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In the absence of an atmosphere to hold in the internally generated heat,
the surface rapidly cooled, forming a thin basaltic crust similar to that of
Venus. Indeed,the Moon and the inner planets offer clues to Earth’s early his-
tory. Among the features common to the terrestrial planets was their ability to
produce voluminous amounts of basaltic lavas. Earth’s original crust has long
since disappeared, remixed into the interior by the impact of giant meteorites
that wereleftovers from the creation of the solar system.
The formative Earth was subjected to extensive volcanism and intense
meteorite bombardment that repeatedly destroyed the crust.A massive meteorite
shower, consisting of thousands of 50-mile-wide impactors, bombarded Earth
and the Moon around 3.9 billion years ago. The other inner planets and the
moons of the outer planets show dense pockmarks from this invasion (Fig. 5).
The meteorite bombardment melted large portions of Earth’scrust, nearly half
of which contained large impact basins up to 10 miles deep.
As the meteorites plunged into the planet’s thin basaltic crust, they
gouged out huge quantities of partially solidified and molten rock. The scars
Figure 4The structure of
Earth, showing the core,
mantle, and crust.7PLANET EARTHInner
coreCCrruusstt
AsthenosphereTransitionzoneLowermantleOutercore