Exploring Triton: Neptune’s Frigid Moon

Triton, the largest moon of Neptune. The strange terrain along the center of the image is called "cantaloupe terrain". The black smears are nitrogen geysers. NASA

When the Voyager 2 spacecraft swept past the planet Neptune in 1989, no one was quite sure what to expect of its largest moon, Triton. Seen from Earth, it's just a tiny point of light visible through a strong telescope. However, up-close, it showed off a water-ice surface split by geysers that shoot nitrogen gas up into the thin, frigid atmosphere. It not only was weird, the icy surface sported terrains never before seen.

Thanks to Voyager 2 and its mission of exploration, Triton showed us just how strange a distant world can be.

Triton: the Geologically Active Moon

There aren't too many "active" moons in the solar system. Enceladus at Saturn is one (and has been studied extensively by the Cassini mission), as is Jupiter's tiny volcanic moon Io. Each of these has a form of volcanism; Enceladus has ice geysers and volcanoes while Io spouts out molten sulfur. Triton, not to be left out, is geologically active, too. Its activity is cryovolcanism — producing the kind of volcanoes that spew ice crystals instead of molten lava rock. Triton's cryovolcanoes spew material out from beneath the surface, which implies some heating from within this moon.

Triton's geysers are located close to what's called the "subsolar" point, the region of the moon directly receiving the most sunlight. Given that it's very cold out at Neptune, sunlight isn't nearly as strong as it is at Earth, so something in the ices is very sensitive to sunlight, and that weakens the surface.

Pressure from material below pushes out cracks and vents in the thin shell of ice that covers Triton. That lets the nitrogen gas and plumes of dust blast out and into the atmosphere. These geysers can erupt for fairly long periods of time — up to a year in some cases. Their eruption plumes lay down streaks of dark material across the pale pinkish ice.

Creating a Canteloupe Terrain World

The ice depots on Triton are mainly water, with patches of frozen nitrogen and methane. At least, that's what the southern half of this moon shows. That's all Voyager 2 could image as it went by; the northern part was in shadow. Nonetheless, planetary scientists suspect that the northern pole looks similar to the southern region. Icy "lava" has been deposited across the landscape, forming pits, plains, and ridges. The surface also has some of the weirdest landforms ever seen in the form of "cantaloupe terrain". It's called that because the fissures and ridges look like the skin of a cantaloupe. It's probably the oldest of Triton's icy surface units, and is made up of dusty water ice. The region probably formed when material under the icy crust rose up and then sank back down again, which unsettled the surface. It's also possible that ice floods could have caused this weird crusty surface. Without followup images, it's hard to get a good feel for possible causes of the cantaloupe terrain.

How Did Astronomers Find Triton?

Triton is not a recent discovery in the annals of solar system exploration. It was actually found in 1846 by astronomer William Lassell.

He was studying Neptune just after its discovery, looking for any possible moons in orbit around this distant planet. Because Neptune is named after the Roman god of the sea (who was the Greek Poseidon), it seemed appropriate to name its moon after another Greek sea god whose was fathered by Poseidon.

It didn't take long for astronomers to figure out that Triton was weird in at least one way: its orbit. It circles Neptune in retrograde — that is, opposite to Neptune's rotation. For that reason, it's very likely that Triton didn't form when Neptune did. In fact, it probably had nothing to do with Neptune, but was captured by the planet's strong gravity as it passed by. No one is quite sure where Triton originally formed, but it's quite likely it was born as part of the Kuiper Belt of icy objects.

It stretches outwards from the orbit of Neptune. The Kuiper Belt is also the home of frigid Pluto, as well as a selection of dwarf planets. Triton's fate is not to orbit Neptune forever. In a few billion years, it will wander too close to Neptune, within a region called the Roche limit. That's the distance where a moon will start to break up due to gravitational influence.

Exploration after Voyager 2

No other spacecraft has studied Neptune and Triton "up close". However, after the Voyager 2 mission, planetary scientists have used using Earth-based telescopes to measure Triton's atmospheric by watching as distant stars slipped "behind" it. Their light could then be studied for telltale signs of gases in Triton's thin blanket of air.

Planetary scientists would like to explore Neptune and Triton further, but no missions have been selected to do so, yet. So, this pair of distant worlds will remain unexplored for the time being, until someone comes up with a lander that could settle down among the cantaloupe hills of Triton and send back more information.