Amalthea, the satellite closest to Jupiter, is 181,000 km from the planet, an important fact which will be discussed later. The table shows that the distances between satellites and the satellite masses in the first group increase. It may be that between Amalthea and Io either there were or there ought to be two more satellites, larger than Amalthea and smaller than Io.
The distance between the first and second groups (that is, between Europa and Ganymede) is 399,600 km, and the distance between Ganymede and Callisto (that is, between the satellites in the second group) is about twice this value (813,000 km).
The second and third groups of Jovian satellites are separated by 9,616,000 km, and the distance between the adjacent satellites of the third and fourth groups (9,250,000 km) is about the same [see table on inset page].
Next, let us consider the inclinations of the satellite orbits with respect to the planetary orbit and the planetary equator (see table on following page).
The further away from the planet the satellites are, the greater are the inclinations of their orbits. This is particularly noticeable for the third and fourth groups.
Now let us compare the densities of the terrestrial planets with the densities of the first group of Jovian satellites: As the table shows, the satellites in the first satellite groups of the two largest bodies in the solar system (the Sun and Jupiter) have nearly identical densities. Thus it may be assumed that the physical situation on the primary, as well as the chemical processes taking place there, were of a similar nature during the evolutionary stages of the Sun and Jupiter in which their first satellite groups were formed.
If the asteroids are considered to represent a fifth, fragmented planet in the first group of the Sun, then the density of this hypothetical planet (3.7) can be compared to that of the Jovian satellite Europa (3.15).
When all these data are studied separately just for the Jupiter system, they are not particularly striking. However, a comparison with the data for the system of Saturn, the second giant planet in the solar system, leads us to draw the following conclusion: certain regularities in the solar system exist which have not as yet been recognized.
Now let us consider the satellite system of Saturn. As the table compiled by the author shows [see inset page], the satellites of Saturn can be divided into four groups in the same way as were the Jovian satellites.
The first group has five members, located (like the first group of Jovian satellites) at distances from 100,000 to 700,000 km from the planet.
The distances between the satellites in this group are found to increase consistently with the distance from the planet, and the same is true of the equatorial diameters of the satellites. Finally, the inclinations of the orbits of these five satellites also indicate that they should be members of a single group: Let us now compare the first satellite groups in the systems of Jupiter and Saturn. The following analogous regularities are observed for these two groups.
1) The first satellite groups of both systems have the same range of distances f^om the center of the system, that is, they are located from 100,000 to 700,000 km from the planet.
2) In both systems the closest satellite is located about the same distance away from the planet: Amalthea is 181,000 km from Jupiter, and Mimas is 185,600 km from Saturn.
3) In both systems the distances between the first-group satellites increase with the distance from the planet.
4) The sizes of the satellites also increase with the distance from the planet.
The first group of the Jupiter system contains three satellites, and that of the Saturn system contains five. If we assume that the Sun and' the planets evolved according to a process which was similar to that according to which the planetary satellites were formed, then it is very likely that the first group of Jupiter also contains five members.
Let us now consider the second satellite group of Saturn, which con tains Titan (VI) and Hyperion (VII), and let us compare it with the second group of Jovian satellites.
Like the second group of Jupiter, the second group of Saturn is located between one and two million km from the planet. The following table compares the characteristics of the two satellite groups.