Voyager glimpsed Triton only briefly, but that glimpse was tantalizing enough. The spacecraft revealed that the moon’s surface is young, with some estimates setting its age as low as 10 million years. The reshaping and repaving of the terrain suggest that something is happening beneath the surface. Whether that process comes from the movement of rocks or the effect of an ocean remains unclear.

One of the moon’s most intriguing features is its cantaloupe terrain — rugged surface features that resemble the skin of the fruit whose name it bears. Planetary scientists think that rising blobs of ice, known as diapirs, cause this terrain as they are pushed upward through the more brittle surface by heating from below.

Voyager also glimpsed plumes of material shooting a few miles above the surface. “At the time, we developed a whole theory about solar-driven nitrogen geysers,” says Hansen, who was on the Voyager team. She describes some of the reasoning as circumstantial, because the geysers appeared where the Sun was nearly directly overhead. As sunlight heated the ices, nitrogen could have jumped from solid to gas to become the plumes.

With the discovery of geysers spouting water from Enceladus and Europa, scientists are taking another look at Triton’s plumes. “Maybe Triton is like Enceladus and Europa, and there could actually be water plumes coming from an interior ocean,” says Mandt.

Material from the plumes has recolored Triton’s surface: Voyager spotted streaks suggesting material fell from previously or currently active geysers. If the geysers draw water from a liquid ocean, samples of the interior may lie on the moon’s surface, ripe for the taking.

Together, these studies suggest the possibility that Triton may be hiding liquid beneath its surface, making it a potentially habitable site in the solar system. “We’ve got some tantalizing clues that it is an ocean world,” says Amanda Hendrix, also of the Planetary Science Institute.