14-8 Neptune’s satellite Triton is an icy world with a young surface and a tenuous atmosphere

Neptune has 13 known satellites, listed in Appendix 3. They are named for mythological beings related to bodies of water. (Neptune itself is named for the Roman god of the sea.) Most of these worlds are small, icy bodies, probably similar to the smaller satellites of Uranus. The one striking exception is Triton, Neptune’s largest satellite (see Table 7-2). In many ways Triton is quite unlike any other world in the solar system.

Triton’s Backward Orbit and Young Surface

Like many of the small satellites of Jupiter, Saturn, Uranus, and Neptune, Triton is in a retrograde orbit, so that it moves around Neptune opposite to the direction of Neptune’s rotation. Furthermore, the plane of Triton’s orbit is inclined by 23° from the plane of Neptune’s equator. It is difficult to imagine how a satellite might form out of the same cloud of material as a planet and end up orbiting in a direction opposite the planet’s rotation and in such a tilted plane. Hence, Triton probably formed elsewhere in the solar system, collided long ago with a now-vanished satellite of Neptune, and was captured by Neptune’s gravity. With a diameter of 2706 km, a bit smaller than our Moon but much larger than any other satellite in a retrograde orbit, Triton is certainly the largest captured satellite.

Figure 14-16 shows the icy, reflective surface of Triton as imaged by Voyager 2. Most of Triton’s ice is water, with some additional nitrogen and methane-ice. There is a conspicuous absence of large craters, which immediately tells us that Triton has a young surface on which the scars of ancient impacts have largely been erased by tectonic activity. There are areas that resemble frozen lakes and may be the calderas of extinct ice volcanoes. Some of Triton’s surface features resemble the long cracks seen on Europa (Section 13-6) and Ganymede (Section 13-7). Still other features are unique to Triton. For example, in the upper portion of Figure 14-16 you can see dimpled, wrinkled terrain that resembles the skin of a cantaloupe.

Figure 14-16: R I V U X G
Triton Several high-resolution Voyager 2 images were combined to create this mosaic. The pinkish material surrounding Triton’s south polar region is probably nitrogen frost. Some of this presumably evaporates when summer comes to the south pole; the northward flow of the evaporated gas may cause the dark surface markings. Farther north is a brown area of “cantaloupe terrain” that resembles the skin of a melon.

(NASA/JPL)

Triton’s tectonically active history is probably related to its having been captured into orbit around Neptune. After its capture, Triton most likely started off in a highly elliptical orbit, but today the satellite’s orbit is quite circular. The satellite’s original elliptical orbit would have been made circular by tidal forces exerted on Triton by Neptune’s gravity. These tidal forces would also have stretched and flexed Triton, causing enough tidal heating to melt much of the interior. The resulting volcanic activity (with lavas made of ice rather than molten rock) would have obliterated Triton’s original surface features, including craters.

There is still enough warmth in Triton’s interior to keep it active with cryovolcanoes (where the lava consists of water that is prevented from freezing due to dissolved ammonia). Voyager 2 observed plumes of dark material being ejected from the surface to a height of 8 km (5 mi). The plumes contain invisible nitrogen gas, along with small and dark dust particles. These plumes may have been generated from a hot spot far below Triton’s surface, similar to geysers on Earth. Alternatively, the energy source for the plumes may be sunlight that warms the surface, producing subsurface pockets of gas and creating fissures in the icy surface through which the gas can escape.

Triton’s surface temperature is only 38 K (−235°C = −391°F), the lowest of any world yet visited by spacecraft. This temperature is low enough to solidify nitrogen, and indeed the spectrum of sunlight reflected from Triton’s surface shows absorption lines due to nitrogen-ice as well as methane-ice. But Triton is also warm enough to allow some nitrogen to evaporate from the surface, like the steam that rises from ice cubes when you first take them out of the freezer. Voyager 2 confirmed that Triton has a very thin nitrogen atmosphere with a surface pressure of only 1.6 × 10⁻⁵ atmosphere, about the same as at an altitude of 100 km above Earth’s surface. Despite its thinness, Triton’s atmosphere was revealed by its winds. Voyager 2 saw areas on Triton’s surface where dark dusty material has been blown downwind by a steady breeze (see Figure 14-16). Dark dusty material ejected from the geyserlike plumes was carried as far as 150 km by high-altitude winds.

Tidal Forces and Triton’s Doom

Just as tidal forces presumably played a large role in Triton’s history, they also determine its future. Triton raises a tidal bulge on Neptune, just as our Moon distorts Earth (recall Figure 10-20). In the case of the Earth-Moon system, the gravitational pull of Earth’s tidal bulge causes the Moon to spiral away from Earth. But because Triton’s orbit is retrograde, the tidal bulge on Neptune exerts a force on Triton that makes the satellite slow down rather than speed up. (In Figure 10-20, imagine that the moon is orbiting toward the bottom of the figure rather than toward the top.) This is causing Triton to spiral gradually in toward Neptune. In approximately 100 million years, Triton will be inside Neptune’s Roche limit, and the satellite will eventually be torn to pieces by tidal forces. When this happens, the planet will develop a spectacular ring system—overshadowing by far its present-day set of narrow rings—as rock fragments gradually spread out along Triton’s former orbit.

Prior to Voyager 2, only one other satellite was known to orbit Neptune. Nereid, which was first sighted in 1949, is in a prograde orbit. Hence, it orbits Neptune in the direction opposite to Triton. Nereid also has the most eccentric orbit of any satellite in the solar system; its distance from Neptune varies from 1.4 million to 9.7 million kilometers. One possible explanation is that when Triton was captured by Neptune’s gravity, the interplay of gravitational forces exerted on Nereid by both Neptune and Triton moved Nereid from a relatively circular orbit (like those of Neptune’s other, smaller moons) into its present elliptical one.