Geoscience Reference
In-Depth Information
12
The Earth in the Solar System
The Earth is, together with Mercury, Venus, and Mars, one of the rocky planets of the
inner Solar System. Our sampling of the Solar System is rather limited. Terrestrial sam-
ples are plentiful and the Apollo astronauts brought back hundreds of kilograms of lunar
rocks. More than 30 meteorites from the Moon and 40 meteorites from Mars (the SNC
meteorites) complete the well-identified planetary material. Achondrites are basaltic rocks
which have been suggested to come from the asteroid Vesta in the asteroid belt, between
Mars and Jupiter. Chondrites derive their name from the presence of abundant mm-sized
molten blebs known as chondrules. They are fragments of much smaller, undifferentiated
asteroids, and are divided into carbonaceous, ordinary, and enstatite meteorites. Carbona-
ceous meteorites are remarkable because they contain hydrous minerals, organic matter,
refractory inclusions, and pre-solar grains (SiC, diamonds). Iron meteorites are actually
Fe-Ni alloys: some of them represent the core of some small planetary bodies disrupted by
impacts.
The composition and the conditions prevailing at the surface of each rocky planet are
very different and distinct from those of the giant gas-rich planets, such as Jupiter and
Saturn, of the outer Solar System. Astronomical observations indicate that the Sun and its
companion planets formed by condensation of a cloud of gas and dust, called the solar
nebula. The topic of this chapter is to describe the major processes that led from the early
stages of the Universe to the formation of the elements, to the birth of the Solar System
and finally to the formation of the Earth and its sister planets.
The speed at which the universe is expanding as measured by the “red shift” of light from
the stars and the nuclear cosmic chronometers indicate that the Big Bang occurred about
14 billion years ago. Because our star, the Sun, is a mere 4.5 billion-year-old youngster, it
is clear that the Solar System recycles material with a long history. The processes leading
to the formation of elements are known collectively as nucleosynthesis. These elements
form in stars by a combination of processes of thermonuclear fusion, neutron capture, and
