Current Theory on the Formation of the Solar System - Part II
In Part I we described how a large cloud of dust collapsed under its own gravity into a rotating accretion disk from which the central star would become our Sun and the growing planetoids would eventually assemble into planets. Here we will discuss how the different planets developed their various strata.
Planet Cores and Other Layers
As the planetesimals grew larger their interiors would heat up because of the increasing pressure due to gravity as well as from the heat of radioactive decay and continuing energetic impacts from collisions. In these hot, molten states the heavier materials, such as iron and nickel would settle toward the center of these planet-sized bodies and form cores. The less dense silicates and oxides would tend to float above the heavier metals creating mantles. The lightest material would rise to the top and form crusts. This process is called chemical differentiation and would form the core, mantle and crust of the planets in the inner solar system. Volcanic out-gassing from these molten interiors and from continuing impacts created atmospheres around the inner planets.
The most is known about the interior of the Earth. Its core is solid, composed mostly of iron and nickel with a small amount of lighter elements. Earth's core measures about 2600 km in diameter. A liquid outer core, also of iron, extends out another 2200 km. Earth may be the only planet with a distinct inner and outer core. A solid mantle, 2900 km thick, is differentiated further into silicon, magnesium, calcium, oxygen, aluminum, and some iron in the deepest portion and compounds of iron, silicon, calcium, aluminum, oxygen and magnesium in the upper mantle. The crust, averaging 40 km thick, is mostly made from lighter compounds of silicon. The Earth's interior serves as a useful model for comparison of the other terrestrial planets since all are probably made of similar compounds and have similar structure.
Mercury has an exceptionally large iron core that is 75% of its diameter and is estimated to be 3600 to 3800 km in diameter. This core, which gives Mercury the highest proportion of iron of all of the terrestrial planets, is possibly due to the high temperatures associated with the inner reaches of the solar nebula, which prevents compounds with low condensation temperatures from proliferating. Another possibility is that a collision with a large planetesimal during the late stages of planet formation stripped Mercury of much of its mantle containing the lighter components. This big core still leaves room for a 600 km thick rocky mantle, which may be partially molten, and a thin silicate crust. Due to its location close to the hot center of the solar nebula, Mercury appears to be the most chemically differentiated of the terrestrial planets. Interestingly, despite its large iron core, iron is not seen in spectroscopic analysis.
The interior of Venus is believed to contain an iron core similar to the Earth's and evidence of recent volcanism leads to the conclusion that its thick rocky mantle is molten. The thickness of Venus's crust is now believed to be thicker than some previous theories assumed.
Despite more intensive investigation and exploration than any other planet, the interior of Mars remains largely unknown. It may possess a dense core about 3400 km in diameter beneath a rocky mantle and a crust that varies from 80 km to 35 km thick.
Unlike the terrestrial planets the four Jovian planets possess huge gas envelopes above a rocky core. Because they have massive layers of hydrogen and helium the gas giants do not have solid surface like the terrestrial planets. Instead their gaseous layers just get denser with increasing depth.
Jupiter probably has a rocky core equivalent to 13 Earth masses that represents 4% of its mass. This core is compressed to a sphere only slightly larger that Earth, about 20,000 km in diameter. The oblateness of these gas giants allows for calculations to estimate what their interior composition may be. In Jupiter's case there is believed to be a 40,000 to 50,000 km thick layer of metallic hydrogen beneath a molecular hydrogen layer that is 10,000 to 20,000 km thick. On top of this huge planet is an atmosphere, compressed by the massive gravity of Jupiter, only 75 km thick.
Saturn is the most oblate of the Jovians but since it rotates slower than Jupiter its oblateness must not be due to its rotational speed but to its interior being distributed in a different manner. Saturn has a rocky core perhaps 15,000 km in diameter under a metallic hydrogen layer 10,000 km thick. Above this is a molecular hydrogen envelope 20,000 km thick. Saturn's atmosphere is believed to be about 300 km deep. Traces of various ices are also present.
One model of the interiors of Uranus and Neptune give them similar structure. They are believed to be composed of about 15% hydrogen, some helium, various ices, and rock. In this model both have rocky cores beneath a thick mantle of liquid or solid water that may also contain ammonia. The top layer is liquid molecular hydrogen and some helium in the liquid state. Neither Uranus nor Neptune have the liquid metallic hydrogen layer that Jupiter and Saturn have. Another theory holds that Uranus's core is distributed uniformly throughout the planet rather than being concentrated in the center. Both planets are denser than the solar nebula model would expect. This could be explained if they took longer to form their cores because they orbited slower due to their greater distance from the center of the solar nebula. Perhaps by the time they were ready to collect large amounts of hydrogen and helium, as Jupiter and Saturn had, the solar wind of the new Sun may have disbursed these light gasses out of the solar system.
Cool temperatures in the outer nebula allowed the retention of light gases. As these planetesimals grew into planets gravity compressed the large amounts of hydrogen into a metallic liquid on Jupiter and Saturn. Denser compounds such as ices of methane, ammonia and water were acquired and retained by Uranus and Neptune. Uranus and Neptune may just have been orbiting too far away from the center of the nebula and therefore moving too slow to participate more fully in planet formation as was experienced by Jupiter and Saturn.
Closer to the protostar the temperatures were hot enough to prevent the condensation of the light elements and volatile compounds so these substances were not available. Hydrogen and helium atoms would remain too energetic to be captured by the planetesimals of the inner nebula. Here conditions did not accommodate the growth and formation of huge gas layers. Instead the inner planets had only rocks and metals with high condensation temperatures available. These substances dominated and so were accreted into the four terrestrial planets. The smaller terrestrials cooled quicker than the larger bodies, lost their internal heat, and became geologically inactive. The larger terrestrial planets, Earth and Venus, still possess molten interiors and remain geologically active.
An important feature of some planets is the presence of a magnetic field. The existence and strength of a planet's magnetic field is strongly related to the composition of its core and other strata. The discussion of magnetic fields will be addressed in Part III. That article will also address the relationship of size and density of the planets to their position in the solar nebula and how those attributes determine whether a planet is classified as a gas giant or as a terrestrial.