The are a number of hard facts about the Earth that have been determined over the years of geological surveys:

1. average density = mass of Earth/volume = 5.5 gm/cc (note that cc is shorthand notation for cm to the 3rd power). Granite has a mean density of 3.0, which is mostly what our crust is made of. Therefore, since the mean density of the whole Earth is larger than granite, the core of the Earth must be made of something much denser than granite -> Iron (Fe) and Nickel (Ni).
   
2. Seismology uses the propagation of seismic waves from earthquakes to study the Earth's interior (similar to refracted light). Two types of seismic waves 1) pressure or P waves and 2) shear or S waves. Shear waves cannot propagate in liquids or gases, since there are no lateral restoring forces.

   Seismic waves travel at about 10 km/sec and, from mapping of the timing and type of wave around the globe, we are able to map the interior of the Earth. Changes in refraction of seismic waves are due to sharp changes in the density = discontinuities due to chemical composition.

   The result is that we know that the interior of the Earth has 4 components:

   1. a thin crust of density 3.3 gm/cc composed of metals, silicates (a substance called basalt)
   2. a semi-solid mantle of density 3.5 to 5.5 gm/cc composed of olivine Fe oxides
   3. a liquid outer core of density 9 to 11 gm/cc composed of molten Fe
   4. a solid inner core of density 17 gm/cc composed of Fe and Ni.

   The temperature of the inner core is 6200 K. The layers of lower density float on top of the the higher density ones, like cork on water. The rocky crust is therefore on the outside.

3. Crustal dating -> radioactive dating (note this is not carbon dating, that's different) - radioactive elements have known half-life's = the time for the material to decay to a lighter atom.

   For example: uranium 238 decays to lead 206 with a half-life of 4.5x10⁹ years. So, if a rock is 1/2 U 238 and 1/2 Pb 206 then its age is 4.5x10⁹ years.

   Oldest rocks on the Earth are 3.8x10⁹ years by radioactive dating. The oldest rocks in meteors are 4.7x10⁹ years. Thus, the crust is not original Solar System material. It has been reprocessed by some method (hint: the age of the ocean floor is only 0.2x10⁹ years).

Earth's Crust:

The surface of the Earth is 71% water and 29% land (we should have named our planet Ocean).

The dry land is composed primarily of:

1. Igneous rock - formed from molten material, such as basalt and granite
2. Sedimentary rock - minerals cemented by pressure, such as sandstone and limestone
3. Metamorphic rock - igneous or sedimentary rock that has been subjected to high temperatures and pressures, such as marble

The lifecycle of these rocks is given in the following diagram:

The crust shaped by:

1. impact cratering in the early solar system

2. erosion - wind, water, slumping (gravity) - most early cratering erased by erosion on planets with thick atmospheres

3. thermal-tectonic activity (plate tectonics) - outflow of heat from core transfered to convective motion in mantle.

There are four (4) types of boundaries between the plates which give rise to particular surface features. For example, colliding plates form mountains.

Young mountain system are sharp and irregular (e.g. Himalayas), old mountain systems are low and rounded (e.g. Appalachians)

An example of tectonic activity in the form of volanic activity on the Earth = Mt. St. Helens:

before

after

As early as the 1920s, scientists noted that earthquakes are concentrated in very specific narrow zones, now known to be plate edges . In 1954, French seismologist J.P. Roth published this map showing the concentration of earthquakes along the zones indicated by dots and cross-hatched areas.

Earth's Magnetic Field:

A field is one of those mathematical conceptual tools to help us understand the behavior of objects with energy. A field assigns to every point in space a strength or force plus direction. A field is used to calculate resulting motion of object within the field and acted on by the field.

The Earth is surrounded by a magnetic field, generated in the core of our planet, in the shape shown below. The field lines (red in diagram below) show different strengths where the lines closest to the Earth are stronger than the lines farther away.

Earth's magnetic field is distorted, compressed on the side facing the Sun, greatly extended on the opposite side. The reason for this is the solar wind, a particle flux from the sun, consisting mostly of protons and electrons.

Our understanding of the origin to magnetic fields in planets is very poor. We know that a conducting fluid in motion generates a magnetic field. The nature of this field and its evolution is governed by the field known as magnetohydrodynamics. The liquid outer core of the Earth is the conducting fluid, free electrons being released from metals, such as Fe and Ni, by friction and heat. Variations in the global magnetic field represent changes in fluid flow in the core. Fossil evidence for field reversals on timescales of 10,000 years indicates that the process of magnetic field generation is unstable.

For the planets it is key to know that a magnetic field indicates that:

1) the planet has a large, liquid core
2) the planet has a core rich in metals (source of free electrons)
3) the planet has a rotation rate

The strength of the magnetic field is telling you something about the combination of the above factors. For example, Mercury has a weak magnetic field. But, since it has a very low rotation rate we conclude that it has a large liquid core. Mars has a high rotation rate (similar to Earth's), but a magnetic field that is 1/800th the strength of the Earth's. Therefore, we conclude that Mars has a very small core.

