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largely dominated by CO 2 and N 2 , and therefore too oxidizing to allow for methane and
ammonia to be stable. Miller and Urey's work fell into near oblivion for decades. This crit-
icism incorrectly assumes that all atmospheric gases would be well mixed and everywhere
in equilibrium. The actual situation is very different since, although the atmosphere was
dominated by carbon dioxide and nitrogen, along mid-ocean ridges, hydrogen, methane,
and ammonia were produced in abundance by serpentinization reactions. Production of ser-
pentine results from the combination of two reactions between water and olivine, a major
mineral of basaltic rocks and a solid solution of Fe and Mg components. First, iron from
the Fe-olivine component (fayalite) is oxidized by water to produce silica, magnetite, and
hydrogen:
3Fe 2 SiO 4 +
2H 2 O
2Fe 3 O 4
+
3SiO 2 +
2H 2
(9.2)
(olivine)
(solution)
(magnetite)
(quartz)
(gas)
Second, silica liberated in this reaction reacts with water and Mg-olivine to form
serpentine:
3Mg 2 SiO 4 +
SiO 2
+
4H 2 O
2Mg 3 Si 2 O 5 (OH) 4
(serpentine)
(9.3)
(olivine)
(silica)
(solution)
Hydrogen liberated by serpentinization reacts with CO 2 and N 2 to give methane and
ammonia:
4H 2
+
CO 2
CH 4
+
2H 2 O
(9.4)
3H 2
+
N 2
2NH 3
The Mid-Atlantic Ridge today shows how this may have happened: reaction of water with
hot igneous material at temperatures
600 C produces hydrogen, which in turn reacts
with CO 2 and N 2 to produce methane and ammonia, respectively. Whether hydrogen was
lost from the atmosphere and methane and ammonia oxidized by atmospheric gases is
not relevant to such a dynamic system: the source of reducing gas was enormous and
steady, and favorable geological sites must have been ubiquitous at the bottom of the ocean.
The floor of mid-ocean ridges, and also the ocean floor on top of the magma ocean in
the early stages of the Earth's evolution, must have represented major production sites
for methane and ammonia. The lack of an ozone layer probably allowed solar ultraviolet
radiation to reach the surface of the Earth. Irradiation of the surface of the ocean by solar
UV wherever deep oceanic upwellings would degas dissolved methane and ammonia into
the atmosphere provided natural conditions for the formation of proteins and the emergence
of life.
How early life could resist oxidizing conditions and find nutrients to keep going is
another issue. Today, the main terrestrial productivity is concentrated at the surface of
the oceans. The most important nutrients, nitrates and phosphates, are, however, brought
to the sea by the rivers. Mid-ocean ridges produce no nitrate, but we do not know which
nitrogen source was used by early organisms, presumably dissolved nitrogen or ammo-
nia. In contrast, modern ridges are a net sink for phosphorus, and if rivers were to dry
out overnight, this element so critical for life would rapidly disappear from the ocean and,
with it, oceanic productivity. It is likely that, at some point, the emergence of continents,
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