3: Planetary Geophysics


• 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.
• 3.1: Orbital Mechanics
Orbital mechanics is a branch of planetary physics that uses observations and theories to examine the Earth's elliptical orbit, its tilt, and how it spins. Observations of the orbital behavior of planets, moons or satellites (orbiters) can provide information about the planet being orbited through an understanding of how these orbital properties are related to gravitational forces.
• 3.2: Layered Structure of a Planet
Once the mass of a planet or moon is known, the next thing a planetary scientist want to know is what is the layered structure inside the planetary object. Does the object have a dense iron core? Or, is the object homogeneous? How thick is observed crustal (or ice) layer that is observed at the surface. This information is more difficult to determine, but with just one more piece of information and some educated assumptions, it is possible to determined a simple layered structure.
• 3.3: Two Layer Planet Structure Jupyter Notebook
An interactive model to explore the layered structure of far-away planetary interiors from the limited information available to us, assuming there are two distinct layers. Build your own model of a planet, and investigate how varying its mass, radius, and density change your model.
• 3.4: Isostasy
Very broad or long wave-length topography is more likely to be compensated than smaller scale topography. This is because large topography is more massive and causes the lithosphere to bend into the deeper viscous mantle. Smaller topographic features can be supported by the strength of the lithosphere, and therefore no compensating mass anomaly forms beneath the topography. Compensated topography is also referred as isostatically compensated topography, where the word isostatic refers to equal
• 3.5: Isostasy Jupyter Notebook
Interactive examples of the principle of isostasy starting with an iceberg, and then moving to more complicated examples of the crust and the mantle, and a crust with multiple thicknesses. See how the thickness and densities of different materials affect the isostasy model.
• 3.5: Observing the Gravity Field
Satellite altimetry provides a very high resolution image of the gravity field. This measurement is so high resolution that it can be converted into a map of seafloor topography and used to locate plate boundaries, fracture zones and seamounts both big and small. It is not high resolution enough to use for navigation of a real boat. However, if you use google maps and turn topography on, and got hunting around off the coast, the background topography images for the globe is the topography derive
• 3.7: Gravitational Potential, Mass Anomalies and the Geoid
Determining the layered structure of a planet requires knowing the gravitational field around a planet and in particular the elliptical shape of the gravity field. Recall from the previous section, that the oblate (ellipsoidal) shape of the planet is a direct result of the viscous flow of a spinning planet.  However, the gravity field can also provide information about density anomalies inside the planet and the viscosity layering in the convecting (mantle) part of the planet. Therefore, here we
• 3.8: Summary

3: Planetary Geophysics is shared under a CC BY-SA license and was authored, remixed, and/or curated by Magali Billen.