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?
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 either migrated from the Kuiper Belt, a region beyond Neptune of icy bodies left over from the early solar system, or it came from beyond the Solar System and was captured by Neptune.
Triton has an unusual atmosphere: 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.)
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.
Those mysterious plumes Voyager 2 spotted are especially intriguing. Plumes seen on Saturn’s moon Enceladus, and possibly present on Jupiter’s moon Europa, are thought to be caused by water from the interior being forced through thick, icy crusts. If an ocean is the source of the plumes on Triton (which lies much farther out in the solar system than Europa and Enceladus), the discovery would give scientists new information about how interior oceans form. Unlike other known ocean worlds, Triton’s potential ocean likely developed after it was captured by Neptune’s gravity.
It would also expand scientists’ understanding of where we might find water. Figuring out what factors lead to a solar system body having the necessary ingredients to be habitable, which include water, is one Trident’s three major goals. The spacecraft would carry an instrument to probe the moon’s magnetic field to determine if an ocean lies inside, while other instruments would investigate the intense ionosphere, organic-rich atmosphere and bizarre surface features.
The proposed launch date in October 2025 (with a backup in October 2026) would take advantage of a once-in-a-13-year window, when Earth is properly aligned with Jupiter. The spacecraft would use the gravitational pull of Jupiter as a slingshot straight to Triton for an extended 13-day encounter in 2038.
And it may seem that time moves slowly in the outer reaches of the solar system, where Neptune’s years are long. Ironically for Triton, the long timeline presents limitations. If Trident arrives before 2040, the team could perform its test of what’s powering the plume activity. Any later, and the Sun moves too far north … for the next hundred years