Geoscience Reference
In-Depth Information
2.4. Summary
The sun formed from the condensation of the solar
nebula about 4.6 b.y.a. Solar radiation incident on the
Earth originates from the sun's photosphere. The pho-
tosphere emits radiation with an effective temperature
near 6,000 K. The solar spectrum consists of UV, visi-
ble, and solar-IR wavelength regimes. The Earth formed
from the same nebula as the sun. Most of the Earth's
growth was due to asteroid and meteorite bombard-
ment. The overall composition of the Earth is similar to
that of stony meteorites. Dense compounds and com-
pounds with high melting points settled to the center of
the Earth. Light compounds and those with low melt-
ing points became concentrated in the crust. The first
atmosphere of the Earth, which consisted of hydro-
gen and helium, may have been swept away by an
enhanced solar wind during early nuclear explosions in
the sun. The second atmosphere, which resulted from
outgassing, initially consisted of carbon dioxide, water
vapor, and assorted gases. When microbes first evolved,
they converted carbon dioxide, ammonia, hydrogen sul-
fide, and organic material to methane, molecular nitro-
gen, sulfur dioxide, and carbon dioxide, respectively.
Oxygen-producing photosynthesis led to the accumula-
tion of oxygen and the production of ozone only after
rocks had become saturated with oxygen. The pres-
ence of oxygen resulted in the evolution of aerobic
respiration, which increased the efficiency of produc-
ing molecular nitrogen, the major constituent in today's
atmosphere.
2.3. Even though the moon is effectively the same dis-
tance from the sun as the Earth, the moon has no effec-
tive atmosphere. Why?
2.4. What is the intensity of radiation emitted by a hot
desert (330 K) relative to that emitted by the strato-
sphere over the South Pole during July (190 K)?
2.5. What prevents the Earth from having magma
oceans on its surface today, even though temperatures
in the interior of the Earth exceed 4,000 K?
2.6. What peak wavelength of radiation is emitted in
the center of the Earth, where the temperature is near
4,000 K?
2.7. What elements do you expect to be most abun-
dant
in
soil
dust
particles
lifted
by
the
wind?
Why?
2.8. Describe the nitrogen cycle in today's atmosphere.
What would happen to molecular nitrogen production
if nitrification were eliminated from the cycle? (
Hint
:
Consider the processes occurring in the preoxygen
atmosphere.)
2.9. Identify at least three ways in which oxygen
improved the opportunity for higher life forms to
develop on the Earth.
2.10. What is the advantage of aerobic respiration over
fermentation?
2.11. What is the source of oxygen during photosyn-
thesis in green plants? Explain.
2.5. Problems
2.1. Explain why the Earth's core consists primarily of
iron, whereas its crust consists primarily of oxygen and
silicon.
2.12. If the Earth's crust contained twice as much iron
as it does, would atmospheric oxygen buildup have been
slowed down or sped up during the past 2.5 billion
years? What might the consequence of this have been
for the evolution of aerobic respiration, the ozone layer,
green plants, and humans?
2.2. Why does the Earth receive radiation from the sun
as if the sun is an emitter with an effective temperature
of near 6,000 K, when, in fact, the hottest temperatures
of the sun are more than 15 million K?
2.13. Explain how the Great Oxygenation Event caused
an ice age.
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