Geoscience Reference
In-Depth Information
Nothing is created from nothing in our observable universe!
Nevertheless, the arrival of water, carried on meteorites and comets,
was not in exact synchrony with the arrival of rocky elements and this
is due to the history of the formation of the solar system. Fervent
research activity carried out by planetologists and astrophysicists
particularly endeavors to explain the differences in composition and
orbit of objects in the current solar system (the Sun, telluric “rocky”,
planets, giant “gas” planets, satellites, asteroid belts, comets, the Oort
cloud). The explanatory model develops particularly in the form of a
historical sequence of several important events such as the formation,
differentiation and the progressive cleaning of the protostellar disk,
the movement of volatile components (including water) associated
with the thermonuclear ignition of the Sun, the formation of telluric
planets, the formation of giant gas planets, the migration of Jupiter
(the “hot Jupiter” hypothesis) and substantial impacts.
It is not possible to give here an overview of the state of current
knowledge in this field; the interested reader can refer to more
specialized works on the formation of the solar system. We can,
however, emphasize a major point for this present chapter: most
water, brought through influx asteroids and comets, accumulated late,
after the principal phase of accretion by rocky material, but probably
in the first 500 million years of the Earth's history. It is this water that
would constitute the majority of the current oceans (93% of water on
earth) and the different reservoirs of fresh water (glaciers and ice caps
above all, then lakes, rivers, groundwater, atmospheric water vapor
and clouds).
1.2.3. The ocean and the end of the “Venus” phase of the Earth's
history (between 4.5 and 4 billion years ago)
During accretion, the heaviest elements were concentrated by the
effect of gravity at the center of the Earth's sphere (especially iron
and nickel, but also other, rarer heavy elements), whereas the lightest
elements became concentrated on its surface. Taking into account
the initial conditions in temperature and radiation, the Earth's very
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