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thought to most closely resemble in composition the cloud of dust of
our solar nebula. These meteorites formed far enough away from the
Sun (in the asteroid belt and beyond) not to have been affected by the
fierce heat that would have blown away most of the volatile elements
from around the Earth's orbit, at and beyond the snow line.
Carbonaceous chondrites contain a lot of water-bearing mineral
matter, and also various carbon compounds, as the name suggests. If
the Earth had just been made of these meteorites, they would have
contained enough water to supply hundreds of Earth-sized oceans
(and so the Earth cannot just be a mass of carbonaceous chondrites).
Much of the water within them is bound up in hydrated minerals,
formed by reaction of a 'dry' mineral with water. Hydrated silicate
minerals of this kind include chlorite and montmorillonite, which
can be found in muds on Earth today, and serpentine, which forms
when basalt minerals react with water.
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With chondrites, though, whether carbonaceous or not, there is
still the question of how and where the water reacted with the sili-
cates to produce the hydrated silicates. It seems that many of these
reactions must take place at or beyond the snow line. There, ice can
coagulate around rock, be melted by the fierce, short-lived radioactiv-
ity (inherited from that recent supernova) within the silicate minerals,
and then react to hydrate those minerals.
The water-bearing rock debris must then be shifted to a closer
orbit, into a collision course with the young Earth. Such orbital dis-
turbances were probably commonplace in this early phase, as the
new planets, including the giant ones, altered the gravitational
dynamics of the solar system, sending debris looping in different
directions.
Albarède's vision is controversial, and has been criticized.
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The
lead isotopes of that late-reading atomic clock could have been reset
by much of the Earth's lead content sinking into the core, not least
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