Titan, the largest satellite in the Saturn system, is a member of the second group of this system. This may be compared with the Jupiter system, in which the two largest satellites (Ganymede and Callisto) are in the second group. Such a comparison shows that, with respect to size, the other satellite in the second group of Saturn, Hyperion, constitutes an exception. In general, however, a comparison of the data for the second groups of Jupiter and Saturn indicates that the satellites in these groups have similar parameters.
So far, the third group of the Saturn system has only one known member, Iapetus. This satellite lies at a distance of 3,562,000 km from the planet. The fourth group of this system also has only one known member, Phoebe, which is 12,961,000 km away from Saturn. The distance between Iapetus and Phoebe is 9,399,000 km, a figure comparable to the distance between the third and fourth groups of the Jupiter system (9,250,000 km).
It is interesting that Phoebe, the fourth-group satellite of the Saturn system, is similar to all four fourth-group satellites of the Jupiter system in that it has retrograde motion. This is another indication of the overall regularity in the formation of the fourth-group satellites for the two planets.
It is thus likely that there are more satellites in the third and fourth groups of the Saturn system, in addition to Iapetus and Phoebe.
Now that we have compared the satellite systems of the two giant planets, we see that there is in fact a great similarity between them, The salient points of this similarity are as follows: 1. Both systems can be divided into four satellite groups.
2. The first and second groups are located within the same ranges of distance from the planet.
3. The second groups of both systems contain two members each, these being (with the exception of Hyperion) the largest satellites in the systems.
4. The distances between the third and fourth groups of both systems are about the same (9,250,000 and 9,399,000 kin).
5. The satellites in the fourth groups of both systems have retrograde motions.
6. For the fourth satellite groups of both systems, the inclinations of the orbits with respect to the planetary orbit and the planetary equator are greater than the inclinations for the other satellite groups.
7. The closest satellites in these two systems (Amalthea and Mimas) lie at nearly the same distances from their planets.
8. In both systems there is an increase in the mass of the satellites and in the distance between satellites for the first group, as we move outward from the planet.
9. In both systems the densities of the second-group satellites are about the same: Ganymede, 2.25; Titan, 2.34; Callisto, 1.56; Hyperion, 1.6 (?).
Consequently, in these two systems the same regularities exist with respect to the distribution of matter according to zones, according to the distances from the centers of the systems, and according to the sizes of the satellites. This all goes to show that the satellites in question were formed in groups. The foregoing data also indicate that these two planets passed through the same four evolutionary stages, leading in each case to the formation of four satellite groups.
The systems of the other two comparatively large planets, Uranus and Neptune, have not yet been studied properly, since these planets are far from the Earth. However, the data which are already available indicate that the same regularities exist in these systems as in the systems of Jupiter and Saturn. For example, the known satellites of Uranus conform to the same rule as the first satellite groups of Jupiter and Saturn. As regards Neptune, however, only two satellites have as yet been discovered, so that it is too early to evaluate the structure of this system.
We shall conclude our study of the planetary systems by considering the Earth and Mars, the remaining two planets possessing satellites, no satellites of Mercury, Venus, or Pluto having been discovered so far.
First let us consider the system of Mars. The two Martian satellites, Phobos and Deimos, lie quite close to the planet: Phobos is 9400 km away, and Deimos is 23,500 km away. These two satellites are both very small, being comparable in size to asteroids: Phobos has a diameter of 16 km and Deimos has a diameter of 8 km.
The orbital inclinations of these satellites to the orbit and equator of the planet are approximately the same as the inclinations of the satellites of Saturn in groups I, II, and III.