Impact Craters

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The number of impact craters can provide a probable lower limit on the age of the Earth. Asteroid strikes that can produce craters on the order of kilometers across are extremely infrequent occurrences; the chance of an asteroid with an Earth-crossing orbit actually striking the planet has been estimated at 2.5 x 10−9 yr−1, and when multiplied by the estimated number of Earth-crossing asteroids this approximates about one collision for every 3.2 million years1.

The collision estimates given here conflict with those given here, from which I quote:

Counting the number of asteroids we see in the sky suggests that over the past 250 million years, Earth should have been hit around 440 times by asteroids larger than one kilometre across. But scientists have found only 38 large impact craters from this period.

Dividing the expected number of impacts(440) by the years that number should have been reached(250,000,000) gives us an estimated impact frequency of one impact(from an asteroid >1km across) every 568,181 years. Half a million is quite a bit different than 3.2 million! Can someone research this further?

If this frequency is correct, the number of impact craters on Earth were it only a few thousand years old should be very few. The most logical number of observable >1km impact craters for a young earth would in fact be something like zero — a number that is completely at odds with the observable evidence, since over one hundred such craters have been discovered2.(3)

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