436 PART 4^ |^ THE SOLAR SYSTEM
similar to water on Earth. Th ose data suggest that there may be
major diff erences in composition among comets. Icy planetesi-
mals that formed far from the sun may be richer in deuterium,
while those that formed closer to the orbit of Jupiter may contain
water with isotope ratios more like those in Earth’s water. Th is is
a subject of continuing research, and it shows that the origin of
Earth’s atmosphere and oceans is yet to be fully understood.
In whatever way Earth’s atmosphere originated, the mix of
gases must have changed over time. Th e young atmosphere
would have been rich in water vapor, carbon dioxide, and other
gases. As it cooled, the water condensed to form the fi rst oceans.
Carbon dioxide is easily soluble in water—which is why carbon-
ated beverages are so easy to manufacture—and the fi rst oceans
began to absorb atmospheric carbon dioxide. Once in solution,
the carbon dioxide reacted with dissolved substances in the sea-
water to form silicon dioxide, limestone, and other mineral sedi-
ments in the ocean fl oor, freeing the seawater to absorb more
carbon dioxide. Th anks to those chemical reactions in the oceans,
the carbon dioxide was transferred from the atmosphere to
seafl oor sediments.
When Earth was young, its atmosphere had no free oxygen,
that is, oxygen not combined with other elements. Oxygen is
very reactive and quickly forms oxides in the soil or combines
with iron and other substances dissolved in water. Only the
action of plant life keeps a steady supply of oxygen in Earth’s
atmosphere via photosynthesis, which makes energy for plants by
absorbing carbon dioxide and releasing oxygen. Beginning about
2 to 2.5 billion years ago, photosynthetic plants in the oceans
had multiplied to the point where they made oxygen at a rate
faster than chemical reactions could remove it from the atmo-
sphere. After that time, atmospheric oxygen increased rapidly.
(Th is topic will be discussed again in Chapter 26.) It is a
Common Misconception that there is life on Earth
because of oxygen. Th e truth is exactly the opposite: Th ere is
oxygen in Earth’s atmosphere because of life. Most life forms on
Earth do not need oxygen (except the minority of creatures that
are animals, including us), and some are even poisoned by it.
An ozone molecule consists of three oxygen atoms linked
together (O 3 ). Ozone molecules are very good at absorbing ultra-
violet photons. Earth’s lower atmosphere is now protected from
ultraviolet radiation by an ozone layer about 15 to 30 km above
the surface that exists because the atmosphere contains abundant
ordinary oxygen (O 2 ) from which ozone is made. Because the
atmosphere of the young Earth did not contain oxygen, an ozone
layer could not form, and the sun’s ultraviolet radiation was able
to penetrate deep into the atmosphere. Th ere the energetic ultra-
violet photons would have broken up weaker molecules such as
water (H 2 O). Th e hydrogen from the water then escaped to
space, and the oxygen formed oxides in the crust. Earth’s atmo-
sphere could not reach its present composition (■ Table 20-1)
until it was protected by an ozone layer, and that required
oxygen.
Climate and Human Effects
on the Atmosphere
If you climb to the top of a high mountain, you will fi nd the
temperature to be much lower than at sea level (■ Figure 20-9).
Most clouds form at such altitudes. Higher still, you would fi nd
the air much colder, and so thin it could not protect you from
the intense ultraviolet radiation in sunlight.
You can live on Earth’s surface in safety because of Earth’s
atmosphere, but modern civilization is altering Earth’s atmo-
sphere in at least two serious ways, by adding carbon dioxide
(CO 2 ) and by destroying ozone.
Th e concentration of CO 2 in Earth’s atmosphere is impor-
tant because CO 2 can trap heat in a process called the■ Figure 20-9
Thermometers placed in Earth’s atmosphere at different levels would register
the temperatures shown in the graph at the right. The lower few kilome-
ters where you live are comfortable, but higher in the atmosphere the tem-
perature is quite low. The ozone layer lies about 15 to 30 km above Earth’s
surface.■Table20-1 ❙ Earth’s AtmosphereGas Percent by Weight
N 2 75.5
O 2 23.1
Ar 1.29
CO 2 0.05
Ne 0.0013
He 0.00007
CH 4 0.0001
Kr 0.0003
H 2 O (vapor) 1.7–0.06100- 200 0 200
Altitude (k^50m)T (°F)100 300
T (K)Ozone layer
Mount
Everest Clouds