The latest results from NASA's New Horizons probe have yielded a big surprise: Pluto may be a volcanic world.
Scientists involved with the mission say that is the most plausible explanation for a large, mound-like feature with a dark central depression located in Pluto's southern hemisphere. The circular feature is 160 kilometres across and strongly reminiscent of large shield volcanoes such as those on Mars.
The difference is that on Pluto, where the average surface temperature is well below -200 degrees C, volcanoes would be made of ice and would presumably erupt some form of slushy material, such as a water-ammonia mix, that can flow at extremely low temperatures.
Scientists left the door open to alternative explanations for the feature but admitted they were having a hard time imagining what those alternatives might be.
"When you see a big mountain with a hole in the top, it basically points to one thing," said Oliver White, a New Horizons team member based at NASA's Ames Research Center near Mountainview, Calif. He added that a volcano on Pluto "is the least weird hypothesis at the moment."
The image was captured when New Horizons sped past Pluto last July 14, and the picture was released on Monday at a gathering of planetary scientists near Washington. A second image from an adjacent region shows another similar volcanic mound. Team members have informally dubbed the features Wright Mons and Piccard Mons. (The names won't become official until they've been approved by the International Astronomical Union.)
Together, the images offers the latest evidence from New Horizons that Pluto is a geologically active world with a strong internal heat source, likely related to the decay of radioactive elements.
This has led some scientists to speculate that Pluto's interior may be quite warm and watery, despite its relatively small size and five-billion-km distance from the sun.
Alan Stern, principal investigator for the mission, said "Pluto has really outsmarted us" in terms of defying scientists' predictions for what New Horizons would find.
Meanwhile, New Horizons is scheduled to conclude a series of manoeuvres this month that are designed to shift its trajectory to intercept a small, icy object called 2014 MU69 in about three years' time. A U.S.-Canadian team of astronomers spotted the 50-km-wide object last year during an exhaustive search for a second target for the mission. It lies in the Kuiper Belt, a region beyond the outer planets, which may hold clues to the formation of the solar system.