Roaming the Cosmos – Uhlanga Regio, Triton

It’s currently late summer in the southern hemisphere of Triton, and it will be for a while. Seasons on Triton, Neptune’s largest moon, last over 40 years, with each pole spending 80 years in sunlight followed by 80 years of darkness.

So where is the best place to spend a (very, very, very) extended summer vacation on Triton?

We suggest Uhlanga, the southern polar region of Triton, named after the marsh from which humanity was born in Zulu mythology. There you will find marvels worthy of any creation myth.

Bring your sunglasses. The icy surface of Triton reflects over 70 percent of the sunlight that hits it. You’ll walk through jutting uplifts of sparkling crystal scattering the light of a seemingly endless day.

But the true wonder is the geysers. Triton is one of only four bodies in the solar system with volcanic activity and it is by far the coldest. Constantly active geysers eject material that snap-freezes in the cold sky and scatters it as glistening nitrogen snow.

This is not a soft settling of snow either. The winds on Triton nearly reach the speed of sound. Thankfully, it’s unlikely to knock you over, as Earth’s atmosphere is 50,000 times more dense than Triton’s.

All of this outgassing creates a constant haze in the summer, extending up to 30 kilometers from the surface. It’s composed largely of hydrocarbons and nitriles created by a methane reaction with both solar and stellar ultraviolet light. The sky is also patched with clouds in the form of nitrogen ice particles. But even through the haze the great blue planet Neptune dominates the sky. Triton is about the same distance from Neptune as Luna is from Earth, but Neptune is 17 times the mass of Earth.

The icy geysers are not the only thing that makes Triton strange place. Its orbit around Neptune is in reverse.

This is unique. Triton is the only large moon in the solar system (and, thus, the only one we know about) that’s in a retrograde orbit. Some outer, irregular satellites of Jupiter, Saturn, and Uranus travel in retrograde, but they’re mostly oddly-shaped, smaller rocks. The absolute largest of them, Phoebe (a pock-marked asteroid revolving around Saturn), has 0.03 percent of the mass of Triton. The other objects in retrograde also tend to be on highly-irregular orbits, but Triton’s orbit around Neptune is a nearly perfect circle, with an eccentricity of almost zero.

This unusual arrangement suggests that Triton was once a dwarf planet, much like Pluto, that was then pulled from the Kupier belt by Neptune’s gravity and captured as a moon.

Pluto, however, will not join Triton. Because Pluto’s orbit occasionally passes within Neptune’s orbit (from 1979 to 1999 Pluto was closer to the Sun than Neptune), many people have wondered if the former member of the Nine will ever become a moon like Triton. But Pluto’s orbit takes it 17 degrees above and below the plane Neptune orbits on and the two never get within 100 million kilometers of each other.

Triton’s capture must have been a chaotic event, and it’s probably why Neptune has so few moons. Jupiter has 67 moons, Saturn has 62, Uranus 27, and Neptune only has 14… and most of those are small. For example, Uranus has four moons with a diameter of greater than 1000 kilometers (Ariel, Oberon, Titania, and Umbriel). Excluding Triton itself, Neptune has none that are even 500 kilometers in diameter.

It’s likely that Neptune was once like the other giants in our solar system, with its own suite of large moons. Then Triton came swinging around Neptune, knocking other moons out of orbit as its oceans of liquid water sloshed around the dwarf planet. It is likely that Triton at one point had liquid water because its post-capture eccentricity probably resulted in severe tidal heating. It could have remained fluid for billions of years as it slowly refroze and drifted into its quiet, nearly-perfect retrograde orbit.

It’s a reminder that the apparent serenity we see now in the solar system is because we’re only seeing a snapshot, a tiny piece of processes that occur on a cosmic scale.

All of our solar system’s planets, and moons – yes, even us – are survivors of this chaos arranged in strange and beautiful fashion, like a backward-orbiting-former-dwarf-planet-moon blasting sun-glowing nitrogen crystals into speed-of-sound-40-year-summer winds.

Sources/Additional Reading:

Triton: In DepthNASA

Seasons Discovered on Neptune’s Moon

MIT researcher finds evidence of global warming on Neptune’s largest moonMIT News

The Atmosphere of TritonWindows to the Universe

Dynamics of Triton’s AtmosphereNature

Captive worlds: Is Neptune’s moon Triton a kidnapped Pluto?

Will Pluto Ever Hit Neptune?

The coupled orbital and thermal evolution of TritonGeophysical Research Letters

Photograph No. 1: Triton’s southern polar region, Voyager 2 spacecraft, Aug. 25, 1989; Photograph No. 2: Neptune (top) and Triton, Voyager 2 spacecraft, Aug. 28, 1989