Chapter 4 - Element Abundances of the Planets

Abstract

This chapter discusses the element abundances of different planets, their time of formation, and the key bits of evidence that led to these descriptions. The solar system contains nine planets that orbit the Sun. The sidereal period is the time it takes to make one orbit around the sun. The orbital characteristics are best explained by the hypothesis that the stellar cloud from which the Sun formed was spinning and that the Sun and the planets were formed from the same cloud around the same time. The mass of each planet is determined from the gravitational influence it exerts on the orbits of its moons and on the space probes sent out from the Earth. The abundances of elements on the Earth, as estimated from their abundances in chondrites, are strongly correlated with the relative abundances of the elements in the universe as measured by their abundances in the solar spectrum. These abundances have been influenced, however, by the tendency of the elements to exist in nonvolatile forms. All elements other than the inert gases have some tendency to take part in chemical reactions. Geologists have spent a great deal of time studying the age of rocks that are believed to have originated on the Earth. In general, rocks found on dry land are far older than those found on the sea floor. The solar abundances of various elements, together with their chemical affinities, are the two factors that explain which elements should dominate the composition of a planet.