Formation of the Satellites and Rings of the Giant Planets

2.3.1 Formation of the Satellites of the Giant Planets

With the exception of Triton, all of the massive satellites of the giants, and many of their less massive satellites, orbit the planet in the same direction as the planet rotates, and in a plane tilted at only a small angle with respect to the equatorial plane of the planet (Table 1.2). This orderly arrangement is strong evidence against separate formation and capture, and strong evidence for formation in a disc of dust and gas around each planet, called a protosatellite disc. In some ways this mimics the formation of the planets from the disc of gas and dust around the proto-Sun, but it differs in one important respect - most of the angular momentum in the giant planet system is in the rotation of the planet and not in the orbits of the satellites. Therefore, there is no need to transfer angular momentum away from the planet, in contrast to the proto-Sun.

In the models, the protosatellite disc is composed of material attracted to the growing giant, but that fails to be incorporated into it. The material forms a cloud of gas, dust, and planetesimals. Interactions within the cloud and between the cloud and the planet cause the cloud to evolve into a thin disc in the equatorial plane of the planet, and orbiting in the same direction as the planet is rotating. Though much of the icy-rocky material in the disc is lost to interplanetary space, coagulation and accretion occur, building up the satellites. The time scale for satellite formation in this way is short, of order 1000 years, resulting in internal satellite temperatures up to about 1000 K, a consequence of the gravitational energy released during accretion. For satellites formed further out, the accretion is slower, so less heat is buried, and the accretion temperatures are correspondingly lower. Remnant gas in the system is lost during the T Tauri phase of the proto-Sun.

The surface temperature of Jupiter reaches about 1000 K - the result of infall of material from the nebula to Jupiter's outer envelope. The luminosity is then high enough for a significant rise in the temperature of the inner part of the protosatellite disc, supplementing the accretional heat. Therefore, the satellites that form close to Jupiter are expected to be more depleted in icy materials than those that form further away. In particular they should be depleted in the proportion of water that they contain, the temperatures in the solar nebula at Jupiter always being too high for appreciable condensation of the more volatile substances. □ Given that water is less dense than rocky materials, why are the densities of the Galilean satellites (Table 1.2) in accord with this prediction of a decreasing proportion of water with decreasing distance from Jupiter? Table 1.2 shows that the densities of the Galilean satellites increase with decreasing distance from Jupiter, which is consistent with a decrease in water content. The lower internal temperatures further out have resulted in Callisto, the outermost Galilean, being undifferentiated, i.e. being a fairly uniform mixture of ice and rock, in contrast to Ganymede and Europa, where rocky materials form a core overlain by water (ice at the surface, liquid deeper down). Io has lost its water.

The known densities of the major satellites of Saturn, Uranus, and Neptune, i.e. with radii greater than a few hundred kilometres, show no trend with distance from the giant, and no density high enough to suggest a lack of icy materials. This could be the result of the lower masses of these giants and the correspondingly reduced heating of the inner protosatellite disc.

The smallest satellites are highly irregular in shape - rocky or icy-rocky bodies need to have radii greater than a few hundred kilometres for their own gravity to pull them close to spherical form. Small satellites are prone to collisional fragmentation, particularly the inner ones where the space is crowded with planetesimals gravitationally attracted to the giant and accelerated to high speeds. Therefore, any newly formed ice-poor satellite of modest mass in the inner region is readily disrupted. Satellites subsequently forming in this region are built of substances from across a wide range of distances from the giant, and compositional differences are consequently diluted. Among the smaller satellites of the giants there are some that seem to be collisional fragments. For example, Jupiter's Amalthea, irregularly shaped with a mean radius of 84km, has a density so low that it must be a pile of rubble created by several collisions. Some of the medium-sized satellites display evidence that they were once disrupted but reformed from the fragments, e.g. Saturn's Enceladus (253 km radius).

Many small satellites far from their planet are in irregular orbits, i.e. with high inclinations and eccentricities, and a high proportion retrograde. For example, Saturn has at least 20 small irregular satellites, most of them in retrograde orbits (most of these are below the size threshold for inclusion in Table 1.2). These properties fit the capture model very well. Moreover, capture is easier far from the planet - large orbits require the captured body to lose a smaller fraction of its orbital energy than do small orbits, which is why the captured satellites are mainly far out. Capture requires the proximity of a third body, additional to the planet and the incoming object. This is typically a satellite already in the system. Disruption of the incomer and of the satellite is a likely outcome.

Just one large satellite might have been captured. This is Triton, one of the largest satellites, 1.6 times the mass of Pluto, and by far the largest satellite of Neptune. It is unique among the large satellites in that it orbits its planet in the retrograde direction (Table 1.2). This is strong evidence that Triton was indeed captured, presumably from the E-K belt. Immediately after capture its orbit would have been eccentric and perhaps inclined at a large angle with respect to Neptune's equatorial plane. Once captured, its gravitational interaction with Neptune would have reduced the eccentricity and the inclination of its orbit in about 500 Ma. Its eccentricity is now indistinguishable from zero. Tidal heating would have caused Triton to differentiate.

The capture of Triton could have been accomplished through its collision with one or more satellites each just a few per cent of Triton's mass. This event would have wreaked havoc with any emerging or fully formed satellite system. The orbit of another satellite of Neptune, Nereid, might bear witness to this. Its large eccentricity and large semimajor axis (Table 1.2) could be the result of the capture of Triton. Nereid's orbit would have remained peculiar because of its large average distance from Neptune. If Triton was captured, then its broad similarity in size and density to Pluto suggests that it might initially have been a large member of the E-K belt. This is also Triton's origin in an alternative explanation, in which Triton was originally one member of a binary E-K object. This system passed so close to Neptune that it was disrupted. As a result, Triton had its speed reduced to the extent that it was captured by Neptune. Its erstwhile companion had its speed correspondingly increased, with the likely outcome that further encounters with the giant planets resulted in its ejection from the Solar System.

Question 2.10

Examine Table 1.2 and list the satellites of the giant planets, additional to Triton, that are likely candidates for a capture origin. Justify your choices.

2.3.2 Formation and Evolution of the Rings of the Giant Planets

All four giants have rings (Figure 2.14), and these are particularly extensive in the case of Saturn (Plate 18). For all giants, the rings are close to the planet and lie in the equatorial plane. The

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