Current Theory on the Formation of the Solar System - Part IV

John Clevenger

In this, the last of a four-part series on the currently accepted solar system formation theory, two of the most obvious features of the planets will be addressed, atmospheres and satellites.


The terrestrial planets are warmer than the Jovians because of their closer proximity to the Sun. The average high temperature on Mercury is around 600 K and on Mars it is about 300 K. At these temperatures the light elements, such as hydrogen and helium, possess too much kinetic energy to be retained by the relatively low gravity of these terrestrial planets. This is consistent with the formation theory where closer to the hot protostar temperatures would have kept the volatile substances such as hydrogen, helium, ammonia, and methane in the gaseous state and prevented them from condensing out as the inner planets were accreting material. In the colder outer solar system where Jupiter sees temperatures around 163 K and Neptune is at only 55 K, these substances would have condensed out and become available to the outer planets.

Planetary mass also plays a major role. Without significant gravity a planet will lose whatever atmosphere it might have acquired during early planet formation or as the result of ongoing volcanic activity. The gas giants can retain light gasses and more volatile substances due to their significant gravity because their location in the solar nebula enabled them to acquire great mass.

Out-gassing from volcanic activity associated with a molten interior and impacting material is responsible for the atmospheres of the terrestrial planets. Because Mercury is small and close to the Sun it has virtually no atmosphere. With the high temperatures close to the Sun, most gasses are sufficiently energetic to escape from its low gravity. Mars has a slightly greater gravity than Mercury but because it is cooler than Mercury it is more successful in retaining an atmosphere, although it is quite thin, comprising 95% carbon dioxide, 3% nitrogen, 1.6% argon and a trace of other gasses.

Earth and Venus have substantial atmospheres with Venus's being 90 times denser than Earth's. The heavier gravity of both planets makes it possible to retain these dense atmospheres. Venus's atmosphere contains 96% carbon dioxide, 3% nitrogen, and 0.003% water vapor. Earth possesses an atmosphere containing 78% nitrogen, 21% oxygen, and 1% argon. Earth's oxygen component is a product of extensive biological activity. It probably had a much higher carbon dioxide content in its past. The carbon dioxide has been washed out of the atmosphere by rainfall and now resides in the oceans and rocks. Venus, without bioactivity or the ability to wash it out has maintained its large carbon dioxide atmosphere.

The Jovian planets retain large amounts of light gasses and other volatile compounds due to their location farther from the heat of the Sun and because of their greater mass. Jupiter and Saturn retain atmospheres of hydrogen, helium, and methane. Because of its great mass Jupiter does not permit a deep atmosphere to exist and compresses its cloud layer into a region only 75 km deep with wind speeds above 600 km per hour. The atmospheric components are 90% hydrogen, 10% helium, and 0.07% methane with ammonia ice, ammonium hydrosulfide and a mixture of ice and water constituting its clouds. Saturn's lesser gravity permits the cloud layer to occupy a region about 300 km deep in an atmosphere made up of 97% hydrogen, 3% helium, and 0.05% methane. It is harder to see striking features in Saturn's atmosphere, as we do with Jupiter, because the clouds are deeper in its atmosphere. The atmosphere of Uranus is about 83% hydrogen, 15% helium and 2% methane. Neptune's atmosphere is 74% hydrogen, 25% helium, and 1% methane. All the Jovian planets have fast moving winds but Neptune's are the fastest at 2000 km per hour.


When comparing the satellites of the terrestrial and Jovian planets there is a noticeable scarcity of terrestrial moons and a plenitude of Jovian satellites. Of the four terrestrial planets Mercury and Venus do not have any satellites. Mars has two very small satellites, Phobos that is less than 28 km in diameter, and Deimos, which is less than 18 km in diameter. Like asteroids they have 10% carbon in their makeup. It is not known whether Phobos and Deimos are captured asteroids or if they formed in orbit around Mars using material leftover from the formation of the asteroids.

Current theory regarding the origin of the Earth's moon believes that a collision occurred between the Earth and a large planetesimal 4.6 billion years ago while the Earth was still forming from the solar nebula and the mass that was ejected from the Earth formed the moon. This collisional ejection theory is supported by much of what is known about the Moon from analysis of its rocks, such as its low iron and water content.

Jupiter possesses 39 known satellites. Four of these, discovered by Galileo, are large, natural satellites of Jupiter. The two innermost Galilean moons, Io and Europa, with densities of 3570 kg/m3 and 3020 kg/m3, are probably composed of rocky material. Ganymede and Callisto have densities below 2000 kg/m3 and are probably composed of a combination of rock and water ice. The presence of these Galilean satellites with their decreasing densities as one moves outward from the primary indicates that their formation may be modeled much like the solar nebula theory for the formation of the planets. Jupiter's inner four moons are small, irregular shaped bodies. The outer eight moons have orbits highly inclined to Jupiter's equator with the outer four in retrograde motion. These outer eight moons may be captured asteroids. The eleven moons just discovered this year are all no more than 2 to 4 kilometers across (assuming their surfaces are very dark).

Saturn has 30 moons (four were discovered in 2000) of which none of the large moons (greater than 1000 km diameter) has a density greater than 1880 kg/m3. This indicates that they are composed of a combination of rock and ice. Like Jupiter, it is believed that at least the seven large moons were created along with the planet. The small irregular moons may have been formed with Saturn or could be captured asteroids. This is most likely true of Phoebe who has a retrograde and highly tilted orbit.

Uranus has 21 known satellites, five of which are larger than 470 km in diameter and all with densities at or below 1350 kg/m3, which indicates rock and ice composition. All but two of these moons have prograde orbits near the plane of Uranus's equator indicating that they may be natural satellites. Neptune has eight known moons of which only Triton is of significant size at 2700 km diameter. It has retrograde motion with a tilted plane (23 degrees) and is probably a captured planetoid. Its density of 2070 kg/m3 suggests that Triton is composed of rock and ice and that it formed in the outer solar nebula.

As we have just seen, in addition to the four terrestrial planets the solar system also contains seven large, terrestrial-like satellites, Io, Europa, Ganymede, Callisto, Titan, Triton, and the Moon, ranging in size from 2700 to 5262 km in diameter (by comparison Mercury is 4879 km in diameter). The Moon, Io, and Europa are composed of rocky material and the others of rock and ice. Additionally, six of these large moons have atmospheres, some very tenuous, either gaseous or frozen. The Earth's moon is the only one of these seven terrestrial-like satellites that does not have an atmosphere.

All the Jovian planets have rings. Jupiter's small rings may be from impact dust from Jupiter's moons. Saturn's spectacular ring system, which is composed of ice and ice-coated rock, may be material that failed to form into a moon. The dark rings of Uranus and Neptune could have originally been ices of methane that have been changed into carbon compounds by incessant bombardment by electrons trapped in the magnetic fields of the two planets in a process called radiation darkening.


This completes our review of the planetary formation theory that comprises the current consensus of how the planets of the Solar System were formed. The temperature gradient between the inner and outer regions of the solar nebula is the key component of the theory, explaining why the terrestrial planets are made of heavy metals and rocky constituents and the Jovians possess rocky cores, ices, and large envelopes of volatile gases.