Overview

Abstract

This volume of the Treatise on Geophysics discusses fundamental aspects of the science of the planets. The solar system (outside of the Sun) contains a myriad of bodies ranging from masses of a few hundred Earth masses (the giant gas and ice planets) to bodies of a sizeable fraction of the Earth’s mass (the terrestrial planets and major satellites) to dust particles. There are planets orbiting the Sun and satellites that orbit planets. Small bodies can be found in orbit around the Sun and in orbit around planets and other small bodies such as asteroids. The number of satellites is small in the inner solar system where terrestrial planets termed after their compositional similarity to the Earth are to be found (Mercury, Venus, Earth, and Mars). The outer solar system with the gas and ice giant planets (Jupiter, Saturn, Uranus, and Neptune) has more than 100 satellites. For most of them ice is a major component aside from rock. The planets are largely of solar composition but differ in their depletion in volatile elements. The degree of depletion increases with decreasing mass and with decreasing distance from the Sun with Jupiter being closest in composition to the Sun and the terrestrial planets being mostly depleted in volatile elements. The solar system is explored with spacecraft, orbiters, landers, and rovers. The surfaces of the terrestrial planets and icy satellites are solid surfaces marked by impact craters and traces of endogenic activity such as volcanism and tectonism. The giant planets present the observer with their outer gas envelopes. The interiors of the planets and larger satellites are likely differentiated with the heavy elements tending to be found near the center and the most volatile elements near the surface. Therefore, density intrinsically increases with depth. There is additional increase of density due to compression. Most planets generate magnetic fields or are likely to have once in their history produced magnetic fields by dynamo action in their interiors. These dynamos are likely powered by thermal energy and gravitational energy. Thermal energy and gravitational energy are also the sources powering endogenic activity. Plate tectonics seems to be unique to the Earth although it cannot be ruled out that other terrestrial planets and satellites (icy and rocky) may have once experienced phases of plate tectonics or other forms of surface mobility and recycling. But thick immobile lids – in places punctuated by volcanic vents – seem to be more commonplace. The bodies in solar system are thought to have originated in a cascading sequence of ever bigger and fewer planetesimals leading to the formation of solid bodies the size and mass of the terrestrial planets in the inner solar system and some tens of Earth masses in the outer solar system. In the outer solar system these solid proto-planets became the cores of the future giant planets onto which the gaseous envelope collapsed.