The Moons of Neptune

By Sabine Stanley, Ph.D., Johns Hopkins University

Neptune is known to have at least 14 moons, but only one of those moons, Triton, is large enough to be spherical. Triton was captured rather than formed from an accretion disk around the planet, hence it is orbiting the planet in a backward, retrograde direction.

Within weeks of the discovery of Neptune, a single moon was found in 1846. Although Uranus and Neptune are quite similar in terms of composition and structure, their systems of moons are very different. Neptune is now known to have at least 14 moons, but only one of those moons, Triton, is large enough to be spherical. Compare that to four round moons at Jupiter, seven at Saturn, and five at Uranus.

Learn more about Mercury, the extreme little planet.

The Size of Triton

In terms of size, Triton is the seventh-largest moon in the solar system. It’s close to twice the diameter of Titania (the largest moon of Uranus) but smaller than Earth’s Moon; and it’s about 10% smaller in diameter than Europa, the smallest of Jupiter’s four round moons. Triton is only about half the diameter of Jupiter’s Ganymede or Saturn’s Titan. However, Triton is about 10% denser than Saturn’s moon Titan, suggesting a larger rock fraction in Triton and a smaller ice fraction—perhaps rock and ice.

Learn more about Venus, the veiled greenhouse planet.

Triton, an Irregular Satellite

The ice is why both Titan and Triton are much less dense than Earth’s Moon. What makes Triton truly extraordinary compared to all the other large round moons in the solar system is that it’s an irregular satellite. That means it didn’t form from an accretion disk surrounding Neptune. Instead, Triton was captured by Neptune’s gravity at some point in the past. We know Triton’s a captured moon because its orbit is retrograde.

Triton is orbiting Neptune in the opposite direction that Neptune is spinning. That’s a key sign that Triton didn’t form from an accretion disk around Neptune. Triton’s orbit is also inclined by about 23° with respect to Neptune’s equator—another sign that Triton is a captured object.

Where Did Triton Come From?

So, where did Triton come from? Most likely, it came from the Kuiper belt. This is the region of the solar system just beyond Neptune’s orbit that’s home to many other known icy bodies similar in size to Triton. The most famous Kuiper belt resident is Pluto. Some Kuiper belt objects, including Pluto, are known to have elliptical orbits that cross the orbit of Neptune. It’s therefore not too surprising that one of those objects may have had an encounter with Neptune in the past and had been captured by Neptune’s gravity.

Learn more about exploring the Earth-Moon system.

The Impact of Capturing Triton on Neptune

Now, capturing Triton would have taken its toll on both Triton and the Neptune system. It might not be a coincidence that Neptune doesn’t have any spherical regular moons.

Imagine if Neptune originally had a system of regular moons that had formed from an accretion disk. Then Triton comes barreling in, and the gravitational interactions between Triton and these moons would likely have disrupted their orbits: perhaps giving them large eccentricities or inclinations, causing them to crash into Neptune or escape the Neptune system altogether.

Nereid, a small moon of Neptune may be evidence of this. Faraway Nereid has an extremely eccentric orbit that may be the result of gravitational interactions during Triton’s capture.

This is a transcript from the video series A Field Guide to the Planets. Watch it now, Wondrium.

The Capturing Impact on Triton

And how did going from a free-flying Kuiper belt object to a captured moon affect Triton? Since its capture, Neptune’s tidal forces have acted to bring Triton’s rotation into a spin-orbit resonance. Triton now keeps one face towards Neptune at all times, just like our Moon does with Earth, and travels in a very circular orbit.

Triton orbits very close to Neptune—about 10% closer than our Moon orbits Earth. And since Neptune is about four times larger in diameter than Earth— considering that Triton is about 20% smaller in diameter than Earth’s Moon—this means that Triton appears almost the same size in the sky from Neptune’s surface as Earth’s Moon does to us. It’s very unusual for an irregular moon to orbit so close to its planet host.

Predictions for Triton’s Orbit

The tidal forces acting on Triton this close to Neptune have two important effects on the moon. First, the tidal forces are changing Triton’s orbit. Triton is slowly spiraling inward towards Neptune. Predictions suggest it will reach Neptune’s Roche limit in about 3.5 billion years. At that point, Triton will begin breaking apart from Neptune’s overwhelming tidal forces.

So in a few billion years, if you were thinking of moving to Triton around the time our Sun turns into a red giant star and our Earth is no longer habitable, don’t be surprised if you find a new ring system around Neptune rather than a moon.

