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The above left figure shows the cratering rate for the Moon in units of number of craters with diameter > 1 km formed per year. The above right figure shows the cratering rate for the Earth. Note the frequency of the dinosaur killer, the KT crater. The cratering rate for the land masses on the Earth is roughly 10 times higher (can you think of reasons why?).
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This jump corresponds to the so-called Late Heavy Bombardment which likely produced the large impacts responsible for the maria and perhaps impacts that may have brought in much of our (the Earth's) oceans. Not apparent in the plot, there was also a smaller increase, around a factor of 2.6 in the impact rate about 290 million years ago. |