Based on a lunar analogy, one might expect several impact craters of at least 500km diameter to have been formed on Earth.
基于月球的对比，可以认为一些直径至少500公里的撞击坑在地球上形成。
Analysis of the lunar samples and impact craters has shown that the Moon's surface was solid 4.3 billion years ago, yet the oldest impact rocks among the samples are 3.9 billion years old.
对月亮石样本和影响的火山口的分析表明月亮表面在43亿年前凝固，然而，样本里面最古老的影响岩石却只有39亿年。
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