Star Stuff Of Life
The early planets consist mostly of compounds of heavier elements. As the planets grow, their gravitational fields increase, drawing in nebular dust, planetesimals, and carbon-rich meteorites. Ultimately, Earth is massive enough and cool enough to retain lighter gaseous compounds of carbon, nitrogen, oxygen, and hydrogen, the star stuff from which life will spring.
Middle: A thick atmosphere shrouds the full-sized Earth. Meteorites from this period are the planet's oldest rocks. (Illustration by Ron Miller, from The History of the Earth by William K. Hartmann and Ron Miller © 1991)
Lower: Planetesimals continue to bombard the growing, half-sized Earth.
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