THE ATMOSPHERE
For the first half billion years, while Earth was spinning wildly on its axis and sur-
face rocks were scorching hot, the planet lacked an atmosphere. It was in a near
vacuum much like the Moon is today. Soon after the meteorite bombardment
began about 4.2 billion years ago, Earth acquired a primordial atmosphere com-
posed of carbon dioxide, nitrogen, water vapor, and other gases spewed out of a
profusion of volcanoes along with gases such as ammonia and methane delivered
by a fussilade of comets.The atmosphere was so saturated with water vapor that
atmospheric pressure was nearly 100 times greater than it is today.
Much of the water vapor and gases came from outer space. Some mete-
orites that visited Earth were stony composed of rock and metal, others were
icy composed of frozen gases and water ice, and many contained carbon as
Figure 9View of a full
Moon taken from Apollo
11 spacecraft, showing
numerous craters and
expansive lava plains.
(Photo courtesy NASA)PLANET EARTH