Structure and Composition of Comets
 |
| TOEFL reading: All about comets |
Passage
Astronomers now have a fairly good idea of what a comet really is. When it is far from the Sun, it is a very small object only a few kilometers across It consists mainly of ices (water, methane, ammonia) with bits of dust embedded in it - a kind of dirty ice ball. As it approaches the Sun, radiation from the Sun vaporizes the icy matter and releases some of the dust. This forms a gigantic halo around the ice ball. This halo - called the coma - extends out tens of thousands of kilometers from the icy core, which is the nucleus of the comet. Sunlight reflected off the dust particles makes the coma visible to observers on Earth. Ultraviolet radiation from the Sun breaks down the vapor molecules into their constituents. These components can be excited by absorbing radiation from the Sun. In returning to lower-energy states, the excited atoms and ions emit light, contributing to the luminosity of the coma.
When the comet approaches the Sun = dirty ice ball + halo (= coma)
When the comet gets even closer to the Sun, one of its most spectacular parts begins to form, the tail. Actually, there are two kinds of tails the dust tail and the ion tail. The dust tail is produced by the light from the Sun reflecting off the dust particles in the coma. A photon carries momentum. In bouncing off a dust particle, it imparts a tiny, but perceptible, momentum change to the dust particle, driving it away from the coma. As the comet sweeps along its orbit, it leaves a curving trail of dust behind in its path. This visible dust tail can extend for tens or hundreds of millions of kilometers out from the nucleus. The dust tail is characterized by its gently curving shape and its yellowish color.
When the comet gets even closer to the Sun = dirty ice ball + halo (= coma) + tails
dust tail =
- produced by the light from the Sun reflecting off the dust particles in the coma
- curving, yellowish trail of dust behind in its path
A different mechanism is responsible for the ion tail. Near the Sun, ultraviolet radiation from the Sun (solar wind) ionizes and excites the atoms in the coma. As the solar wind sweeps through the coma, the high-velocity charged particles of the solar wind interact with the electrically charged excited ions in the coma, driving them away from the head of the comet. In returning to lower-energy states, these excited ions emit photons and form a luminous, bluish-colored tail extending out from the comet directly away from the Sun. Since both kinds of tails are produced by radiation streaming out from the Sun, they extend out from the coma in the general direction away from the Sun. A comet may exhibit several tails of each kind.
ion tail =
- produced by the interaction between solar wind and ions in the coma
- bluish-colored tail extending out from the comet directly away from the Sun.
Although the nucleus is of the order of a few kilometers in size, the diameter of the coma may be tens or hundreds of thousands of kilometers, the tails typically extend out tens or hundreds of millions of kilometers away from the coma.
nucleus < coma < tail
A comet leaves a trail of matter behind it as it moves through the inner solar system. Some of this debris may get strewn across Earth's orbit around the Sun. When Earth passes through this part of its annual path, it sweeps through the dust trail. The particles enter Earth's atmosphere at high velocity. The air friction can cause one of these bits of matter to produce a brief streak of light as it burns up in the atmosphere.
Some left behind comet matter can enter the earth when the earth passes through the comet's orbit path
Paragraph 6. Since a comet loses matter on each pass by the Sun, eventually it will be depleted to the point where it is no longer visible. Comets that approach the Sun have finite lifetimes. Given the typical sizes of comets and the typical rates at which they lose matter, astronomers have concluded that the lifetimes of comets with orbits that bring them near enough to the Sun to be seen from Earth are very much shorter than the age of the solar system. Where do the new comets come from to replace the old ones that dissipate and vanish from view?
Comets visible from the earth have much shorter life than the age of the solar system
Paragraph 7. Dutch astronomer Jan Oort proposed that a giant cloud of matter left over from the formation of the solar system surrounds the Sun and extends out to about 50,000 astronomical units. This cloud contains large chunks of matter like the nuclei of comets. The gravitational influence of a passing star can be sufficient to perturb the orbit of one of these chunks to send it toward the inner solar system and bring it near the Sun.
Oort proposed that a giant cloud made of the remnant material of the solar system surrounds the edge of the solar system. the material = nuclei of comet. The gravitational influence of a passing star can cause some material to move toward the Sun.
Hard Questions
Q: What does paragraph 6 imply about comets that are visible from Earth?
- A.The visible comets grow brighter as they grow older
- B.The comets that are visible from Earth today have not been orbiting close to the Sun throughout the history of the solar system
- C.There were fewer visible comets during the early history of the solar system than there are today.
- D.The average lifetime of the visible comets is decreasing.
Analysis
P6 says that, since a comet loses matter on each pass by the Sun, it will eventually lose all the material and become invisible. It also states that, for this reason, comets visible from the earth have much shorter life than the age of the solar system. From this we can infer that visible comets are not the "permanent" fixtures of the solar system: comets die. So if we can see a comet, then the comet must have been created more recently in the history of the solar system (= it must have been born way after the birth of the solar system).
Answer: B
Q: According to paragraph 7, how did astronomer Jan Oort contribute to our understanding of comets?
- A.He theorized that comets form when passing stars push chunks of debris from the outer to the inner solar system
- B.He proposed a way of comparing comets in our own solar system with those that orbit other stars.
- C.He explained that the lifetime of a comet depends on its distance from the Sun at the time the solar system was formed.
- D.He proved that old comets do not really disappear; they just move beyond the limits or the solar system.
Analysis
Oort proposed that a giant cloud made of the remnant material of the solar system surrounds the edge of the solar system and some of the material can be sent to the inner orbit by the movement of a passing star. So we can infer that this (a star causing some material to move toward to the Sun) is how comets were born.
Answer: A
Dr. Byrnes' comments: this question is not good since it relies on the definition of "comet," which is not provided in the passage. if we think of comets in terms of their raw materials, then they were made of the leftover material when the solar system was formed, as said in paragraph 7. So comets could not have been formed by the passing-by stars. But if we think of comets as those that we can see, then they are formed by the passing-by stars. To make A correct, the q should have been like this:
how did astronomer Jan Oort contribute to our understanding of comets that are visisble to us?