Like Jupiter and Saturn, both Uranus and Neptune have extensive moon systems, each consisting of a few large moons, long known from Earth, and many smaller moonlets, most of them discovered by Voyager 2.

URANUS'S MOONS

William Herschel discovered and named the two largest of Uranus's five major moons, Titania and Oberon, in 1789. British astronomer William Lassell found the next largest, Ariel and Umbriel, in 1851. Gerard Kuiper found the smallest, Miranda, in 1948. In order of increasing distance from the planet, they are Miranda (at 5.1 planetary radii), Ariel (at 7.5), Umbriel (at 10.4), Titania (at 17.1), and Oberon (at 22.8). Ten smaller moons discovered by Voyager 2 all lie inside the orbit of Miranda. Many of them are intimately related to Uranus's ring system. All these moons revolve in Uranus's skewed equatorial plane, almost perpendicular to the ecliptic, in circular, tidally locked orbits. Their properties are listed in Table 13.1. Because these satellites share Uranus's odd orientation, they experience the same extreme seasons as their parent planet.

In September 1997 two new moons were discovered orbiting Uranus. Interestingly, they were found not by visiting spacecraft (there have been none since Voyager 2, and none are planned), nor by the latest generation of telescopes, such as Hubble or Keck. Instead, these newest satellites were discovered by astronomers using one of the oldest instruments still in daily use—the 5-m Hale Telescope on Mount Palomar in California. (Sec. 5.1) Orbiting much farther (about 6 million km) from Uranus than the other known moons, on retrograde, highly inclined orbits, these moons, like the outer moons of Jupiter, and Phobos and Deimos of Mars, are most likely debris captured from interplanetary space following a grazing encounter with the planet's atmosphere. S/1997U1 and S/1997U2 are just 80 km and 160 km in diameter, respectively, and are the faintest moons ever to have been imaged with a ground-based telescope.

The five largest Uranian moons are similar in many respects to the six midsized moons of Saturn. Their densities lie in the range 1100—1700; kg/m³, suggesting a composition of ice and rock, like Saturn's moons, and their diameters range from 1600 km for Titania and Oberon, to 1200 km for Umbriel and Ariel, to 480 km for Miranda. Uranus has no moons comparable to the Galilean satellites of Jupiter, nor to Saturn's single large moon, Titan. Figure 13.11 shows Uranus's five large moons to scale, along with Earth's Moon for comparison.

Figure 13.11 The five largest moons of Uranus, to scale. In order of increasing distance from the planet, they are Miranda, Ariel, Umbriel, Titania, and Oberon. Earth's moon is shown for comparison. The appearance, structure, and history of Titania and Oberon may be quite similar to those of Saturn's moon Rhea. The smallest details visible on both moons are about 15 km across. Umbriel is one of the darkest bodies in the solar system, although it has a bright white spot on its sunward side. Ariel is similar in size but has a brighter surface, with signs of past geological activity. Resolution is approximately 10 km.

The two outermost moons, Titania and Oberon, are heavily cratered and show little indication of geological activity. Their overall appearance (and quite possibly their history) is comparable to that of Saturn's moon Rhea, except that they lack Rhea's wispy streaks. (Sec. 12.5) Also, like all Uranus's moons, they are considerably less reflective than Saturn's satellites, suggesting that their icy surfaces are quite dirty.

One possible reason for this may simply be that the planetary environment in the vicinity of Uranus and Neptune contains more small "sooty" particles than does the solar system closer to the Sun. An alternative explanation, now considered more likely by many planetary scientists, cites the effects of radiation and high-energy particles that strike the surfaces of these moons. These impacts tend to break up the molecules on the moons' surfaces, eventually leading to chemical reactions that slowly build up a layer of dark, organic material. This radiation darkening is thought to contribute to the generally darker coloration of many of the moons and rings in the outer solar system. In either case, the longer a moon has been inactive and untouched by meteoritic impact, the darker its surface should be.

The darkest of the moons of Uranus is Umbriel. It displays little evidence of any past surface activity; its only mark of distinction is a bright spot about 30 km across, of unknown origin, on its sunward side. By contrast, Ariel, similar in size to Umbriel but closer to Uranus, does appear to have experienced some activity in the past. It shows signs of resurfacing in places and exhibits surface cracks a little like those seen on another of Saturn's moons, Tethys. However, unlike Tethys, whose cracks are probably due to meteoritic impact, Ariel's activity probably occurred when internal forces and external tidal stresses (due to the gravitational pull of Uranus) distorted the moon and cracked its surface.

Strangest of all Uranus's icy moons is Miranda, shown in Figure 13.12. Before the Voyager 2 encounter, astronomers expected that Miranda would resemble Mimas, the moon of Saturn whose size and location it most closely approximates. However, instead of being a relatively uninteresting cratered, geologically inactive world, Miranda displays a wide range of surface terrains, including ridges, valleys, large oval faults, and many other tortuous geological features.

Figure 13.12 Miranda, an asteroid-sized moon of Uranus photographed by Voyager 2, has a strange, fractured surface suggestive of a violent past, but the cause of the grooves and cracks is presently unknown. The resolution here is about 1 km.

In order to explain why Miranda seems to combine so many different types of surface features, some researchers have hypothesized that this baffling object has been catastrophically disrupted several times (from within or without), with the pieces falling back together in a chaotic, jumbled way. Certainly, the frequency of large craters on the outer moons suggests that destructive impacts may once have been quite common in the Uranus system. It will be a long time, though, before we can obtain more detailed information to test this theory.

