Geoscience Reference
In-Depth Information
hypothesis. Even if the Earth accreted slowly,
compared to cooling and condensation times, the
later stages of accretion could involve material
that condensed further out in the nebula and
was later perturbed into the inner solar system.
A drawn-out accretion time does not imply a cold
initial condition. Large impacts reset the ther-
mometer.
The early history of planets was a very vio-
lent one; collisions, radioactive heat and core
formation provided enough energy to melt the
planet. Cooling and crystallization of the planet
over timescales of millions of years resulted in its
chemical differentiation -- segregation of mate-
rial according to density. This differentiation left
most of the Earth's mantle different in compo-
sition from that part of the mantle from which
volcanic rocks are derived. There must be mate-
rial that is complementary in composition to the
materials sampled by volcanoes.
The Earth and the Moon are deficient in the
very volatile elements that make up the bulk
of the Sun and the outer planets, and also the
moderately volatile elements such as sodium,
potassium, rubidium and lead. Mantle rocks con-
tain some
primordial noble gas isotopes
.
(
Reminder
:
primordial noble gas isotopes
is a Googlet. If it is typed into a search engine it will
return useful information on the topic, including defi-
nitions and references. These Googlets will be sprinkled
throughout the text to provide supplementary infor-
mation
.) The noble gases and other very volatile
elements were most likely brought in after the
bulk of the Earth accreted and cooled. The
40
Ar
content of the atmosphere demonstrates that the
Earth is an extensively degassed body; the atmo-
sphere contains about 70% of the
40
Ar produced
by the decay of
40
K over the whole age of the
Earth. This may imply that most of the K and
other incompatible elements are in the crust and
shallow mantle.
as it forms. Even small objects can melt if they
collide at high velocity. The mechanism of accre-
tion and its time scale determine the fraction of
the heat that is retained, and therefore the tem-
perature and heat content of the growing Earth.
The 'initial' temperature of the Earth was likely to
have been high even if it formed from cold plan-
etesimals. A rapidly growing Earth retains more
of the gravitational energy of accretion, particu-
larly if there are large impacts that can bury a
large fraction of their gravitational energy. Evi-
dence for early and widespread melting on such
small objects as the Moon and various meteorite
parent bodies attests to the importance of high
initial temperatures, and the energy of accretion
of the Earth is more than 15 times greater than
that for the Moon. The intensely cratered surfaces
of the solid planets provide abundant testimony
of the importance of high-energy impacts in the
later stages of accretion.
During accretion there is a balance between
the gravitational energy of accretion, the energy
radiated into space and the thermal energy pro-
duced by heating of the body. Latent heats asso-
ciated with melting and vaporization are also
involved when the surface temperature gets high
enough. The ability of the growing body to radi-
ate away part of the heat of accretion depends
on how much of the incoming material remains
near the surface and how rapidly it is covered
or buried. Devolatization and heating associated
with impact generate a hot, dense atmosphere
that serves to keep the surface temperature hot
and to trap solar radiation. One expects the early
stages of accretion to be slow, because of the
small gravitational cross section and absence of
atmosphere, and the terminal stages to be slow,
because the particles are being used up. The tem-
perature profile resulting from this growth law
gives a planet with a cold interior, a tempera-
ture peak at intermediate depth, and a cold outer
layer. Superimposed on this is the temperature
increase with depth due to self-compression and
possibly higher temperatures of the early accret-
ing particles. However, large late impacts, even
though infrequent, can heat and melt the upper
mantle. Formation of 99% of the mass of Earth
probably took place in a few tens of millions of
Magma ocean
A large amount of gravitational energy is released
as particles fall onto an accreting Earth, enough
to evaporate the Earth back into space as fast
