Based on a lunar analogy, one might expect several impactcraters 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。
2
Analysis of the lunar samples and impactcraters 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.