Demonstrate the relationship between mass, volume, and density and show how this relationship can be used to calculate planetary density.
Introduce the concept of moment of inertia and use it to calculate planetary mass distribution.
Investigate physical parameters that affect seismic wave velocity.
Examine lines of evidence for the existence of a core.
Geophysicists believe that the Earth is divided into numerous shells, each with different physical and chemical properties. Because we can’t collect samples from the center of the Earth, we base our models on indirect evidence, such as data from seismic events.
To construct a model of the Earth’s interior, we use a variety of geophysical observations. Estimates of the Earth’s dimensions are combined with measurements of gravitational acceleration to determine planetary density. Precession rate is used to calculate the Earth’s moment of inertia, a measure of how mass is distributed within the planet. Arrival times of seismic waves at different seismometers on the Earth’s surface are used to infer the physical properties of rocks and how they vary with depth. Shadow zones of seismic waves show that the Earth has two distinct layers, a mantle and a core (Fig. 1).

Fig. 1: Internal structure of the Earth.
In the tradition of Sherlock Holmes, we pool different lines of evidence to create a comprehensive model of the Earth’s interior. We must reevaluate our model whenever investigations bring to light new clues about the Earth’s interior.


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