In this lecture period we discuss:
09/21/2008 Evolution of the Solar SystemThe planets are byproducts of the formation of the Sun.
Star birthClick here for a gallery of famous stars. Stars form from dense interstellar clouds of gas and dust. These clouds are typically ~30 light years in dimension and ~10,000 times larger than our Sun. When mutual gravitational attraction dominates, regions of the cloud start to condense into stars. Fusion reactions start when energy provided by further gravitational collapse is large enough to heat the core to ~10,000,000 K (1E7K).
How were the Planets
formed?
The Nebula HypothesisThe planets of our Solar System formed due to two properties of interstellar clouds: rotation and turbulence.
|
Planet |
Diameter (km) |
Distance from Sun
|
Surface temperature |
Density |
Main atmospheric constituents |
1,392,000 |
- |
5,800 |
|
- |
|
4,880 |
58 |
260 |
5.4 (rocky) |
- |
|
12,100 |
108 |
480 |
5.3 (rocky) |
CO2 |
|
12,750 |
150 |
15 |
5.5 (rocky) |
N2, O2 |
|
6,800 |
228 |
-60 |
3.9 (rocky) |
CO2 |
|
143,000 |
778 |
-150 |
1.3 (icy) |
H2, He |
|
121,000 |
1,427 |
-170 |
0.7 (icy) |
H2, He |
|
52,800 |
2,869 |
-200 |
1.3 (icy) |
H2, CH4 |
|
49,500 |
4,498 |
-210 |
1.7 (icy) |
H2, CH4 |
|
2,300 |
5,900 |
-220 |
2.0 |
CH4 |
The reason for the difference between the rocky dense inner planets and the icy/gaseous outer planets is : the composition of each planet is determined by the type of material that can survive in the solid form given the temperature of the particular part of the Nebula: Condensation theory.
|
Sequence of condensation of minerals in the nebula as a function of temperature. At temperatures above about 1300K, metals and silicates can condense and become solid dust grains. At lower temperatures more volatile minerals become solids, and at temperatures of less than ~400K, hydrogen-bearing gases such as methane and ammonium become solids. Hydrogen and helium remain gases. For the inner planets, at high temperatures, the planet-building dust grains were made up of rocky materials (silicates, iron, etc.). The hydrogen and helium could have been blown away by the solar wind. For the outer planets, the hydrogen and helium was retained by a combination of the larger gravity for these massive bodies and the formation of ice. |
Apart from the thin outer regions of atmosphere, ocean and crust, the Earth is composed of three main compositional layers. The mantle is about 2900km thick and makes up around 65% of Earth's total mass. Below the mantle is a dense core, which has an outer liquid region and an inner solid region. The table summarizes the properties of these various regions.
Constituents of Earth
Component |
Average Thickness (km) |
Average Density (x103
kg/m3) |
Fraction of Total (%) |
Principal Constituents |
Atmosphere |
- |
- |
0.00009 |
N2, O2 |
Oceans |
4 |
1.03 |
0.024 |
H2O |
Crust |
45 |
2.8 |
0.5 |
Silicates and other oxides |
Mantle |
2900 |
4.5 |
67 |
Mg silicates |
Core |
3400 |
11.0 |
30 2 |
Fe, +/-S (liquid) Fe-Ni (solid) |
The layered structure of the Earth (with density increasing
with depth) can only be interpreted as being the product of differentiation.
Differentiation is the gravitational separation of materials according to
their specific gravities in a liquid mixture that was originally homogeneous
throughout.
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