Geoscience Reference
In-Depth Information
proto-planets
.
The result was the formation of an orbital disk,
followed by the progressive cleaning of this disk to form the Moon
[KOK 00].
However, another scenario could have involved the instantaneous
introduction of a significant quantity of water into the magma mantle
of this protoplanet, a precursor to Earth in the right conditions, an
event that did not occur, or which occurred differently, in the
formation of Venus [BIB 09]. As we now know, thanks to the study of
plate tectonics on Earth, the filtration of light components (including
water) into the mantle leads to a decrease in its viscosity [BIL 01,
DIX 04] and to a chemical differentiation between the continental
crust and the ocean along the subduction zones. Together, these two
phenomena favor the maintenance of an active tectonic system, at
least as we know it on Earth.
Some results of the exploratory mission
Venus Express
published
in 2009 by the European Space Agency [ESA 09] suggest that a
tectonic system nonetheless existed at the beginning of Venus'
history, but it would have ceased to develop due to the lack of ocean.
The primordial water was in fact systematically vaporized with a
positive retroactive effect on the greenhouse effect, leading to its
enhancement and to the gravitational escape of hydrogen.
In the case of the Earth, one hypothesis is that a much more active
tectonic system, initiated by the giant impact that led to the formation
of the Earth-Moon duo [RUI 11], could have absorbed some of the
primordial water via subduction, sufficiently slowing the enhancement
of the greenhouse effect so that sizeable oceans could form. After that,
the reduction of the initial greenhouse effect by the precipitation of
carbonates could have become considerable and the first stage of
abiotic regulations of the Earth system was established. This is
illustrated in Figure 1.2.
