though millions of tons of coal rained down from the heavens. Perhaps these
carbon-rich meteorites bore the seeds of life, which might have existed in the
universe eons before Earth came into being. Comets, composed of rock debris
and ice, also plunged into Earth, releasing large quantities of water vapor and
gas.These cosmic gases were mostly carbon dioxide, ammonia, and methane.
Most of the water vapor and gases originated within Earth itself. Magma
contains large quantities of volatiles, mostly water and carbon dioxide, which
made it more fluid.Tremendous pressures deep inside Earth kept the volatiles
within the magma. However, when the magma rose to the surface, the drop
in pressure released the trapped water and gases, often explosively. The early
volcanoes erupted violently because Earth’s interior was much hotter and the
magma contained higher amounts of volatiles.
The early atmosphere contained up to 1,000 times the current level of
carbon dioxide. This was fortunate because the Sun’s output was only about
three-quarters of its present value, and a strong greenhouse effect kept Earth
from freezing solid.The planet also retained its warmth by a high rotation rate
and by the absence of continents to block the flow of ocean currents.
Oxygen originated directly by volcanic outgassing and meteorite
degassing. It was also produced indirectly by the breakdown of water vapor
and carbon dioxide by strong ultraviolet radiation from the Sun. All oxygen
generated in this manner quickly bonded to metals in the crust, much like the
rusting of iron. Oxygen also recombined with hydrogen and carbon monox-
ide to reconstitute water vapor and carbon dioxide. A small amount of oxy-
gen might have existed in the upper atmosphere, where it provided a thin
ozone screen.This would have reduced the breakdown of water molecules by
strong ultraviolet rays from the Sun and prevented the loss of the entire ocean,
a fate that might have visited Venus eons ago (Fig. 10).
Nitrogen, which comprises 79 percent of the present atmosphere, orig-
inated from volcanic eruptions and from the breakdown of ammonia, a mol-
ecule with one nitrogen atom and three hydrogen atoms. Ammonia was a
major constituent of the primordial atmosphere. Unlike most other gases,
which have been replaced or recycled, Earth retains much of its original nitro-
gen.This is because nitrogen readily transforms into nitrate, which easily dis-
solves in the ocean, where denitrifying bacteria return the nitrate-nitrogen to
its gaseous state.Without life, Earth would have long ago lost its nitrogen and
possess only a fraction of its present atmospheric pressure.THE OCEAN
During the formation of the atmosphere, Earth’s surface was constantly in
chaos. Winds blew with a tornadic force. Fierce dust storms on the dry sur-Historical Geology