April 21, 1998
Second Year Seminar Paper
Johns Hopkins University
The Planets After Formation
Differentiation and Internal Structure:
After the era of planetary formation, the planets were undifferentiated, homogeneous masses. (For more info on the planet-forming process, see my web paper Planetary Formation and Our Solar System.) But for bodies with diameters greater than a few kilometers, sources of internal heat are great enough to cause partial or total melting of the interiors. The major sources of such heating are internal pressure, radioactive decay of elements in the planet's interior, heating by meteorite impacts, and residual heat left over from formation. This melting allows the materials of the planet to seperate according to density, rather like how oil forms a seperate layer on top of water. The "heavy" materials are those that bond to iron, forming iron-bearing minerals; these are called the siderophiles, ie. "iron-lovers". The siderophiles sink to the center of the planet, forming cores. The "light" materials are those the bond to silicates; they're called the lithophiles. These rise to the upper layers of the planet, forming crusts like a scum on a boiling pot. This seperation of materials according to density allows a complicated layered structure to form inside a planet or other large body.
B) Internal Structure of the Earth
The figure below shows the layered internal structure of the Earth.
(Adapted from Beatty and Chaikin, eds. The New Solar System, 1990)
1) Inner core: 1.7% of total mass of the Earth
The inner core is composed mainly of an iron and nickel alloy. Even though the temperature in this region is very high,
the core is solid due to the high pressure it is under.
2) Outer core: 30.8% of total mass
This layer is again mainly composed of iron-nickel alloy, but with larger amounts of oxygen and sulfur mixed in.
It is liquid, due to the high temperature and lower pressure. Convective motions are present in the liquid, and as this layer is electrically
conducting, it plays an important role in the magnetic dynamo that generates the Earth's magnetic field.
3) D" layer: 3% of total mass
This layer is often called part of the next layer, the lower mantle. However, it is thought to be chemically different.
4) Lower mantle: 49.2% of total mass
This layer is composed of the lighter elements silicon, magnesium, oxygen, and small quantities of heavier iron, calcium, and aluminum.
5) Transition region: 7.5% of total mass
The transition region, or middle mantle, is thought to be the source of basaltic magmas. (These magmas are those rich in calcium and aluminum.)
When the material of the middle mantle is hot, it forms these basalts and has a fairly low density.
When cold, it forms garnet, an aluminum-bearing silicate mineral, which has a higher density.
This complex behavior with temperature is important in the tectonic process, which will be discussed later.
6) Upper mantle: 10.3% of total mass
This layer is mainly composed of olivine and pyroxene, which are magnesium-bearing silicates.
These materials are crystalline at high temperatures, and thus tend to settle out of rising magmas.
Part of the upper mantle may be partly molten.
7) Crust: 0.47% of total mass
This extremely thin shell over the Earth comprises the entire world to us. Under the oceans, it is 0-10 km thick.
Under the continents, it is 0-50 km thick. It is composed of low density materials, like quartz (silicon dioxide) and other metal-poor silicates (feldspars).
The above layering classification scheme is based primarily on differences in chemical composition.
There is another important layering based on a physical characteristic of the material that will be useful in explaining the tectonic process later.
This word is derived from the Greek "asthenes", meaning weak. It is part of the upper mantle that is weak and fluid over long time scales.
It can support convective motions.
This word is derived from the Greek "lithos", meaning rock. It referrs to the crust and the top of the upper mantle that is solid and brittle.
It is about 100 km thick.
C) Internal Structure of Other Planets
The moon has a low density and iron content; presumably, it has little or no core. It is smaller than the Earth and thus cooled rapidly after formation.
This rapid cooling produced a thick crust, about 60 km thick. The brittle lithosphere is 1000 km thick.
This planet is thought to have no asthenosphere now; instead, a thick lithosphere reaches down to a molten iron core.
Again, due to the small size of Mercury, it cooled rapidly after formation. During this rapid cooling, its radius decreased by 2 km, compressing the surface
and leading to thrust faults, or cracks, in the surface.
Due to their similar size and composition, Venus's interior is expected to similar to Earth's. However, it is not well understood, as will be
described in more detail later.
Mars is in between the Moon/Mercury and Venus/Earth. It has a thick lithosphere, about 2000 km thick. There is little or no asthenosphere now.
During early heating and cooling of the planet, there was a 5-10 km change in diameter, leading to fractures in the surface of the planet.
Mars is thought to have an iron or iron sulfide core.
These worlds have similar interior structures to the planets previously discussed, but different compositions. Their overall densities are lower.
They have rocky or muddy cores, instead of iron ones, and icy surface crusts, instead of rocky ones. The states of their asthenospheres are not well known.
D) Heat Flow in Planets
There are several sources of energy that heat the interiors of planets. They are ...
Gravitational energy from contraction during formation. The virial theorem states that half the gravitational potential energy of a contracting body
goes into heating the material. (The other half must be radiated away or otherwise lost.)
Radioactive decay of elements with long half-lives. The radioactive elements with short half-lives will have already decayed before incorporation into
a solid planet.
Tidal heating. Due to the finite size of a planetary body (body A), the gravitational force due to the larger body being orbited (body B) will
be stronger on the side of body A facing body B. Since any planetary body has some flexibility, this gradient in force will stretch body A,
causing tidal bulges. If body A is in an eccentric orbit, the stretching will be greatest when body A is closest to body B, and least when A if
furthest from B. This repeated stretching and relaxing heats the body by friction, like kneading a lump of silly putty does.
This effect is very important for Io.
Electromagnetic heating. A planet with a metalic core is a conductor moving through the magnetic field of the Sun. This generates a current
in the conductor, which heats it, like a toaster element. This effect could have been important during the T Tauri phase of the Sun, when it had
strong magnetic fields. Now, the Sun's magnetic field is most likely too weak for this to be a significant source of heating.
The energy absorbed by a body from the Sun, Ls, plus its internal heat flow, Li, must equal the total energy radiated by the body, or its
temperature will start to increase. Treating the body as an approximate blackbody (perfect absorber/radiater), the total energy radiated L is
proportional to the equilibrium temperature to the fourth power. The equilibrium temperature for the Earth is 255 K.
In the terrestrial planets, Ls >> Li. This is consistent with radioactive decay being the primary internal energy source.
In the Jovian (gas giant) planets, Li > Ls. This is consistent with heat from gravitational contraction being the primary internal energy
For Io, Li = 1.3 Ls. This world's primary internal heat source is tidal stretching.
So, with all this heat inside planets, there must be a mechanism to transport it to the surface, where it may be radiated away.
There are two possible ways: conduction or convection. Conduction isn't too interesting; here, the energy is transported simply
through contact of hot material and cooler material. However, convection is a more efficient method of energy transport, where hot material
literally rises up to cooler regions where it can then radiate its energy or conduct it more efficiently. Once the material is cool, it sinks
back down to the hotter regions and is replaced by fresh hot material, forming convection currents. If this mechanism occurs inside a planet,
interesting things can happen, which will be discussed in the next section.