Geoscience Reference
In-Depth Information
10
2
10
1
s
r
s
10
0
r
10
-1
10
-2
10
-3
p
10
-4
10
-5
10
-6
80
100
120
140
160
180
200
A
Figure 12.6
Breakdown of the solar abundances in components produced by the different nucleosynthetic
12.2 The formation of the Solar System
The Solar System is composed of a medium-sized star - the Sun - and its retinue of planets.
Some 99.9% of the mass of the Solar System is concentrated in the Sun and the dominant
element is hydrogen. Our idea of how the Solar System formed goes back in its principle
to Laplace. We start with a cloud of gas and dust formed from material lost by unstable
large stars: grains of silicon carbide, diamonds, and various silicates presenting uncanny
isotope compositions for all their elements are found in some chondritic meteorites and
attest to the pre-solar history of the Solar System material. This is the solar nebula, which
quickly assumes the shape of a disk. Material swirling around the center of mass (occu-
pied by the nascent star), atoms, molecules, and particles fell to the equatorial plane of the
disk. Thermal agitation caused countless collisions. Loss of momentum during these colli-
sions and interaction of matter with the magnetic field of the Sun caused material to spiral
inward toward the Sun. Although most of the mass ended up at the heart of the system, the
orbiting part that was to form the planets condensed into rock fragments (planetesimals),
which coalesced through successive collisions. The planets toward the inner part of the
Solar System are more rocky because the high temperatures caused by intense radiation
emanating from the Sun did not allow gases, such as water vapor and methane, to con-
dense. It is commonly assumed that the “snow line,” i.e. the imaginary limit of ice stability,
was located somewhere near the asteroid belt between Mars and Jupiter. The outer planets
such as Jupiter and Neptune formed in a colder environment and, in addition to their rocky
cores, also collected gaseous species such as hydrogen and methane. All the observations
of young stars and the astrophysical models concur that the residual gas was blown off
in 3-5 million years by the solar wind, the intense electromagnetic radiation (ultra-violet,