The Surface of Triton

Tidal forces are not only pulling Triton inward. These tidal interactions also stretch and flex Triton’s interior, causing heating. This heating has resulted in geologic activity on Triton’s surface. Our best views of Triton came from the Voyager 2 flyby. The closest approach was at 40,000 kilometers from the moon, and Voyager was able to image 40% of Triton’s surface.

The other 60% has remained a mystery. But that 40% reveals a world that is geologically rich and varied. Over half of the surface is covered with frozen nitrogen. The remainder is a combination of water ice and frozen carbon dioxide—that’s dry ice.

Pinkish Hue or Tholins of Triton

The surface has a pinkish hue thanks to the presence of organic compounds on the surface. These may be tholins like we saw on Saturn’s moon Titan. They can be caused when methane is broken apart by sunlight and its components are then reassembled into larger organic molecules.

Craters and Other Geologic Changes on Triton’s surface

The surface is covered with signs of geologic change, but there are not many craters on Triton. Crater counting indicates Triton’s surface is very young, with the oldest regions around 50 million years old. The youngest regions are estimated to be about six million years old. Yet Triton does display ridges, troughs, cliffs, and volcanic plains we see on other worlds.

Cantaloupe like Shapes in the Western Hemisphere of Triton

But there is also some terrain that is not seen anywhere else in the solar system. In the part of the western hemisphere that has been imaged, Triton looks like a cantaloupe! An irregular pattern of pits or depressions, about 30 kilometers across, and ridges several hundred meters high cover the surface here. In some places, the ridges are reminiscent of the surface of Jupiter’s moon Europa and indicate the tectonic movement of the ice shell.

We don’t know how the cantaloupe terrain formed. One hypothesis is that the ridges and pits form where plumes of warmer solid ice pushed up against the surface, causing the raised areas surrounding the pits.

The Different Terrain of South Polar Region of Triton

The southern polar region of Triton shows a very different terrain. Here, we see a frozen nitrogen ice cap covered in geysers. These geysers spew nitrogen gas into Triton’s atmosphere. Voyager 2 even caught eruptions in progress, with plumes reaching eight kilometers high! These nitrogen eruptions put Triton in a select group of solar system worlds where we have observed active eruptions. The others are Jupiter’s moons Io, which erupts sulfur-rich magma; and Europa, which erupts water ice; and Saturn’s moon Enceladus which erupts water ice; and of course, Earth, which erupts magma.

Evidence of Winds and Clouds on Triton

Voyager 2 also saw spots of nitrogen clouds about a kilometer above Triton’s surface. We also have evidence of winds on Triton. We can’t measure the winds directly, but the plumes of nitrogen gas from the geysers on Triton are blown by the winds.

The plumes collect dust and organic particles from the atmosphere, then get blown by the wind and eventually fall to the surface. We can see trails of that wind-blown dust around 150 kilometers long from the geysers. And the trails are all on the same side of the geysers, which means we can even measure the wind direction on Triton.

The Seasons of Triton

Triton also experiences Neptune’s very long seasons, each about 40 Earth-years long. Because Triton has an inclined orbit about Neptune, the geometry works out so that during parts of Neptune’s year, Triton’s pole points towards the Sun. This keeps one hemisphere in constant daylight and the other in the constant night. This is similar to the scenario of Uranus and some of its moons, but it goes on even longer.

Observations of Seasonal Changes on Triton

We have already observed hints of seasonal changes on Triton. First, the moon appears to be getting paler, or less red. This may be due to new frozen nitrogen layers falling and covering the tholins on the surface. There are also hints that the atmosphere is getting thicker, perhaps due to the warming of the surface that causes evaporation of more nitrogen.

The Other Moons of Neptune

Triton and Nereid were the only moons known to orbit Neptune until Voyager 2 visited the planet. Voyager found six more small moons, then others have been discovered using Earth-based telescopes. The four innermost moons of Neptune orbit within Neptune’s rings may have some shepherding influences on keeping some of the rings quite narrow, similar to what is seen in some of the rings of Saturn and Uranus.

The Rings of Neptune

Neptune’s rings are quite dark, similar to the rings of Uranus. But the particles making up Neptune’s rings are micrometer-sized dust rather than the basketball to house-sized ring particles at Uranus. This makes Neptune’s rings more similar to those at Jupiter. The accidental discovery of Uranus’s rings from occultation studies in 1977 is what motivated scientists to use a similar method to try and find rings around Neptune.

But detecting rings at Neptune proved to be more challenging for the occultation studies. Eventually, Voyager 2’s flyby in 1989 conclusively found five rings around Neptune. Three of the rings are narrow, and the other two are much broader.

Common Questions about the Moons of Neptune

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