A Walk Through Time

Star Stuff Of Life

4500 Million Years Ago

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.

Top: The early Moon, still quite close to Earth, casts a stunning shadow. (Illustrations, top and bottom, by William K. Hartmann, from The History of the Earth, by William K. Hartmann and Ron Miller © 1991)

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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