Previous: Lecture Six
(pp 30-36 Christiansen & Hamblin)
Many of the properties of a planet are controlled by the amount of
heat trapped in the interior, and by the rate at which this heat leaks
out. For example, the surface of the Earth is relatively
young---typical rocks have an age of about 1 billion years. Moon rocks
are typically three billion years old or older. The difference arises
because the Earth's mantle is still hot and therefor plastic, so that
the surface is constantly being destroyed and rebuilt by tectonic processes.
Similarly, Neptune is currently losing more heat than Uranus,
indicating that the interior of Neptune is hotter than that of
Uranus. The warm core of Neptune is believed to be responsible for Neptune's
rather strong magnetic field.
Heat Loss Mechanisms
- Conduction. Heat is the random motion of atoms. Collisions
between atoms will transport this random motion. This is the least
efficient of the major heat loss mechanisms, but in opaque rigid
matter is the operative one.
- Convection. Most fluids (air, water under most conditions,
magma) become less dense when they are heated, since the more vigorous
atomic motion means that each atom occupies more space. In a
gravitational field, lighter fluids surrounded by heavier fluids are
buoyant. Since planets tend to be hot in the center and cool at the
surface, it can happen that hot, less dense fluids lie below cool,
more dense fluids. In that case, the hot fluids will rise toward the
surface of the planet, carrying heat with them. This motion is called convection.
- Radiation (Light). Photons escaping from the visible surface of
a planet remove energy from the planet. The rate at which energy is
lost by this mechanism is proportional to the temperature of the
surface to the fourth power (T4). The result is that the surfaces
of hot bodies cool very efficiently.
The amount of heat or energy in a body is proportional to the body's
temperature, and to the total mass of the body (since more stuff means
more stuff in motion). The total mass of a spherical body is
proportional to the radius cubed (R3). The rate of heat loss of the
same body is proportional to the surface area, or R3. The cooling
time tcool is the ratio of the amount of heat to the rate of heat
loss, so tcool~ R3/R2~ R. Large bodies take longer to
cool off. If you cook, you are familiar with the inverse result---cooking a
large roast takes longer than cooking a small roast.\hfill\break
The efficient loss of heat from the surface of terrestrial planets
ensures that a crust forms rapidly. In larger bodies like Earth and
Venus, the large reservoir of heat ensures that the crust is pushed
around by convective flows for billions of years. In smaller bodies
like the moon, the crust rapidly thickens, and after a billion years
or so all tectonic activity ceases.
Results of Solar System Formation
At the end of the formation period, which lasted about 100,000,000
- The sun burning H into He to produce heat.
- Terrestrial planets formed in the hot near solar region, devoid
The terrestrial planets were hot enough (due to accretion
heating) that they could differentiate (with denser material moving to
the center and less dense material moving to the surface). Surface
temperatures are estimated to be
Initial Surface Temperatures
- Gas giant planets with rock/ice cores surrounded by H and He.
- Large number of planetesimals---asteroids, comets, Pluto and etc.
Physically Differentiated Zones
Planets have different physical properties at different radii:
- Core. Metallic, either solid (in small bodies) or molten (in
- Asthenosphere. Plastic or ductile material. Convection can
occur in this region.
- Lithosphere. Rigid material (``solid Earth'').
- Biosphere. Where life is.
- Hydrosphere. Fluid regions. The atmosphere of Earth, and the
bulk of Jupiter are two examples.
- Photosphere. The region from which photons can escape to
space. This is what we see from the outside of the planet.
- Magnetosphere. The region outside the photosphere where the
magnetic field of the planet dominates the physics of the
Chemically Differentiated Zones
Planets also have different chemical properties at different radii;
the different chemical zones do not necessarily correspond to the
physically determined zones described above.
- The core. Siderophile metals
- The mantle. Siderophile and Chalcophile elements, silicates rich
in Fe and Mg (``mafic'').
- The crust. Lithophile elements.
- Siderophile (``iron loving''); Fe, Ni, Co
- Chalcophile (``sulfur loving''); Cu, Zn, S
- Lithophile (``oxygen loving''); Na, K, Ca, Si, Al
- Atmosphile; N, O, H, He
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