In July 2015, NASA’s New Horizons spacecraft gave humanity its only close-up view of Pluto. Far from being a featureless and frigid sphere, the spacecraft discovered that the dwarf planet was a showy world with epic impact craters, methane ice and nitrogen snow.
Two of the mountains, Wright Mons and Piccard Mons, were suspected to be volcanoes. But instead of spewing molten rock, they would be built from and able to erupt ice in a process known as cryovolcanism.
After years of scrutinizing data from New Horizons, scientists believe they may have seen evidence of recently erupted ice lava, a sign that Pluto is home to cryovolcanoes that were active in its immediate geological past.
“We tried to find another way to explain it, but it just didn’t work,” said Kelsi Singer, a senior researcher at the Southwest Research Institute in Boulder, Colorado, and author of the study, which was published Tuesday in the journal Nature. communication.
If the team is right, the implications are significant. Every erupting volcano requires a source of molten fuel. If these ice lava deposits are young, then the underworld just below this spot on Pluto’s icy shell was, at least to some extent, hot and liquid very recently. And such a finding gives credence to the hypothesis that present-day Pluto is an ocean world, improbable as that may seem for a small, icy sphere so far from the sun.
Possible cryovolcanic features have been observed across the solar system over the past four decades, from the asteroid belt dwarf planet Ceres to Neptune’s wintry moon Triton.
An icy lava eruption has never been observed, but seeing a lava eruption on a world like Pluto would be downright surreal: The icy lava can slowly emerge from the vent or crevice of a dome as a yellow, slimy mass. – something similar to Silly Putty but made from one or more frozen chemical compounds – and largely retains its lumpy shape in an extremely cold, low-gravity environment.
The discovery of Wright Mons and Piccard Mons on Pluto, two icy mountains with central pits that looked like eruptive vents, suggested it was also a member of the cryovolcanic club. But whether these ice volcanoes really were what they looked like and whether they were still active proved difficult to determine.
After their forensic examination of the New Horizons flyby images, Dr. Singer and her colleagues argue that Wright Mons and Piccard Mons are not two large cryovolcanoes. Instead, they are likely colonies of smaller volcanic domes, usually created when very gloopy lava erupts slowly and forms squat piles. It seems that many such domes grew in the same area and squashed together, creating two mountainous masses.
But is this cryovolcanic colony still alive?
The team noted that much of the region is covered in irregular, undulating patches, mostly made of water ice. This terrain is nothing like twisted deposits elsewhere on Pluto, formed by erosion. That leaves a likely explanation: It was made by water ice lava flows. And since their deposits do not have craters, the flows erupted close to the geological present.
Glacial eruptions may be foreign to this planet, but some geological foundations are likely familiar: Just as Earth’s active volcanoes sit atop reservoirs of partially molten rock, Pluto’s volcanic domes may sit atop partially liquid reservoirs of water ice. But in the case of the dwarf planet, these reservoirs can feed on one large ocean.
Previous studies have suggested that Pluto has a subterranean ocean, although its existence is difficult to explain.
A world this small would have lost the heat from its formation long ago, and it doesn’t contain enough radioactive, heat-releasing elements in its rocky core, so all the liquid beneath its icy outer shell must be frozen. Also various hypotheses about the retention of heat cannot be proven at the moment.
It is not fanciful to imagine frozen worlds containing oceans. Europa, a moon of Jupiter, and Enceladus, orbiting Saturn, both have undeniable internal oceans. But the gravitational interactions with their planets and nearby moons that are thought to warm them don’t apply to Pluto.
And yet Pluto’s youthful cryovolcanic terrain supports the case for its own subterranean seas, while adding evidence to two theories: that ocean worlds are commonplace in the cosmos, and that scientists can’t always explain how their existence is possible.
Ultimately, the data from New Horizons and other spacecraft researchers is constantly teaching the same lesson. “We needed a better imagination,” said Jani Radebaugh, a planetary scientist at Brigham Young University who was not involved in the study.