NEPTUNE'S MOONS

From Earth we can see only two moons orbiting Neptune. William Lassell discovered the inner moon, Triton, in 1846. The outer moon, Nereid, was located by Gerard Kuiper in 1949. Voyager 2 discovered six additional moons, all less than a few hundred kilometers across and all lying within Nereid's orbit. Neptune's known moons are listed in Table 13.2.

In its moons we find Neptune's contribution to our list of solar system peculiarities. Unlike the other jovian worlds, Neptune has no regular moon system—that is, no moons on roughly circular, equatorial, prograde orbits. The larger moon, Triton, is 2700 km in diameter and occupies a circular retrograde orbit 355,000 km (14.3 planetary radii) from the planet, inclined at about 20 to Neptune's equatorial plane. It is the only large moon in our solar system to have a retrograde orbit. The other moon visible from Earth, Nereid, is only 340 km across. It orbits Neptune in the prograde sense, but on an elongated trajectory that brings it as close as 1.4 million km to the planet and as far away as 9.7 million km. Nereid is probably similar in both size and composition to Neptune's small inner moons.

Voyager 2 approached to within 24,000 km of Triton's surface, providing us with essentially all that we now know about that distant, icy world. Astronomers redetermined the moon's radius (which was corrected downward by about 20 percent) and measured its mass for the first time. Along with Saturn's Titan and the four Galilean moons of Jupiter, Triton is one of the six large moons in the outer solar system. Triton is the smallest of them, with about half the mass of the next smallest, Jupiter's Europa.

Lying 4.5 billion km from the Sun, and with a fairly reflective surface, Triton has a surface temperature of just 37 K. It has a tenuous nitrogen atmosphere, perhaps a hundred thousand times thinner than Earth's, and a surface that most likely consists primarily of water ice. A Voyager 2 mosaic of Triton's south polar region is shown in Figure 13.13. The moon's low temperatures produce a layer of nitrogen frost that forms and evaporates over the polar caps, a little like the carbon dioxide frost responsible for the seasonal caps on Mars. The frost is visible as the pinkish region on the right of the figure.

Figure 13.13 The south polar region of Triton, showing a variety of terrains, ranging from deep ridges and gashes to what appear to be frozen water lakes, all indicative of past surface activity. The pinkish region at the right is nitrogen frost, forming the moon's polar cap. Resolution is about 4 km.

Overall, there is a marked lack of cratering on Triton, presumably indicating that surface activity has obliterated the evidence of most impacts. There are many other signs of an active past. Triton's face is scarred by large fissures similar to those seen on Ganymede, and the moon's odd cantaloupe-like terrain may indicate repeated faulting and deformation over the moon's lifetime. In addition, Triton has numerous frozen "lakes" of water ice (Figure 13.14), which are believed to be volcanic in origin.

Figure 13.14 Scientists believe that this lakelike feature on Triton may have been caused by the eruption of an ice volcano. The water "lava" has since solidified, leaving a smooth surface. The absence of craters indicates that this eruption was a relatively recent event in Triton's past. The nearly circular feature at the center of this image spans some 200 km in diameter; its details are resolved to a remarkable 1 km. The insert is a computer-generated view illustrating the topographic relief of the same area.

Triton's surface activity is not just a thing of the past. As Voyager 2 passed the moon its cameras detected two great jets of nitrogen gas erupting from below the surface, rising several kilometers into the sky. It is thought that these "geysers" result when liquid nitrogen below Triton's surface is heated and vaporized by some internal energy source, or perhaps even by the Sun's feeble light. Vaporization produces high pressure, which forces the gas through cracks and fissures in the crust, creating the displays Voyager 2 saw. Scientists conjecture that nitrogen geysers may be very common on Triton and are perhaps responsible for much of the moon's thin atmosphere.

The event or events that placed Triton on a retrograde orbit and Nereid on such an eccentric path are unknown, but they are the subject of considerable speculation. Triton's peculiar orbit and surface features suggest to some astronomers that the moon did not form as part of the Neptune system but instead was captured, perhaps not too long ago. Other astronomers, basing their views on Triton's chemical composition, maintain that it formed as a "normal" moon but was later kicked into its abnormal orbit by some catastrophic event, such as an interaction with another similar-sized body. It has even been suggested that the planet Pluto may have played a role in this process (see Section 13.10), although no really convincing demonstration of such an encounter has ever been presented.

The surface deformations on Triton certainly suggest fairly violent and relatively recent events in the moon's past. However, they were most likely caused by the tidal stresses produced in Triton as Neptune's gravity circularized its orbit and synchronized its spin, and they give little indication of the processes responsible for the orbit.

Whatever its past, Triton's future is fairly clear. Because of its retrograde orbit, the tidal bulge Triton raises on Neptune tends to make the moon spiral toward the planet rather than away from it (as our Moon moves away from Earth). (Sec. 7.6) Thus, Triton is doomed to be torn apart by Neptune's tidal gravitational field, probably in no more than 100 million years or so, the time required for the moon's inward spiral to bring it inside Neptune's Roche limit. (Sec. 12.4) By that time, it is conceivable that Saturn's ring system may have disappeared, so Neptune will then be the planet in the solar system with spectacular rings!