1.4 Gas Planets, Rocky Planets, Icy Planets: The Chemical Composition of the Solar System

The substances that make up the planets are divided into three compositional groups: gases, rocks and ices, based on their melting points.

Gases – hydrogen and helium are those with melting points near absolute zero, also known as zero degrees Kelvin. These two gases were the most abundant constituents of the solar nebula.

Rocks – are principally silicate minerals and metallic iron, which have melting points that exceed 700 degrees Celsius.

Ices – include ammonia, methane, carbon dioxide, and water. They have intermediate melting points, for example, water has a melting point of 0 degrees Celsius.

The terrestrial planets are dense, consisting mostly of rocky and metallic substances, with minor amounts of ices. The Jovian Planets, on the other hand, contain large amounts of gases, hydrogen and helium, and ices, mostly water, ammonia, and methane. This accounts for their low densities. The Jovian Planets also contain substantial amounts of rocky and metallic materials, which are concentrated in their central cores.

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