as a result of differences in pressure. Pressure in the cavities is far greater than the pressure outside in space. This means that carbon dioxide gas is ejected from the cavities at incredibly high speeds. It is not only gases that spurt out of the jets; dust and water ice are also ejected……
Professor Frink would like to announce that there are four features of a comet nucleus that we should all know about.
He emphasizes that these four features exist only when a comet’s orbit takes it near the sun.
1) Professor Frink’s Guide to comets- the coma
The material forced out through jets forms a ‘coma’ surrounding the nucleus. The coma is a comet’s atmosphere and can be thousands of times bigger than the nucleus.
The coma of Comet Holmes has a distinctly green appearance…
…while comet Ison has an impressively bright coma.
2) Professor Frink’s Guide to comets- the hydrogen cloud
The warming of the nucleus leads to the melting and evaporation of water ice from the surface of the comet. Water vapour is also ejected into the coma through the jets.
Water molecules in the water vapour absorb ultra violet light from the sun. Water molecules then break up; this leads to the creation of hydrogen molecules. Hydrogen molecules form the ‘hydrogen envelope’ which surrounds the ‘coma’.
The shape of the hydrogen envelope is distorted by the influence of solar winds. The envelope gets bigger as the comet approaches the sun and smaller as it recedes for the sun.
The envelope cannot be seen from earth but it can be detected by space telescopes.
The blue image below, captured by the Hubble space telescope, shows Comet Hyakutake’s hydrogen envelope. At 8 700 miles (14 000 kms) in diameter its hydrogen envelope is massive! The red image shows sunlight reflected off particles of dust in the coma.
3) Professor Frink’s Guide to comets- the dust tail
The dust tail (colored brown in this diagram) always points away from the sun and always curves towards the comet’s orbital path.
Dust tails are made of tiny dust particles about the same size as particles in cigarette smoke.
High speed solar winds ‘blow’ dust away from the coma leading to the formation of the dust tail. Because it reflects sunlight the dust tail is often the easiest part of the comet to see.
As the nucleus approaches the sun, dust tails become extended and can be millions of miles long. The image below, captured from Chile in 2007, shows the dust tail of Comet Mcnaught over the Pacific Ocean.
4) Professor Frink’s Guide to comets- the gas tail
The gas tail forms after solar winds from the sun collide with atoms of gas (including water and carbon dioxide atoms) in the coma.
Following these collisions atoms of gas in the coma become electrically charged and start to glow. Gas tails (also called ion or plasma tails) can be seen because they generate their own light. This image shows the long gas tail of Comet Swan.
The same process which forms gas tails also forms auroras over the polar regions of the earth. This is the Aurora Australis over the Southern Hemisphere. (Also read Earth as a Magnet )
Not only is a comet nucleus small in size but its mass is also small. (See Understanding Gravity and Mass)
A comet nucleus has little mass in its cavities and the ‘stuff’ inside it, such as water and carbon dioxide ice, is not at all dense. As a result of having a low density interior, its force of gravitational attraction is weak.
For instance, comet Hartley’s force of gravitational attraction would allow Bart Simpson to stand on top of the comet and not float off into space. However if he decided to jump he would float off never to return….
A comet’s gravitational field is therefore too weak to have a stable coma’ (or atmosphere) surrounding it. Gas and dust ejected through the jets completely overwhelm the comet’s gravitational field and leak into space.
The problem is made worse by the absence of any protective magnetic field that could shield the comet from harmful solar radiation.
‘Comet Bart Simpson’ is heading straight for Springfield!
The residents of Springfield are horrified about the impending disaster that awaits!
The Simpsons remain calm and seek out an advantageous spot from which to view the incoming comet!
Professor Frink hatches a cunning plan to destroy the comet with a ground to air missile. The missile is fired but misses it target!
Disaster is averted after the comet breaks up in the polluted atmosphere above Springfield. Only a small lump of rock (as big as a chihuahua dog’s head) survives! Springfield and its inhabitants are saved!!