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Crust (0-50 km)
0
4000
8000
12000
0
Upper mantle
Upper mantle (50-700 km)
Crust
1000
Inner core
Density (kg m -3 )
Lower mantle
(700-2900 km)
Lower mantle
2000
3000
Outer core
(2900-5100 km)
4000
5000
Outer core
Inner core
(5100-6371 km)
6000
0
4000
8000
12000
Figure 2.7. (a) Diagram of the Earth's interior. (b) Variation of pressure, temperature, and density in the Earth's
interior.
density. Temperatures, densities, and pressures at the
center of the Earth are estimated to be 4,300 C, 13,000
kg m 3 , and 3,850 kbar, respectively (1 kbar
occurred early on, and the new ocean was hot. Sizable
oceans have been present during almost all the Earth's
history (Pollack and Yung, 1980). In 1955, geochemist
William W. Rubey (1898-1974) calculated that all the
water in the Earth's oceans and atmosphere could be
accounted for by the release of water vapor from vol-
canos that erupted throughout the Earth's history. The
oceans are a critical part of today's hydrologic cycle. In
this cycle, ocean water evaporates, the vapor is trans-
ported and condenses to form clouds, rain from clouds
precipitates back to land or ocean surfaces, and water
on land flows gravitationally back to the oceans.
=
1,000
10 8 Pa; for comparison, surface air
pressures are about 1 bar).
10 8 Nm 2
bar
=
=
2.3.2. Prebiotic Atmosphere
Earth's second atmosphere evolved as a result of out-
gassing from the Earth's mantle. As temperatures
increased during the molten stage, hydrogen and oxy-
gen, bound in crustal minerals as hydroxyl molecules
(OH), became detached, forming the gas-phase
hydroxyl radical. The hydroxyl radical then reacted with
reduced gases, such as molecular hydrogen [H 2 (g)],
methane [CH 4 (g)], ammonia [NH 3 (g)], molecular nitro-
gen [N 2 (g)], and hydrogen sulfide [H 2 S(g)], to form
oxidized gases, such as water [H 2 O(g)], carbon monox-
ide [CO(g)], carbon dioxide [CO 2 (g)], nitrogen dioxide
[NO 2 (g)], and sulfur dioxide [SO 2 (g)]. As the molten
rock rose to the Earth's surface during convection, oxi-
dized and reduced gases were ejected into the air by vol-
canos , fumaroles (vents near volcanos), steam wells
(vents near geothermal reservoirs), and geysers (springs
near volcanos with intermittent ejecting of water and
vapor).
After the crust and mantle solidified, outgassing con-
tinued. The resulting secondary atmosphere contained
no free elemental oxygen. All oxygen was tied up in oxi-
dized molecules. Indeed, if any free oxygen did exist, it
would have been removed by chemical reaction.
Outgassed water vapor in the air condensed to form
the oceans, starting around 4 b.y.a. Most outgassing
2.3.3. Biotic Atmosphere before Oxygen
Table 2.3 shows an approximate timeline of important
steps during the evolution of the Earth's atmosphere.
Living microorganisms have been responsible for most
of the changes.
Living microorganisms first developed about 3.5
b.y.a. from amino acids ,the building blocks of life.
Amino acids evolved by abiotic synthesis ,the process
by which life is created from chemical reactions and
electrical discharges in the absence of oxygen. Abiotic
synthesis was first hypothesized by Oparin (1938), and
then demonstrated in 1953 by the American chemist
Stanley Miller (1930-2007) while working in the labo-
ratory of Harold Urey (1893-1981). Miller discharged
electricity (simulating lightning) through a flask con-
taining H 2 (g), H 2 O(g), CH 4 (g), NH 3 (g), and boiling
water. He let the bubbling mixture sit for a week and,
after analyzing the results, found that he had produced
complex organic molecules, including the amino acids
 
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