The Solar System is host to amazing worlds that always surprise and deliver a sense of awe. How do the planets and their moons appear the way that they do? Is it even possible? There is a moon that is keeping one of the biggest secrets of the Solar System. It has no problem keeping out of the spotlight as it is located at the far edge of the Solar System in eternal twilight and not easily accessible by spacecraft.

Triton is hiding something. This largest moon of Neptune is not behaving like normal big moons. Neptune has sixteen known moons; all, but Triton, are very small. There are two classes of moons: regular and irregular. The regular moons orbit close to Neptune with nearly circular orbits in Neptune’s equatorial plane whereas the irregular one’s orbit further away with elongated and tilted orbits. Triton is the oddball. It was quickly discovered by William Lassell on October 10, 1846, only 17 days after Neptune was discovered. It is the biggest of Neptune’s moons at 1680 miles in diameter, over 200 times bigger than all the other moons of Neptune combined. It is also an irregular moon orbiting backward to Neptune’s spin and inclined steeply at 67º to Neptune’s equator. Despite this, the orbit is one of the most perfectly circular of any moon in the Solar System.

Triton had a close encounter when Voyager 2 flew past it on August 25, 1989, at a distance of 25,000 miles. Before the flyby, scientists analyzed the sunlight reflected off it and found a strong signal of nitrogen that hinted the possible presence of a vast sea of liquid nitrogen. It would have been the only other world besides Earth at that time to have liquid on its surface. The discovery of liquid methane on Saturn’s moon, Titan, was still nearly two decades away. The Sun at Triton is 900 times dimmer than on Earth and appears like a brilliant star, still dangerous to look at from 2.8 billion miles away.

Voyager 2 discovered a cap of frozen nitrogen at Triton’s south pole disproving that liquid nitrogen could exist on the surface. The ice cap is patchy with a sharply defined border and is vaporizing in the constant sunlight of the southern hemisphere’s summer. The vaporizing polar ice cap is creating a thin, but deep nitrogen atmosphere that is 500 miles thick (compared to Earth’s 62 miles) with a pressure only 1/100,000 Earth’s. At the bottom of this deep atmosphere is a 15-mile-deep haze of methane and hydrocarbon compounds. Triton is a brutally frigid world with a surface temperature of -392ºF, therefore nitrogen cannot exist as a liquid (freezing point -346ºF). The dim Sun heats the atmosphere enough to create winds that flow from the warmer, sunny south pole to the dark, colder north pole. The proof is that Triton is covered with dark streaks originally thought to be wind-scoured material. Voyager 2 revealed the truth only a few days after the encounter. Triton has active geysers! The geysers are powered by pockets of liquid nitrogen that are kept warm and under pressure beneath the ice cap. As the ice cap thins in the sunlight, the liquid nitrogen erupts into towering geysers up to five miles high. At that height, the winds sheer off the plumes and carry them downwind for nearly 100 miles. The geysers may be the strong nitrogen signal that was seen from Earth before the Voyager 2 flyby since no seas were seen anywhere on Triton.

A large region of Triton’s mid-latitudes is covered in bumpy, wrinkled features nicknamed cantaloupe terrain. Another area is a broad, featureless plain dotted with massive calderas. Most obvious is the lack of lots of craters, indicating a young surface capable of erasing craters by an active process. The ability to resurface is not common in the Solar System and is a sign that Triton still has a partially warm interior. Triton is an oddball with all these characteristics, but how and why did it get this way? It is possible that Triton was captured by Neptune. It may have been a Kuiper Belt object more like Pluto or Eris, unlike the other moons. Triton may have been in an orbit on its own around the Sun, got too close to Neptune and was captured. It could have also collided with one of Neptune’s original moons and lost enough energy for Neptune to capture it. The capturing of Triton explains its strange orbit. If it formed with Neptune, it would be in a regular orbit instead of orbiting Neptune backwards and would orbit close to the same plane as Neptune’s equator instead of at a whopping 67º angle. If Triton were captured early enough it could still have enough time to be shaped into a circular orbit through multiple collisions with debris and smaller moons. Unfortunately, its backward orbit has sealed Triton’s fate as atmospheric drag and tidal forces from Neptune will eventually cause it to disintegrate into a new ring in about 3-4 billion years.

Triton remains warm because it has enough radioactive elements to release heat. The heat turns water, carbon dioxide, and nitrogen ices into slush, keeping the surface young and active like Pluto’s giant heart-shaped nitrogen glacier field. Triton may be warm enough to have a liquid water ocean beneath its crust. It could be classified as an Ocean World like Europa, Enceladus, Ganymede, Callisto, and possibly Pluto. This new class of worlds warrant further exploration. Triton begs for a closer look; the lure was strong soon after the Voyager 2 encounter for a Neptune orbiter/probe mission that would have launched in 2002 and arrived in 2021. The cost of the mission prohibited approval. Other missions, all flybys and less expensive, were planned, but none were approved. Trident was the latest mission to nearly get approved as it was a finalist but was shot down in 2021 for approval of a pair of Venus missions, Veritas and Davinci. It would have been launched this year and fly past Triton in 2038. It would have been a quick visit like Voyager 2 but would have had state of the art technology and cameras that would have given scientists sharp views and detected any changes in the ice cap and geysers. It now rests on a future Neptune orbiter since, if a spacecraft is going to go that far away taking at least a decade to get there, it is best for it to stay there.

NASA is investing in a Uranus orbiter/probe mission set to launch in the 2030’s followed by an Enceladus orbiter/lander the following decade. A mission to Neptune is expected to be proposed after these two missions fly. However, China is advancing its deep space exploration program with a proposed mission to orbit Neptune, deploy a probe into its atmosphere, and study Triton, possibly launching around 2039. The spacecraft would orbit Neptune for at least four years with several extremely close flybys of Triton where the geysers could be sampled. A penetrator would slam into Triton’s crust giving close-up measurements of its surface. Until the next spacecraft explores Neptune and Triton, the secret as to how Triton came to be and if it is truly an Ocean World will remain.