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Impact Craterin1 - space the more craters it will have If...

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Impact Cratering Impact cratering was especially prevalent for the first several hundred million years after the planets formed as the planets swept up left-over material. The last stage of that "sweeping up" is called the late heavy bombardment occurred from about 4.1 to 3.8 billion years ago. Impacts as large as the one that led to the demise of the dinosaurs in much more recent history were happening about once a month. Most of the impact basins---craters measured in hundreds of kilometers---were made during this time. It is noteworthy that about the time the heavy bombardment ended, life took hold. The oldest fossil evidence of ancient organisms dates back to 3.5 billion years ago and evidence for biological activity based on isotopic ratios of carbon may date back to about 3.85, even up to 4.2 billion years ago, though the carbon isotope ratio evidence is controversial. The number of craters per unit area on a surface can be used to determine an approximate age for the planet or moon surface if there is no erosion. The longer the surface has been exposed to
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Unformatted text preview: space, the more craters it will have. If you know how frequently craters of a given size are created on a planet or moon, you can just count up the number of craters per unit area. This assumes, of course, that the cratering rate has been fairly constant for the last few billion years. The heavy bombardment of about 3.8 billion years ago must be taken into account when using the crater age dating technique. For example, the highland regions on the Moon have ten times the number of craters as the maria, but radioactive dating (explained in the next chapter ) shows that the highlands are approximately 500 million years older than the maria, not ten times older. At a minimum crater-age dating can tell you the relative ages of surfaces (which surface is older than another). Careful studies of how the craters overlap other craters and other features can be used to develop a history or sequence of the bombardment on the moons and planets....
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