The Planets and Chemical Compositions
Since scientists cannot directly determine the chemical composition of the deep interior of even the earth, how is it possible that astronomers can know the composition of distant planets? We don't know it by means of experimentation, but we believe we know something about it by means of a theoretical argument. The argument proceeds in the following fashion: Whatever composition the planets had at their birth has clearly not changed over the span of their lives since no significant influx of material from outside has been added to the planets' mass and no known processes are at work to change the original chemical composition into something else as there are in the case of stars.
What then was the original composition of the planets, and is it likely that it is not the same for each planet? As the solar system formed, it is probable that the sun was well along in the process and consequently reasonably hot by the time the planets began to form. Thus the temperature of the matter from which the planets would form was higher closer to the sun, probably 1000 K or so, and declined rapidly outward to about 100 K. Since that matter would have to be solid or solidlike, astronomers find that they can divide it into three groups on the basis of the ease with which they vaporize: The first group consists of the materials that are gases at temperatures above a few tens of degrees Kelvin, such as hydrogen and helium. Next are the icy materials, like methane, ammonia, carbon dioxide, and water (containing such light elements as carbon, nitrogen, and oxygen besides hydrogen), which do not vaporize until the temperature is over a couple of hundred degrees. Finally there are rocky compounds, such as iron and its oxides, silicates, and sulfides, which do not vaporize until the temperature is several thousand degrees. Hence close to the sun, where the terrestrial planets formed, the rocky materials would have been the only ones not in a gaseous form. Iron and the elements near it in the periodic table should dominate the compositions of the terrestrial planets, as they seem to do, while lighter elements, like hydrogen, helium, carbon, nitrogen, and oxygen, should be the principal constituents of the Jovian planets, as evidence from thei r atmospheres confi rms.
