Planetary Density and Planetary Interiors
We study the Earth's interior and use this
information to infer the nature of the other planets given their density.
- We must view planets from the outside
and cannot directly investigate their interiors.
- The perspective, to some extent, holds for the Earth as well.
- The deepest hole ever drilled into the Earth is 12 km deep (1/500 of the Earth's radius) and is equivalent to drilling 1/10th the way through the skin of an apple.
- We can, however, easily measure a planet's average
- Densities of common substances in the solar system.
- Iron - 8 grams / cubic centimeter
- Rock (Silicates) - 3 grams/cc
- Water/Ice - 1 gram/cc
- Highly Compressed Gas - 1-2 gram/cc
On Earth we have the luxury of examining how the Earth vibrates
in response to earthquakes.
- The mean density of the Earth is 5.5 grams/cc.
- The Earth's surface is mostly rock (density = 3 grams/cc) but its density is intermediate between that of rock and that of iron.
- From this we infer the Earth contains a significant amount of
Seismic studies reveal that the interior of the Earth has an
- Earthquakes create two types of waves which travel through the
- Pressure/primary waves are like sound waves and result from the compression of material (P-waves).
- Shear/secondary waves are like waves created by shaking a rope (S-waves).
- P-waves can pass through liquids. S-waves cannot.
- Boundaries in density and composition refract (bend) earthquake waves in the same way that glass refracts light
- The iron is not mixed throughout the Earth, but has
collected in an iron core at the center making up about 1/2 the diameter of the Earth.
- Temperature increases with depth into the Earth reaching 6000K at the center.
- Despite temperatures well above the melting point of rock or iron at the Earth's surface, the mantle and inner core are solid due to the high pressure.
- Release the pressure on the mantle rock and it liquifies.
- The Earth's "lightweight" crust is a thin layer which floats on
top of the slightly denser mantle material.
- The crust varies in thickness from 5-10 km under the oceans to 20-70km under the continents.
- Why the organized structure? Gravity causes dense material (e.g. iron) to separate from
lighter material (e.g. rock) of a liquid planet.
- Shortly after they formed, most planets
due to radioactive heating and the heat from impacts.
- The dense material (iron) sunk toward the center.
The lighter material (rock) floated to the top.
- This process of gravitational separation of light and dense
material in a liquid planet is called differentiation.
- Thus the Earth has a core of iron and a crust composed of relatively low density rock.
- Iron and rock are two of the most abundant
substances in the inner solar system.
- Terrestrial planets have iron cores and rocky mantles.
- The satellites in the outer solar system have rocky cores and
Revised April 4, 2001