Module 2 - Planetary Formation and Differentiation

Density

Density is the measure a substance’s mass per unit volume.  It is most often represented by the Greek letter rho, ρ.  The equation for density is:

ρ = m /V

 where m is the mass and V is the volume. Typical units are grams per cubic centimeter (g/cm3), or kilograms per cubic meter (kg/m3).
 
Relative differences in density will cause a denser, less buoyant substance to sink, while a lighter, more buoyant one will rise.  This causes layering (also called density stratification) to occur; the densest stuff will be at the bottom, or closest to the center of gravity, while the lightest will be at the top, or farthest from the center of gravity. 

For the case of a differentiated rocky body, its center is also its center of gravity.  Solid iron metal is the densest and is thus found at the center of gravity. Crusts of terrestrial planets are commonly rich in a low-density silicate mineral called plagioclase feldspar, and are found farther away from the center of gravity. 

One of the lines of evidence that the Earth has a dense core is that the mass of the Earth is much greater than can be attributed to rocks seen at the surface.  Basalt is a typical crust rock for terrestrial planets.  The density of basalt (3.2 g/cm3) is significantly less than the mean density of the Earth (5.52 g/cm3).  Therefore, there must be a very dense core.
 

Relationship between density and temperature

The volume of a substance correlates with temperature; when a substance heats up, its volume expands, and when the substance cools down, the volume contracts. An example of this phenomena is your car tires. In the winter, it is common for the air in your car tires to contract from the cold, causing your tire pressure to go down and the warning light on your dash to go on.  But once you drive around for awhile, your tires warm up, and the tire pressure light will go away. (Unless you really do have a flat tire!) 
 
So now let’s think about how the changing volume with temperature affects density.  Density is mass per unit volume.  With increasing temperature, the mass stays constant, but it is spread out over a larger volume, causing density to decrease.  Conversely, with decreasing temperature, mass stays constant, but it is squeezed into a smaller volume, and density increases.  Density is therefore inversely proportional to temperature (when temperature goes up, density goes down).
 
This relationship between temperature, density and buoyancy is really well illustrated by a lava lamp.  Be groovy and impress your friends by teaching them how a lava lamp works:

For the planets, changes in density related to changes in temperature drive important geologic processes, including
  1. the rise of hot magma (molten rock) relative to surrounding colder rock,
  2. the rise of hot fluids and eruptions of geysers on Earth, or Saturn’s moon Enceledes,
  3. mantle convection and plate tectonics, which in turn governs the distribution of volcanoes and earthquakes on the Earth.  (This is a really, really  important process for the Earth, so stay tuned for the next module.)
The next sections discuss the major layers of terrestrial bodies, ordered from most to least dense:

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