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Earth's atmosphere is primarily composed of nitrogen and oxygen, with a small percentage of other gases, and extends up to hundreds of kilometers but is densest within 30 km of the surface. The early atmosphere was likely made up of hydrogen and helium, which later evolved into a denser atmosphere due to volcanic outgassing, leading to the formation of water and the oceans. Over time, the atmosphere transitioned to one dominated by nitrogen and gradually increased in oxygen concentration due to photodissociation.

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9 views2 pages

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Earth's atmosphere is primarily composed of nitrogen and oxygen, with a small percentage of other gases, and extends up to hundreds of kilometers but is densest within 30 km of the surface. The early atmosphere was likely made up of hydrogen and helium, which later evolved into a denser atmosphere due to volcanic outgassing, leading to the formation of water and the oceans. Over time, the atmosphere transitioned to one dominated by nitrogen and gradually increased in oxygen concentration due to photodissociation.

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Earth’s atmosphere is a relatively thin, gaseous envelope that comprises mostly nitrogen and

oxygen, with small amounts of other gases, such as water vapor and carbon dioxide (CO 2).
Nestled in the atmosphere are clouds of liquid water and ice crystals. Although our atmosphere
extends upward for many hundreds of kilometers, it gets progressively thinner with altitude.
Almost 99 percent of the atmosphere lies within a mere 30 km (19 mi) of Earth’s surface (see
Fig. 1.3). In fact, if Earth were to shrink to the size of a beach ball, its inhabitable atmosphere
would be thinner than a piece of paper. This thin blanket of air constantly shields the surface
and its inhabitants from the sun’s dangerous ultraviolet radiant energy, as well as from the
onslaught of material from interplanetary space. There is no definite upper limit to the
atmosphere; rather, it becomes thinner and thinner, eventually merging with empty space,
which surrounds all the planets.

1.2a The Early Atmosphere

The atmosphere that originally surrounded Earth was probably much different from the air
we breathe today. Earth’s first atmosphere (some 4.6 billion years ago) was most likely hydrogen
and helium—the two most abundant gases found in the universe— as well as hydrogen
compounds, such as methane (CH4) and ammonia (NH3). Most scientists believe that this early
atmosphere escaped into space from Earth’s hot surface.

A second, more dense atmosphere, however, gradually enveloped Earth as gases from
molten rock within its hot interior escaped through volcanoes and steam vents. We assume that
volcanoes spewed out the same gases then as they do today: mostly water vapor (about 80
percent), carbon dioxide (about 10 percent), and up to a few percent nitrogen. These gases
(mostly water vapor and carbon dioxide) probably created Earth’s second atmosphere. As
millions of years passed, the constant outpouring of gases from the hot interior—known as
outgassing—provided a rich supply of water vapor, which formed into clouds. It is also believed
that when Earth was very young, some of its water originated from numerous collisions with
small meteors that pounded Earth, as well as from disintegrating comets.

Rain fell upon Earth for many thousands of years, forming the rivers, lakes, and oceans of the
world. During this time, large amounts of carbon dioxide (CO 2) were dissolved in the oceans.
Through chemical and biological processes, much of the CO 2 became locked up in carbonate
sedimentary rocks, such as limestone. With much of the water vapor already condensed and
the concentration of CO2 dwindling, the atmosphere gradually became dominated by molecular
nitrogen (N2) , which is usually not chemically active.

It appears that molecular oxygen (O2), the second most abundant gas in today’s atmosphere,
probably began an extremely slow increase in concentration as energetic rays from the sun split
water vapor (H2O) into hydrogen and oxygen during a process called photodissociation. The
hydrogen, being lighter, probably rose and escaped into space, while the oxygen remained in
the atmosphere.

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