Geoscience Reference
In-Depth Information
first atmosphere was probably dominated by gaseous and stable
components, formed by the chemical combination of light elements
such as hydrogen, carbon, oxygen and nitrogen - the most likely
candidates being carbon dioxide (CO
2
), dinitrogen (N
2
) and water
(H
2
O). This is the reason why, in the absence of natural archives
from the era, the current theoretical models propose an initial
atmosphere that was very dense, dominated, apart from water, by
around 98% CO
2
and 2% N
2
; these proportions can vary slightly
depending on the models used [GAR 06]. Certain minor or trace
components (such as gaseous SiO
2
) are not excluded from these
hypotheses, particularly because of the very turbulent conditions
caused by enormous, repeated meteoric impacts or by the mega-
impact that gave rise to the formation of the dual Earth-Moon
system.
The primordial atmosphere was probably very dense (from 60 to
160 times the current atmospheric pressure) because it contained, in
the form of gaseous CO
2
, the carbon that is present today in the form
of solids (carbonated rocks and sediments and organic matter, notably
fossils). In such conditions, the greenhouse effect was enormous and it
kept the Earth's surface temperature at several hundred degrees
Celsius. This phase is sometimes called the Earth's “Venus” phase by
analogy with the conditions that prevail today on Venus.
Water, which arrived on Earth progressively carried by meteorites
and comets, progressively accumulated, at first in gaseous form,
increasing the greenhouse effect, then in liquid form when the
saturation pressure was reached. It is therefore very probable that
the first reservoirs of liquid water contributed considerably to the
reduction of the greenhouse effect [LIU 04] as they constituted
chemical reactors by which gaseous carbon in the form of CO
2
could
be transformed into solid carbon in the form of carbonates
(Metal
2+
CO
3
2-
), via the equilibriums between the different forms of
carbon mineral dissolved that coexist in the aqueous phase, CO
2
,
HCO
3
-
and CO
3
2-
[ZEE 01].
It is helpful to note that this general process is constantly occurring
today, but through biologically mediated processes (biomineralization
of calcium shells and skeletons) and via the alteration of
