Mountains on Pluto – at least one set rising higher than any on Earth outside the Himalayas – have been revealed by reanalysis of images taken by the New Horizons spacecraft in 2015. These peaks look like nothing else in the Solar System, and could not be formed from rock, planetary scientists believe – instead, being made of frozen ammonia, nitrogen, methane, and water ice. Nevertheless, their origins are poorly understood.
Pluto is so small and far from the Sun it was once assumed to be frozen all the way through. However, the complex geology observed by New Horizons – including the absence of impact craters from parts of the dwarf planet – indicates recent geological activity has reshaped some areas.
Planetary scientists have spent the last seven years trying to make sense of the images collected as New Horizons shot past, and understand the forces that produced what we can see.
Pluto's most obvious feature is its heart, technically known as Sputnik Planitia, an ice sheet around 1,050 kilometers (620 miles) across, thought to fill an ancient impact basin. Its flanks include substantial rises composed of many small hillocks. A detailed study in Nature Communications reveals the largest, to the south-west, is 7 kilometers high (4.2 miles), and around 225 kilometers (135 miles) wide.
The largest peak has been named Piccard Mons, but it was in darkness by the time New Horizons took its most detailed images. Consequently, the paper focuses attention on Wright Mons, seen just before local sunset.
Dr Kelsi Singer of the Southwest Research Institute and colleagues conclude both are ice volcanoes, but rather than each being formed in a single dramatic eruption they probably represent many smaller outbursts located close together. Other, lower, mountains are nearby, but their volcanic status is less certain.
At 4 kilometers (2.49 miles) high and 150 kilometers (93.2 miles) across, Wright Mons has around the same internal volume as Hawaii's Mauna Loa, which is much larger than Everest but is based far below sea level. However, rather than being single peaks, Wright and Piccard may be formed from multiple eruptions so close together the mountains merged to create the hillocky pattern.
The absence of impact craters in the area indicates it has been resurfaced relatively recently. If so, Pluto has retained enough heat to be able to drive such eruptions, probably from an internal ocean made from a slushy mix of water, ammonia, and other forms of antifreeze.
Nevertheless, how this ocean survived, let alone had the energy to produce such eruptions, remains a puzzle. Pluto certainly isn't getting the warmth from the Sun, being -230º C (-382º F) at the surface.
The Earth's core is molten thanks to the heat generated by radioactive decay, but Pluto's rocky core is so small it shouldn't contain sufficient radioactive elements. Tidal interactions between Pluto and Charon would have provided a heat source after Charon's formation in a large impact, but this too is thought to have died away as the distance between the two objects grew.
Nevertheless, the evidence for recent activity is too strong to deny; something created the energy to raise these ice mountains.
A puzzling feature of Piccard and Wright for scientists was the giant depressions near their peaks. These look like caldera in volcanoes on Earth or Mars, but are much larger, even relative to the peaks that enclose them. Singer concluded the knobbly "walls" of these craters represent the multiple peaks produced by a series of small eruptions, rather than the outline of a single cladera.
Although the pair were considered candidates for ice volcanoes soon after New Horizon passed, it's taken this long to work out what we have about these mountains partly because we missed the chance to see Piccard clearly. Moreover, the complex topography in this area has proven a challenge to understand.
Just because you're not a planet, doesn't mean you can't be full of surprises, as Ceres proved.