Evolution of TP surfaces:

The evolution of a planetary surface is dominated by the following processes:

impact cratering

tectonic activity

erosion

Note that this list is also in temporal order since impact cratering occurs first, followed by tectonic activity and then erosion. Also note that all the planets receive the same amount of impacts from remnant debris in the early Solar System. But that the amount of tectonic activity and erosion varys from planet to planet.

Impact cratering:

After the formation of the planets some 4.5 billion years ago there was a tremendous amount of material leftover. This material was in the form of icy rocks that had various orbits out to the cometary Oort cloud. Often these orbits intersected with the forming planets and hence would impact on the newly formed surfaces with a great deal of kinetic energy.

While the surfaces were molten, these impacts would have just added more material to the planet (in fact, some of the H₂ and CO₂ in the mantle comes from early comet impacts). But as the planets cooled, the crust would have cooled and solidified first. Later impacts would have either 1) created craters or 2) burst through the crust to the mantle to release lava to form basins. Note that as time pasts and the planet cools, crust becomes thicker and impacts that form basins become rarer. Basins will be filled in, partially, with later cratering.

Planets with old surfaces have large amounts of impact cratering. Planets with young surfaces (young meaning later changes) have little evidence of the early epoch of cratering. Most impact basins were later destroyed due to more impacts (the smooth terrain was cratered) with the exception of the Moon, whose nearside was shielded by the Earth.

Tectonic Activity:

The amount of tectonic activity on a planet is controlled by the amount of heat stored in the planets interior after formation. The larger the amount of heat, the more energy stored that is transfered to the surface in the form of geological activity. Although the process of tectonic activity is still mostly unknown (see a Geology course), the connection between interior heat and activity is supported by the observations of the Galilean satellites where the inner moons, which are heated by tidal friction with Jupiter, are also the most geologically activity.

The amount of heat stored in a planet's interior comes from two sources, 1)the energy of formation of the planet and 2) heat generated by the decay of radioactive elements.

Formation energy or leftover heat is due to the fact that the debris and gas that the planet forms from coalese's into a ball. The potential energy from gravity of this infalling material is converted to kinetic energy (heat) as the debris falls together. Thus, the higher the mass of the planet, the greater the amount of energy deposited on it during formation, the greater the heat and, therefore, the greater the amount of tectonic activity.

The amount of heat from radioactive materials is also proportional to the mass of the planet. Again, more mass = more radioactive material = more heat from radioactive decay.

Tectonic activity displays itself in the following ways:

gravity (slumping)

Depending on the mass of the atmosphere, this list is in order of strength. Atmospheric erosion has short timescales, on order of hundreds of thousands of years. Tectonic activity can take on order of millions of years. Gravity slumping is only visible on airless worlds with timescales of billions of years.

Note that large features, such as impact basins or extremely large impact craters can not be eroded away even after 100's of millions of years. Such large features on the Earth were eroded by tectonic activity, i.e. the crust was recycled by plate motion such that those ancient impact basins are gone.

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Planetary Interiors:

Can not examine the interiors of planets directly (even our own). Thus, we build computer models which contain the following parameters:

seismic activity

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Chemical Fractionation:

The most important process early in the formation of a planet that influences its structure of its interior is gravity. Gravity causes heavier elements to sink to the core of a planet, this is called chemical fractionation.

Since this is a slow process, the planet may solidify before chemical fractionation can fully develop. Thus, large, massive planets, like the Earth and Venus, are molten long enough for a Fe and Ni core to form. Whereas, smaller planets, like Mars, cool faster and solidify before the heavier elements sink to the core. Thus, elements like Fe are over abundant in the soil, giving Mars its red color.

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Crust development:

The thickness of a planet's crust is directly proportional to the rate at which the planet cooled in the distant past. A fast cooling rate (i.e. a small planet) will result in a thick crust. For the major terrestrial worlds, the crust thickness is proportion to the diameter of the planet is:

Note that the cooling rate is proportional to the total mass of the planet. Large worlds cool slower, have thinner crusts. High cooling rates also determine the interior structure. Slow cooling rates imply planets that still have warm interiors now. Warmer interiors imply more diversified structure (inner core, outer core, semi-solid mantle, etc.)

Note also that a thicker crust means less tectonic activity.

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Summary of Terrestrial Planet Interiors:

The make-up of planet interiors is dominated by the physics of materials under high temperatures and pressures. Starting with cold, low pressure regions, rocky materials are straight solids. As one goes deeper into a planet the temperature and pressures go up. Solids become semi-solid, plastic-like materials. With higher temperatures and pressures, semi-solids become liquids. With even higher temperatures and pressures liquid or molten rocky materials undergo a phase change and become solids again. That is why the very inner cores of the Earth and Venus are solid, surrounded by liquid outer cores.

Just from examination of mean densities, the primary terrestrial worlds fall into two classes. High mean density worlds, Earth, Venus and Mars, with values around 5 gm/cc. And low mean density worlds, Mercury and the Moon, with values around 3 gm/cc. The high density worlds have Fe/Ni cores and clearly defined interior structure. Low density worlds have weaker structure, i.e. no strong cores. Cross sections of the major terrestrial worlds are found below. Areas marked in red are in a liquid or semi-solid state.

Several trends should be noted:

The lack of tectonic activity for the outer worlds has left them with warm interiors and large, liquid ice mantles.

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