Why is Neptune’s moon Triton so strange?

By | September 16, 2023

Sometimes, astronomy is like a forensic investigation: we can’t simply reconstruct the past, so we have to study the clues to understand what happened. And the outer solar system contains a surprising crime scene that holds a great mystery: how the environment around Neptune got the shape of the moon Triton.

Neptune it has 14 known moons. Except for Triton, everyone Neptune’s moons they are very small and come in two general flavors: regular and irregular. The regular ones orbit close to the planet, while the irregular ones are generally farther from Neptune, with all sorts of crazy orbits.

And then there’s Triton. It was discovered by the English trader and amateur astronomer William Lassell, who, in 1846, spotted the moon just 17 days after the discovery of Neptune itself. But it wasn’t until August 25, 1989 that we were able to observe Triton up close for the first time. Traveler 2 the probe flew on the final leg of its historic “grand tour” of the outer solar system. Although Voyager 2 only mapped 40% of Triton’s surface, the mission revealed just how strange Neptune’s moon is.

Related: The James Webb Space Telescope captures a stunning image of Neptune’s rings and moons

For one thing, Triton is big. It is the seventh largest moon on the planet solar systemand it is over 200 times larger than all of Neptune’s other moons combined, making it truly exceptional.

Secondly, Triton is one of the irregular moons. It orbits backwards relative to Neptune’s rotation, and its orbit is tilted a full 67 degrees, nearly perpendicular to its parent planet. Despite its irregularity, Triton’s orbit is surprisingly circular: in fact, one of the most perfectly circular orbits of any solar system object.

Surface-wise, it looks like a melon. Much of Triton’s mid-latitudes are covered in bumpy, wrinkled features nicknamed, appropriately enough, melon terrain. Another large section features large featureless plains dotted with massive calderas, while the southern part is dominated by a vast nitrogen ice capdotted with dozens of cryovolcanoes, volcanoes that emit jets of water.

What Triton’s surface doesn’t have are many craters, meaning it is capable of resurfacing and obliterating them. This ability to resurface is rare in the solar system and is a sign that the Moon is still partially hot.

The main suspects

NASA's Voyager 2 space probe captured dark streaks produced by geysers visible on the icy surface of Triton's south polar region.

NASA’s Voyager 2 space probe captured dark streaks produced by geysers visible on the icy surface of Triton’s south polar region.

So we gathered the facts of the crime scene investigation: this moon of Neptune is much larger than it should be, has a completely shaky, non-traditionally formed orbit, and has a young, dynamic and active surface.

What’s the story behind these oddities? The best answer is that Triton is not a mere moon but rather the victim of an interplanetary abduction. Maybe it really is a Kuiper belt closest object Pluto OR Eris than to the other moons of the solar system. Perhaps a long time ago, Triton fell in the vicinity of Neptune and was captured by the planet severity and was forced to spend the remaining billions of years in orbit in its unwelcome home.

Either Triton was simply unlucky and took the wrong orbit to land near Neptune, or it suffered a fatal collision with one of Neptune’s original moons and, in the process, lost enough energy to stay in orbit. Another possibility is that Triton originally formed as a small binary system, just like many other Kuiper belt objects, and a close encounter with Neptune sent Triton’s twin flying away and left Triton himself trapped.

Triton’s capture explains its strange orbit. If it did not form organically in the Neptunian system, it would have no reason to share the orbital plane with regular moons. And if it were caught early enough, it would have to swim through the debris surrounding the still-forming Neptune, which would be enough to shape its orbit into a near-perfect circle.

Related stories:

—Next stop, Triton? Here are two crazy ideas for exploring Neptune’s strangest moon

—Wild mission concepts would melt into icy moons, surf on sunlight to ice giants, and more

—The 10 strangest moons in the solar system

Triton stays hot because, like other Kuiper Belt objects, it has enough radioactive elements to release heat. That heat turns water, carbon dioxide and nitrogen ices into slush, keeping the surface young and active (just as Pluto has a giant, muddy field of nitrogen ice glaciers). In fact, Triton may be so hot that it hosts an ocean of liquid water beneath its crust.

More than a generation has passed since our last brief encounter with Triton. Those images captured by Voyager 2 are the only ones we have. Unfortunately, no mission concepts for returning to the Neptunian system have advanced beyond the internal proposal stage NASAthe financing structure. Given the agency’s interest in the moons of Jupiter AND Saturn (which, to be fair, are also home to vast oceans of liquid water and are much, much closer than Neptune), there probably won’t be any follow-up missions in our lifetimes.

And so this crime scene will have to wait until the next generation of explorers is fully investigated.

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