Pluto’s icy volcanoes

Giant cryovolcanoes on Pluto appear to be remarkably recent features, indicating that Pluto’s subsurface remained warm much longer than previously thought – long enough that it might still be fueling volcanic activity, scientists say.

Cryovolcanoes are ice volcanoes, formed not from molten rock but from molten ice: i.e., water. When that water emerges onto super-cold surfaces like Pluto’s (where temperatures can be as low as -243°C), it rapidly solidifies into rock-hard ice.

Pluto’s cryovolcanoes, which reach as high as 7,000 metres, were imaged by NASA’s New Horizons spacecraft on its 14 July 2015 flyby.

Spreading across a region southwest of a giant basin known as Sputnik Planitia, they rise not in sharp cones, like Japan’s famous Mt Fuji, but in spreading domes up to 225 kilometres across. “These ice volcanoes are different than any other geologic features in the Solar System,” says Kelsi Singer, a planetary scientist at the Southwest Research Institute in Boulder, Colorado, US.

In a paper in this week’s issue of Nature Communications, Singer’s team compiled images taken from multiple directions during New Horizon’s flyby to create 3D renderings that allowed them to be studied in detail.

The biggest volcano, called Wright Mons, is similar in scale to Mauna Loa in Hawaii, which from its seabed base is one of the largest volcanoes on Earth. But Wright Mons isn’t as isolated as the volcanoes that form the Hawaiian Islands. Instead, Singer says, it appears to be part of a vast complex of overlapping volcanoes.

All told, tens of thousands of cubic kilometres of icy “cryolava” appears to have oozed from Pluto’s interior at multiple locations in a succession of eruptions, creating a gigantic field of overlapping cryovolcanoes, rather than individual mountains.

“[These volcanoes] have a very rough and bumpy texture all over them, and no obvious calderas,” Singer says. “We think there had to be many vent locations and that the vents were buried by the volcanic mounds that formed over them.”

Furthermore, these eruptions all appear to have been fairly recent. How recent isn’t certain. “It is hard to put an exact date on the formation of these features because we don’t have any samples,” Singer says. But the entire volcanic landscape is devoid of impact craters, she says, indicating that they are young enough not to have had time to be peppered with incoming asteroids.

Best guess? “These features formed in the last few hundred million years,” Singer says, “which is short on geologic timescales.” In fact, she says, “we cannot rule out that [they] could still be forming. The events might be episodic over long timescales, just like volcanoes on Earth are sometimes dormant for a long time before erupting again.”

It’s an important find, because it indicates that Pluto may have more near-subsurface heat than previously believed.

Prior studies suggested that Pluto might still have a liquid ocean at least 100–200km below its icy shell. But the new finding suggests that pockets of liquid water might be present at shallower depths than that. Not that this means these liquid pockets might be contain life, or even be habitable to life as we know it. “There are still a lot of challenges for any organisms trying to survive there,” Singer says. “They would still need some source material of continual nutrients and if the volcanism is episodic, and thus the heat and water availably is variable, that [might be] tough.”