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The last stage of that "sweeping up", called the 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.
Terrestrial planets have hard surfaces that can be re-shaped by several different processes: impact cratering, volcanism, erosion, and tectonics.
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The craters on all of the moons except Io, Mercury, and most of the ones on Mars are from impacts.
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 rock on the surface of the planet or moon is bent backward, upward, and outward so the amount of material ejected is much larger than the projectile.Large craters will have a central peak formed by the rock beneath the impact point rebounding upward and they may also have terracing of the inner walls of the crater from the collapsing of the crater rim inward.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.