英文阅读--八大星系知识
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Mercury（水星） In 1974, for the first time, as U.S. spacecraft passed within 400 miles of the planet Mercury. The name of the spacecraft was mariner 10. There were no people on the spacecraft, but there were cameras that could take clear pictures from a long distance. What the pictures showed was interesting. They showed that Mercury's surface was a lot like the surface of the Moon. The surface of Mercury is filled with huge holes called craters. A layer of fine dust covers Mercury. This, too, is like the dust on the Moon. There is no life on either Mercury or the Moon. Venus（金星） For many years, no one knew much about Venus. When people looked through telescopes, they could not see past Venus' clouds. Long age, people thought the clouds covered living things. Spacecraft radar has shown this is not true. Venus is too hot for life to exist. The temperature on Venus is about 900 degrees! Remember how hot you were last time it was 90 degrees? Now imagine it being 10 times hotter. Nothing could exist in that heat. It is also very dry on Venus. For life to exist, water must be present, because of the heat and dryness, we know there are no people, plants or other life on Venus. Earth（地球） One planet in out solar system certainly supports life—Earth. Our planet is the third planet from the sun and it takes 365 days, or a year, to orbit the sun. This rotation makes it possible for most of our planet to have four seasons—winter, spring, summer and fall. Besides being able to support life, our planet is unique in another way—Earth is 75% covered by water. No other planet has that much, if any liquid on its surface. This liquid and its evaporation help provide the cloud cover and our climate patterns. Earth has one natural satellite—the moon. Scientists and other experts all over the world have created and sent into orbit other satellites used for a variety of purposes—communication, weather forecasting, and so on. Mars（火星） The U.S. has sent many unmanned spacecrafts to Mars since 1964. (Unmanned means there were no people on the spacecraft.) That is why scientists know a lot about this planet. Mars has low temperatures. There is no water on Mars. There is only a gas called water vapor. There is also ice on Mars. Scientists have also learned that there is fog on Mars in the early morning! Do you remember when you last saw fog here on Earth? Scientists say the fog on Mars looks the same. As on Earth, the fog occurs in low-lying areas of the ground. Another interesting thing about Mars is that it is very windy. The wind blows up many dust storms on this planet. A spacecraft called mariner 9 was the first to take pictures of dust storms. Later, the unmanned Viking spacecraft landed on the surface of Mars. Jupiter（木星） Jupiter, the fifth planet from the sun, is circled by a ring of dark particles. It takes this planet almost 12 years to orbit the sun. Jupiter's ring is very difficult to see from Earth without using special equipment. Jupiter is the largest planet in our solar system. It is 11 times bigger than Earth! Scientists have been able to learn much about this planet because of the information received from voyager 1 in 1979. They know that we cannot send a spacecraft to land on the surface of Jupiter as we have done with the moon. The surface of Jupiter is not solid. The outer"shell"of Jupiter is gas. Saturn（土星） Have you looked at Saturn through a strong telescope? If you have, you know it has rings. Saturn is the most beautiful planet to see! It is bright yellow. It is circled by four rings. Two bright rings are on the outside of the circle. Two dark rings are on the inside. The rings of Saturn are made of billions of tiny bits of rocks. The rocks travel around the planet in a swarm. They keep their ring shape as the planet travel around the sun. These rings shine brightly, and so does the planet Saturn. Both reflect the rays of the sun. The sun is 885 million miles away from Saturn. It takes Saturn 29.5 years to travel around the sun! Uranus（天王星） William Herschel discovered the planet Uranus in 1781. As has happened many times throughout history with other scientists, inventors and explorers, he didn't realize he had found a planet—he thought it was a comet. Scientists didn't know too much about this planet, though, until 1986 when the U.S. spacecraft voyager 2 flew past it. Do you think the planet Earth is big? Well, the planet Uranus is four times bigger! Uranus is another planet that has rings. While Saturn's rings are made of ice, the rings of Uranus are made of dark particles the size of boulders. Earth has one natural satellite—the moon—but Uranus has 15 natural satellites. It takes Earth 1 year to circle the sun, but Uranus 84 years! Uranus is the seventh planet from the sun. Neptune（海王星） 本文来源：https://www.wddqw.com/doc/a2ef12bbde88d0d233d4b14e852458fb770b3862.html