2024年3月24日发(作者：小天才平板电脑)

Planets in Our Solar System

The Sun is the hub of a huge rotating system consisting of nine planets, their satellites,

and numerous small bodies, including asteroids, comets, and meteoroids. An estimated

99.85 percent of the mass of our solar system is contained within the Sun, while the

planets collectively make up most of the remaining 0.15 percent. The planets, in order of

their distance from the Sun, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus,

Neptune, and Pluto. Under the control of the Sun's gravitational force, each planet

maintains an elliptical orbit and all of them travel in the same direction.

The planets in our solar system fall into two groups: the terrestrial (Earth-like) planets

(Mercury, Venus, Earth, and Mars) and the Jovian (Jupiter-like) planets (Jupiter, Saturn,

Uranus, and Neptune). Pluto is not included in either category, because its great distance

from Earth and its small size make this planet's true nature a mystery.

The most obvious difference between the terrestrial and the Jovian planets is their size.

The largest terrestrial planet, Earth has a diameter only one quarter as great as the

diameter of the smallest Jovian planet, Neptune, and its mass is only one seventeenth as

great. Hence, the Jovian planets are often called giants. Also, because of their relative

locations, the four Jovian planets are known as the outer planets, while the terrestrial

planets are known as the inner planets. There appears to be a correlation between the

positions of these planets and their sizes.

Other dimensions along which the two groups differ markedly are density and

composition. The densities of the terrestrial planets average about 5 times the density of

water, whereas the Jovian planets have densities that average only 1.5 times the density

of water. One of the outer planets, Saturn, has a density of only 0.7 that of water, which

means that Saturn would float in water. Variations in the composition of the planets are

largely responsible for the density differences. The substances that make up both groups

of planets are divided into three groups—gases, rocks, and ices—based on their melting

points. The terrestrial planets are mostly rocks: dense rocky and metallic material, with

minor amounts of gases. The Jovian planets, on the other hand, contain a large

percentage of the gases hydrogen and helium, with varying amounts of ices: mostly water,

ammonia, and methane ices.

The Jovian planets have very thick atmospheres consisting of varying amounts of

hydrogen, helium, methane, and ammonia. By comparison, the terrestrial planets have

meager atmospheres at best. A planet's ability to retain an atmosphere depends on its

temperature and mass. Simply stated, a gas molecule can "evaporate" from a planet if it

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reaches a speed known as the escape velocity. For Earth, this velocity is 11 kilometers per

second. Any material, including a rocket, must reach this speed before it can leave Earth

and go into space. The Jovian planets, because of their greater masses and thus higher

surface gravities, have higher escape velocities (21-60 kilometers per second) than the

terrestrial planets. Consequently, it is more difficult for gases to "evaporate" from them.

Also, because the molecular motion of a gas depends on temperature, at the low

temperatures of the Jovian planets even the lightest gases are unlikely to acquire the

speed needed to escape. On the other hand, a comparatively warm body with a small

surface gravity, like Earth's moon, is unable to hold even the heaviest gas and thus lacks

an atmosphere. The slightly larger terrestrial planets Earth, Venus, and Mars retain some

heavy gases like carbon dioxide, but even their atmospheres make up only an

infinitesimally small portion of their total mass.

The orderly nature of our solar system leads most astronomers to conclude that the

planets formed at essentially the same time and from the same material as the Sun. It is

hypothesized that the primordial cloud of dust and gas from which all the planets are

thought to have condensed had a composition somewhat similar to that of Jupiter.

However, unlike Jupiter, the terrestrial planets today are nearly void of light gases and

ices. The explanation may be that the terrestrial planets were once much larger and

richer in these materials but eventually lost them because of these bodies' relative

closeness to the Sun, which meant that their temperatures were relatively high.

1. According to the passage, each of the

following statements comparing terrestrial planets with Jovian planets is true EXCEPT:

○Terrestrial planets are closer to the Sun than Jovian planets.

○Terrestrial planets have smaller diameters than Jovian planets.

○Terrestrial planets have smaller masses than Jovian planets.

○Terrestrial planets travel in a different direction than Jovian planets do.

Paragraph 4: Other dimensions along which the two groups differ markedly are density

and composition. The densities of the terrestrial planets average about 5 times the density

of water, whereas the Jovian planets have densities that average only 1.5 times the

density of water. One of the outer planets, Saturn, has a density of only 0.7 that of water,

which means that Saturn would float in water. Variations in the composition of the planets

are largely responsible for the density differences. The substances that make up both

groups of planets are divided into three groups—gases, rocks, and ices—based on their

melting points. The terrestrial planets are mostly rocks: dense rocky and metallic material,

with minor amounts of gases. The Jovian planets, on the other hand, contain a large

percentage of the gases hydrogen and helium, with varying amounts of ices: mostly water,

ammonia, and methane ices.

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