This is an image showing the cratered surface of the planet Mercury.
Click on image for full size
In addition to being hot, the surface of the Earth was being cratered.
Even though the solar system was finished forming, there were still probably a lot of smaller planetesimals
debris around, too.
gravity of the large planets would attract nearby planetesimals,
which would hit the planets and leave a crater on the planet's
When we look at images of many of the planets such as Mercury, shown here, we see all sorts of
circular craters on the planet surfaces. Most of these craters
were probably formed at this period of history. The cratering eventually tapered off, even though craters are still formed on planetary surfaces today.
Subsequent evolution of a planet's surface, driven by activity within the
planet itself (if the planet were still active), would wipe out any
evidence of this period of cratering. Thus scientists can often trace a planet's age by counting the amount and size of the craters on the surface.
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