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The Solar System contains well over 150 planetary satellites, but, as might be expected from the wide range of sizes and compositions, these seem to have arisen in several different ways (Figure 1.20).

As mentioned above, Earth's Moon is thought to have been born during a massive, grazing collision between the young Earth and a Mars‐sized planetary embryo. The mixture of debris from both objects formed a ring around the scarred Earth, eventually accreting into a large satellite.

Other satellites may also have been created by sizeable impacts early in the Solar System's history. For example, the Pluto‐Charon system may have originated during a collision between two large, icy planetesimals over 4 billion years ago. Simulations show that some of the debris from the collision would be blasted into orbit around the surviving protoplanet, eventually coalescing to form Charon and several smaller satellites.

Most of the major satellites seem to have followed a less traumatic path, gradually accreting from a protoplanetary disk, much like the planets. The most obvious example is the Jovian system, with its family of four Galilean moons. The inner pair, Io and Europa, are smaller but denser (with a higher proportion of rock) than the outer pair, ice‐rich Ganymede and Callisto. All of them orbit Jupiter in the same direction and in more or less the same plane.

These characteristics can be explained if the moons were born from a spherical cloud of dust and gas being drawn inward from the solar nebula by a fledgling planet. As time went by, the cloud flattened into a disk around the protoplanetary core. This disk was hotter and denser near the center, allowing condensation and accretion of the less volatile materials. Further out, the icy volatiles could also condense and accrete to form Ganymede and Callisto.

Although the Saturnian family of satellites is dominated by planet‐sized Titan, none of the members are particularly rocky, with many only slightly denser than water. Titan itself is similar in density to Ganymede and Callisto. If the proto‐Saturn was surrounded by a collapsing cloud, it seems to have been only about one quarter the mass of Jupiter's. This suggests that the cloud contained less silicate (rocky) material and more ice than its counterpart in the warmer environs of Jupiter.

Certainly, there is a general increase in size and mass moving outward from Saturn toward Titan, with a marked decrease in both properties beyond Titan. This has led theorists to suggest that Titan grew sufficiently quickly to collect much of the solid material in the disk around Saturn, leaving only a modest amount for the medium‐sized satellites to accumulate.

A modified version of the accretion scenario has been proposed by Robin Canup and William Ward of the Southwest Research Institute. They suggested that a growing satellite's gravity induces spiral waves in a surrounding disk of gas, primarily hydrogen. Gravitational interactions between these waves and the satellite cause the moon's orbit to contract. This effect becomes stronger as a satellite grows, so that the bigger a satellite gets, the faster its orbit spirals inward toward the planet. They proposed that the balance between the inflow of material to the satellites and the loss of satellites through collision with the planet implies a maximum size for a satellite of a gas giant.

Figure 1.20 The most significant satellites in our Solar System are shown beside the Earth, with their correct relative sizes and colors. Ganymede and Titan are larger than Mercury and eight satellites are larger than Pluto. Earth's Moon is the fifth largest, with a diameter of 3,476 km. Most of them are thought to have formed from a disk of gas and dust in orbit around their home planet. However, Triton and many of the smallest satellites are thought to be captured asteroids or Kuiper Belt objects that formed elsewhere in the Solar System. Earth's Moon, and possibly the moons of Mars, Uranus, and Pluto, are thought to have formed as the result of major impacts.

(NASA)

Numerical simulations and analytical estimates of the growth and loss of satellites showed that multiple generations of satellites were likely, with today's satellites being the last surviving generation to form as the planet's growth ceased and the gas disk dissipated.

The origin of the Uranian (and Neptunian) satellites is open to debate. Models suggest that the ice giants grew more slowly than their larger cousins. By the time they were large enough to gather a disk of material, most of the gas and dust in the protosolar disk had been dispersed, probably after the young Sun entered its active T Tauri phase. If the regular satellites of Uranus and Neptune could not have formed through large‐scale accretion from a circumplanetary disk, how did they come about?

One idea is that the planets were larger and hotter during their accretion phase. As they subsequently cooled and contracted, they left behind a “spinout disk” from which small satellites could grow by accretion.

One complication is the fact that the Uranian satellites orbit in circles close to the planet's equator, even though it spins on its side. Neptune's rotation axis is also tilted quite markedly, aligned at about 30° to its orbital plane, while the orbits of its small satellites are circular and near‐equatorial. This suggests that the planets were involved in major impacts early in their histories, and that the satellites were born during or after these collisions.

It may be that impacts with planet‐sized objects blasted out clouds of hot material that formed orbiting disks around the ice giants. When the material cooled and condensed, the ice‐rock ingredients were available for medium‐sized satellites to form.

The major exception is Triton, the largest satellite of Neptune. One clue to its origin is that most of its bulk properties are very similar to those of Pluto, one of the largest known members of the Kuiper Belt. Furthermore, unlike the other Neptunian moons, it follows a retrograde path which is quite steeply inclined to the planet's equator. This unusual orbit has led to speculation that Triton was a Kuiper Belt object that ventured too close to Neptune and was somehow captured.