How does the solar nebula theory explain the present densities and chemical compositions of the planets?

Question

What will be an ideal response?

Answer 1

According to the solar nebula theory, the observed pattern of planet densities originated when solid grains first formed, a process called condensation. Solid particles condensed from the gas of the nebula as it cooled. The kind of matter that could condense in a particular region depended on the temperature of the gas there. In the inner regions, close to the Sun, the temperature was evidently 1500 K or so. The only materials that can form grains at that temperature are compounds with high melting points, such as metal oxides and pure metals, which are very dense. Farther out in the nebula it was cooler, and silicates (rocky material) could also condense, in addition to metal. These are less dense than metal oxides and metals. Mercury, Venus, Earth, and Mars are evidently composed of a mixture of metals, metal oxides, and silicates, with proportionately more metals close to the Sun and more silicates farther from the Sun. Even farther from the Sun there was a boundary called the ice line beyond which water vapor could freeze to form ice particles. Yet a little farther from the Sun, compounds such as methane and ammonia could condense to form other types of ice. Water vapor, methane, and ammonia were abundant in the solar nebula, so beyond the ice line the nebula would have been filled with a blizzard of ice particles, mixed with small amounts of silicate and metal particles that could also condense there. Those ices are low-density materials. The densities of Jupiter and the other outer planets correspond to a mix of ices plus relatively small amounts of silicates and metal.