Proposed NASA Mission Would Visit Neptune's Curious Moon Triton
This global color mosaic of Neptune's moon Triton was taken in 1989 by Voyager 2 during its flyby of the Neptune system. Credit: NASA/JPL/USGS

When NASA’s Voyager 2 spacecraft flew by Neptune’s strange moon Triton three decades ago, it wrote a planetary science cliffhanger.

Voyager 2 is the only spacecraft ever to have flown past Neptune, and it left a lot of unanswered questions. The views were as stunning as they were puzzling, revealing massive, dark plumes of icy material spraying out from Triton’s surface. But how? Images showed that the icy landscape was young and had been resurfaced over and over with fresh material. But what material, and from where?

How could an ancient moon six times farther from the Sun than Jupiter still be active? Is there something in its interior that is still warm enough to drive this activity?

A new mission competing for selection under NASA’s Discovery Program aims to untangle these mysteries. Called Trident, like the three-pronged spear carried by the ancient Roman sea god Neptune, the team is one of four that is developing concept studies for new missions. Up to two will be selected by summer 2021 to become a full-fledged mission and will launch later in the decade.

Investigating how Triton has changed over time would give scientists a better understanding of how solar system bodies evolve and work.

The oddities of Triton could fill an almanac: As Neptune rotates, Triton orbits in the opposite direction. No other large moon in the solar system does that. And Triton’s orbit lies at an extreme tilt, offset from Neptune’s equator by 23 degrees. About three-quarters the diameter of our own Moon, Triton isn’t where it used to be, either. It likely migrated from the Kuiper Belt, a region beyond Neptune of icy bodies left over from the early solar system.

Triton has an unusual atmosphere, too: Filled with charged particles, a layer called the ionosphere is 10 times more active than that of any other moon in the solar system.

That last trait is especially strange, because ionospheres generally are charged by solar energy. But Triton and Neptune are far from the Sun – 30 times farther from the Sun than Earth, so some other energy source must be at work. (It takes 165 Earth years for Neptune to complete one orbit around the Sun.)

And Triton’s climate is dynamic and changing, with a steady flow of organic material, likely nitrogen, snowing onto the surface.

“Triton has always been one of the most exciting and intriguing bodies in the solar system,” said Louise Prockter, director of the Lunar and Planetary Institute/Universities Space Research Association in Houston. As principal investigator, she would lead the proposed Trident mission, and NASA’s Jet Propulsion Laboratory in Southern California would manage it. “I’ve always loved the Voyager 2 images and their tantalizing glimpses of this bizarre, crazy moon that no one understands,” Prockter added.