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发帖数: 1994 | 1 Welcome to Planet Earth
Credit: Apollo 17 Crew, NASA
Explanation: Welcome to Planet Earth, the third planet from a star named the
Sun. The Earth is shaped like a sphere and composed mostly of rock. Over 70
percent of the Earth's surface is water. The planet has a relatively thin
atmosphere composed mostly of nitrogen and oxygen. The above picture of
Earth, dubbed Blue Marble, was taken from Apollo 17 in 1972 and features
Africa and Antarctica. It is thought to be one of the most widely
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