The coldest world in the solar system is active: Voyager 2 data from 1989 still puzzles scientists

Neptune's moon Triton, long thought to be a frozen world, actually shows signs of geological activity, and the only images proving it were captured by the Voyager 2 probe back in 1989. The spacecraft then recorded dark plumes rising nearly eight kilometers above the surface and carried 150 kilometers by the wind. This happened on the coldest known surface in the solar system, where temperatures hover around minus 235 °C.

Before that, it was believed that at such frigid conditions, when most nitrogen freezes as frost, no activity could occur. However, observations of nitrogen geysers forced a rethink of processes in extreme cold. It remains unclear how kilometer-long plumes form, and a 2022 research paper examining various hypotheses reached no final conclusion.

Voyager 2, launched in 1977 and now more than 21 billion kilometers from Earth, is still operational: five of its scientific instruments continue to work. During its flyby of Triton, the probe managed to map only about 40 percent of the moon's surface. The remaining 60 percent has never been photographed up close, as no spacecraft has returned to that system since.

NASA's 2020 Trident mission concept highlighted key knowledge gaps left by the flyby: the origin of the plumes and the reasons for the surface's youthful appearance. Triton has almost no craters, which may suggest it is only about 10 million years old. This implies the surface was geologically reshaped relatively recently — in an environment previously considered "frozen."

Karl Mitchell, project scientist for Trident, noted that Voyager 2's early-1970s technology was "essentially a television camera connected to a fax machine." Yet, despite the outdated equipment, those images remain the only close-up views of Triton ever obtained.