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# 3.0: Introduction

Planetary geophysics is the study of the internal structure of the objects in our solar system and other solar systems, including planets, moons, and asteroids. It does not include the structure of the star (sun), but these studies do sometimes get into questions of planetary formation which occurs at the same time star/sun forms. Several remote observations of planets and moons can be used to determine their internal structure.

In this chapter we will first focus on two calculations used to determine the basic structure of a planet (mass and core/mantle layering from the moment of inertia) using observations of the that can be made remotely (orbital properties, spin and precession). Since orbiters are needed for the some of these observations, we will also consider the question of how long it takes to send a space craft from one planet to another (the Hohman Transfer Orbit).

Next, we will explore the gravitational field of the Earth, how it is related to plate tectonics and mantle convection and how we measure the variations in gravity at the surface of the Earth. The image below shows the "anomalies" in the gravitational field. If the Earth was only made up of concentric layers, there would not be any anomalies. Therefore, these anomalies give us information about lateral variations in density and the viscosity inside the mantle. We will finish up by considering the concept of isostasy and how it relates to the gravitational field.

## Characterizing Planetary Structure

Before we can consider whether another planet (or moon) might have plate tectonics or mantle convection, it is necessary to gather basic information about the structure of the planet. This information can be organized in to different levels of knowledge. First order information is the most basic and most fundamental. Second order information adds details or refinement to the first order information, and so on.

In terms of planetary structure, we can divide up the information into three levels (see figure below):

• 1st Order: Mass and radius: from the radius we can determined the volume, and then together with mass determined the bulk/average density $$\rho=\frac{m}{v}$$
• 2nd Order: Layers (stratification): what is the radius of the mantle $$R_m$$ and core $$R_c$$? This requires information about the moment of inertia and possible densities of the mantle and core $$\rho_m,\rho_c\rightarrow\rho_{bulk}$$
• 3rd Order: Internal (non-layered) density anomalies. If there is mantle convection, we expect regions of dense ($$\Delta P$$, cold, and/or composition) material and buoyant (-$$\Delta P$$, hot, and/or composition) material. These density variations can be detected as perturbations (anomalies) in the gravity field.
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