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allowed heavy iron to separate from silicates, therefore releasing even more gravitational
energy (about 2000 K for the Earth). Differences in 182 W abundances between chondrites
and the Earth dates completion of core segregation to within the first 30 Ma of Solar
System formation. Differences in 142 Nd abundances between meteorites and the silicate
Earth, and also between modern rocks and the 3.85-Ga-old Isua rocks, date the crystal-
lization of the magma ocean to the first tens of Ma of the planet's history. The period that
still remains mysterious is the transition to plate tectonics: when did it happen? Which
parameter (water?) made the Earth, and only the Earth, evolve towards plate tectonics?
Why is the Earth endowed with a dynamo?
12.6 The Moon
The Moon's radius is 1738 km, its gravity a mere 17% of that of the Earth, and its density
3344 kg m 3 (compare with the Earth's 5515 kg m 3 ). Thanks to the Apollo missions,
we have several hundred kilograms of lunar samples from six localities on the visible side
of the Moon and a wealth of observations made by the astronauts. Antarctic and desert
meteorites have added more than 30 extra lunar samples, some of which are suspected
to come from the far side. Although oxygen isotopes tell us that the Earth and the Moon
formed from the same mass of debris left by the collision of a body the size of Mars with
the proto-Earth, our satellite has a very different history and geology from that of our
planet. A consequence of the small gravity is that the Moon does not retain most gases
and therefore has no atmosphere. Gravimetric and altimetric data sent back by the satellite
Clementine tell us that the Moon has a crust some 60-100 km thick, much thicker than
the Earth's continental crust. A metal core was probably present during the early history
of the Moon, but the absence of a magnetic field shows that it has now solidified. With
respect to the Earth, the Moon is depleted in volatile elements and is extremely reduced,
so much so that metal iron is a common mineral in magmatic rocks. It is also depleted in
highly siderophile elements, notably Ni and the platinum group elements, which suggests
that by the time of the giant impact, the supply of late veneer had already dried up. The
Moon is a dead body whose last volcanic eruptions occurred 3.2 billion years ago. During
the first billion years following accretion, the surface of the Moon was subjected to intense
bombardment by planetary objects of different sizes, which gave our satellite its charac-
teristic heavily cratered aspect. There is apparently no element of the original surface left
untouched by cratering.
Lunar rocks are classified into three main groups: (1) intrusive rocks of the lunar high-
lands, especially anorthosite, with its high plagioclase content, and gabbro, which, in
addition to plagioclase, contains olivine or pyroxene; (2) KREEP basalt, an acronym indi-
cating that it is very rich in potassium, rare-earths, and phosphorus; and (3) lunar mare
basalts. The intrusive rocks are 3.8-4.5 Ga old, KREEP basalts 3.6-3.8 Ga old, and mare
basalts 3.2-3.9 Ga old. The feldspathic highlands dominate the far side. Very large impacts,
such as the one that excavated the Mare Imbrium at around 3.8 billion years, form a nebular
super-event known as the Late Heavy Bombardment (LHB). An idea popular at this time is
 
 
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