### Planetary Density and Planetary Interiors

• 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 density

• 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

• We study the Earth's interior and use this information to infer the nature of the other planets given their density.

• 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 iron.

• On Earth we have the luxury of examining how the Earth vibrates in response to earthquakes.

• Earthquakes create two types of waves which travel through the Earth.

• 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

• Seismic studies reveal that the interior of the Earth has an organized structure.

• 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 were molten 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 water/ice mantles.
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Revised April 4, 2001