Current Theory on the Formation of the Solar System - Part III
In Parts I and II we learned how the planets coalesced from the solar nebula and developed their structure (relating to their cores, mantles, and crusts). An important feature of planets, magnetic fields, are related to the composition of the planet as discussed in Part II, will be developed here. Also, we will continue the discussion on how their formation was very dependent on their distance from the Sun and this dependency leads to their classification and many of their primary features.
Most of the planets have magnetic fields. Those planets that have sufficient metallic components, iron cores or metallic hydrogen, and a mechanism to move those components may develop a magnetic field. Of the terrestrial planets, Earth has the strongest magnetic field which is tilted at 11.3 degrees to the axis of rotation. Earth's strong field strength is attributed to the combination of a high iron and nickel content in a solid core with an iron-rich liquid, outer core. This produces convective currents in the liquid outer core containing the metallic components and these currents move electrons; thus creating the magnetic field. The planets' rotational speed contributes to the movement of the electrons adding to the strength of the magnetic field. Despite its large iron content, Mercury has a magnetic field strength only 1% that of the Earth. Its interior may have only a small portion that is molten. This would minimize the metallic material available for convective currents to be generated. Its slow rotation on its axis (58.6 days) may also help explain its weak magnetic field.
Venus has no magnetic field. Its interior is not well understood but is believed to have a molten interior. This is evidenced by parts of its surface being only 10 million years old or less and the replenishing of sort-lived sulfur compounds in the atmosphere by volcanic action. Despite its molten interior the lack of a magnetic field may be due to its slow rotation of 243 days.
Mars may have had a global magnetic field in its past. Spacecraft indicate that Mars now has an incomplete magnetic field that is too weak to measure. Its low density indicates that it probably has a low iron content. Its small size means that it cooled more rapidly than the larger Earth and lost its molten interior. It is known to have had a molten interior in its past as evidenced by its inactive volcanoes; however, as the planet continues to cool it looses its molten interior and its magnetic field with it. Notice that Mercury, which is smaller than Mars, may still maintain some internal heat due to its close proximity to the Sun where tidal effects may cause tidal heating of the planet's interior.
Molten interiors containing metals such as iron and nickel are part of the reason that the terrestrial planets have magnetic fields. These materials were gathered during early planet formation and having remained molten they provide a way for metal-rich components to move by convective currents. In order for a planet to have a magnetic field it must have a mechanism to keep the electrons in motion.
All of the Jovians have significant magnetic fields. Among them, Jupiter has the strongest magnetic field. This field is so large that it extends beyond the orbit of Saturn and maintains very intense radiation belts within it. As described in the section on planetary interiors in Part II, Jupiter and Saturn have deep layers of liquid metallic hydrogen that supplies the metallicity needed in a fluid state combined with an iron core and rapid rotation to produce substantial magnetic fields. Uranus and Neptune lack the liquid metallic hydrogen layers but a conductive layer of water probably provides the necessary electron source. The Jovians collected all of these electrically conductive materials during the planet formation period.
Jovian planets' magnetic fields are all aligned in the reverse direction as the Earth's. The north magnetic poles on the Jovians are closest to their respective geographic north pole. On Earth the north magnetic pole is aligned less than 12 degrees from the axis of rotation and located closest to the south pole. Jupiter's magnetic pole is aligned 10 degrees from its axis of rotation and Saturn's is not tilted at all. The magnetic fields of both Uranus and Neptune have large tilts with respect to their rotation axis, 59 and 47 degrees respectively. Additionally, they are offset from the centers of the planets. One theory is that these two gas giants are experiencing magnetic pole reversal although the reasoning behind why both planets are experiencing it at the same time, due to coincidence or not, is somewhat dubious without explaining the mechanism responsible.
Size and Density
The classification of the planets as either terrestrial or Jovian is based on whether they more closely resemble the Earth or Jupiter. The terrestrial planets all have solid surfaces with features such as craters, hills, valleys, and either active or dead volcanoes. The Jovians possess large cores that contain rocky material and ices but they also have very dense layers of gases such as hydrogen, helium, methane or ammonia. As such, the Jovians do not offer a solid surface on which one could stand. Most of the size and mass of the Jovians are the result of these dense, gaseous envelopes.
The four terrestrial planets constitute the four inner planets of the solar system; Mercury, Venus, Earth, and Mars. The four Jovian planets are Jupiter, Saturn, Uranus, and Neptune. It is no coincidence that the terrestrials are the four planets nearest the Sun while the Jovians are in the outer solar system. The composition, size and location of all of the planets were determined during planet formation in the solar nebula. Pluto, the ninth planet out from the Sun is most likely one of the Kuiper belt objects. Evidence for this conclusion is that Pluto orbits in the most eccentric orbit of all the other planets, is inclined more to the elliptic than any other planets, is very small, and with a low density more similar to cometary material than to the planets. With a rock and ice composition it is neither a gas giant nor a true terrestrial. It will be considered as a Kuiper belt object and not included here.
The relationship between size, composition, and density of both classes of planets are strongly related. The ratio between the largest and smallest terrestrial planet (Earth and Mercury) and the largest and smallest Jovian planets (Jupiter and Neptune) are quite similar at 2.6 and 2.8, respectively, based on equatorial diameter. However, there is a step increase when comparing the sizes of the terrestrial planets, where Earth is the largest with a diameter of 12,756 km, and with the Jovian planets, where Neptune being the smallest, has a size almost 4 times larger than Earth.
Despite the fact that the Jovians are much larger than the terrestrial planets they are not nearly as dense. This lower density is good supporting evidence that the solar nebula theory is correct. Metals and rock minerals such as silicates could exist as solids in the hotter inner solar system and so the planets that formed near the Sun contain a large proportion of these dense substances. The lighter gases and ices which are stable in the colder regions away from the central star would be available to make up the planets forming in those farther orbits. The least massive Jovian, Uranus, is 14 times more massive than Earth, the most massive of the terrestrial planets. The terrestrials show an increasing mass for increasing size much as would be expected, that is, the larger the planet the more mass it contains. This is only generally true for the Jovians. Jupiter is the most massive planet followed by the smaller Saturn. However, Neptune, the smallest of the Jovians is actually slightly more massive than Uranus, which may be due to its larger core. The terrestrials, with their high proportion of dense elements and compounds stable at high temperature, possess densities much higher than the Jovians. Iron and iron compounds, nickel and its compounds, silicates and other rock-forming materials comprise the bulk of the terrestrial planets. During their growth period, the Jovians acquired huge amounts of lighter substances, grew enormously in size but low in average density.
In the next and final part of this four part series, two prominent features of the planets - their atmospheres and satellites, including rings - will be described.