It is impossible to estimate the age of the universe.
Many compositional features of the earth, including its abundant surface water, are unique in the solar system.
The starting composition of the solar nebula can be inferred from studies of stars.
It is impossible to know the internal composition or structure of the earth because geologists cannot sample the interior.
The least dense minerals are found in earth's core.
Once internal differentiation was complete, the earth's surface remained essentially unchanged to the present.
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