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Mountains on the surface of Pluto, a scientific mystery

NASA | Image shows details of the surface of Pluto taken from the New Horizons probe

NASA scientists were not surprised this week when their New Horizons spacecraft sent back images confirming that Pluto had a polar ice cap. But photos showing mountains on the dwarf planet have left them scratching their heads.

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Mountains on the small planet measuring a mere 7,232 km (4,494 miles) in circumference are, in theory, not supposed to exist. But exist they do. Several of the first new photographs of Pluto released by NASA on July 15 show mountains rising several thousand metres above its surface.

“This is the first time we have observed significant landforms on a dwarf planet, which theoretically shouldn’t be there,” admitted François Forget, an astronomer and research director at France’s National Centre for Scientific Research.

Forget, who has personally collaborated on the New Horizons mission, said mountains were the norm on large planets like Earth or Mars. “Their internal heat and tectonic movements give way to volcanic phenomena that are at the origin of [the mountains’] creation,” he told FRANCE 24.

There is another category of celestial bodies that feature mountains, and that is the moons of even larger planets. In the second case the mountains are the result of intense and varying gravitational forces that create movement and friction that give way to volcanoes. “Io, one of Jupiter’s moons, is constantly rocked by volcanic eruptions,” Forget explained.

However, Pluto is small compared to Earth and has nothing in common with moons. It is isolated (no big planets nearby), it is small and it is cold: the perfect combination for a relatively flat surface. Yet “from these first pictures, it looks like there might even be mountain ranges,” Forget said.

Radioactive?

Evidence suggesting that the mysterious icy peaks are also relatively recent, or “some tens of millions of years old,” has further confounded scientists, Forget said.

Images taken by the New Horizons probe show no visible craters on its surface. Impact marks from asteroids and other space debris crashing on the surface are inevitable for a 4 billion-year-old planet like Pluto. Forget says this means Pluto’s surface must have been reworked before the creation of its mountains.

The relatively new mountains of Pluto imply the planet must be active, meaning heat must emit from its centre, or that it was active until recently. These ideas contradict everything scientists have taught us about Pluto in the past.

The discovery of the mountains is “going to send a lot of scientists back to the drawing boards”, Alan Stern, New Horizons’ principal investigator, told a press conference in Maryland on Wednesday.

His colleague, John Spencer, even advanced a theory amid the barrage of questions by reporters: “Pluto has to have silicate rocks in its interior. Some thorium and uranium and various radioactive elements give off heat as they decay… The naive theory was a world this size was not big enough to get enough heat to drive this amount of activity.”

These working hypotheses may be refined in the coming days as New Horizons continues to take photos and send them back to Earth.

“For now, these mountains are an enigma, but don’t forget that we are only at the beginning of a trove of information that New Horizons will collect,” Forget said. For the French scientist, the possibilities seem endless, including a “small revolution” in our understanding of how mountains are made.

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