A landmark on Pluto that was previously designated as an impact crater may actually be the caldera of a supervolcano that has exploded in the past few million years, new research suggests.
When NASA’s New Horizons mission flew by Pluto in 2015, it revealed a geologically rich world, rather than the cold, dark landscape many had anticipated. Almost immediately, researchers identified two features, called Wright Mons and Piccard Mons, that were strongly suspected to be icy volcanoes, and further study confirmed their identity.
But not every cryovolcano was easy to spot. The suspected supervolcano, Kiladze, was initially classified as an impact crater. However, now scientists suspect it’s something else.
“We evaluated the possibility of the depression as a cryovolcanic caldera versus having an impact crater origin,” said Al Emran, a planetary scientist at the Jet Propulsion Laboratory in California. Emran presented his team’s results in July at the Progress in Understanding the Pluto System: 10 Years After Flyby conference in Laurel, Maryland.
“We think it’s more like Yellowstone Caldera in Wyoming,” Emran said. At least two of Yellowstone’s eruptions, millions of years ago, reached supervolcano status.
Impact crater or caldera?
Kiladze remains listed as a crater. But the rich supply of water ice surrounding the bowl-shaped feature sparked Emran’s curiosity, and he wondered if it might be a cryovolcano instead.
At first glance, the elongated oval bears a strong similarity to an impact crater. It’s large, with an average diameter of 2.5 miles (4 kilometers). Its walls are irregularly shaped, and the complex features it would require could easily have been eroded by Pluto’s active surface processes.
The landscape itself is marked by pits and other geological features, many of which have collapsed. If an incoming impactor broke through the surface and exposed veins of frozen lava beneath, it could have created the explosive distribution of water ice seen on the surface.
But when Emran dug into the topography maps of Pluto created by the New Horizons team, he realized there was a problem: The crater was too deep. Across the solar system, crater depth scales with crater diameter in a predictable way, and the same law appeared to hold true for other craters on Pluto — but for not Kiladze.
At best, estimates put an impact crater of its size at 1.7 miles (2.74 km) wide. But with the activity flowing across Pluto, material would have been more likely to fill in the crater over time, making it even shallower. Haze particles would have piled up, and melting or slumping ices would have fallen inward.
However, Kiladze isn’t shallower than projected; it’s deeper. Parts of the basin reach 2.5 deep, and the entire site averages nearly 2 miles (3 km) in depth.
For these reasons, Emran and his colleagues suspect that Kiladze is a caldera, a massive depression created by the eruption and subsequent collapse of a volcano. Magma — or cryomagma — spewing from the surface rapidly over a short period of time can weaken the supporting material, causing it to collapse inward on itself.
Despite the supervolcano’s collapse, the eruptive power of Kiladze would have been impressive. Emran and his colleagues calculated that the explosion could have ejected as much as 240 cubic miles (1,000 cubic kilometers) of icy cryomagma across the surrounding region, achieving the definition of a supervolcano. Although Yellowstone has erupted more than 80 times over its lifetime of more than 2 million years, only two explosions have been classified as supervolcanic.
Kiladze may have blasted out its cryomagma in a single explosive event, or it may have spread its eruptions over time. Either way, its most recent event spewed water ice at least 60 miles (100 km). Emran suspects that estimate is low, however, as more water ice is likely visible at resolutions smaller than New Horizons could reach.
“One or more cataclysmic explosive eruptions that resulted in the excavation and collapse of what is seen as the Kiladze caldera would be expected to scatter the water-ice cryomagma widely for a thousand or more kilometers, leaving exposures too small to be seen in the data at hand,” the authors wrote in a paper recently published in The Planetary Science Journal.
Clues in the ice
Kiladze sits just north of Sputnik Planetia, the icy heart to Pluto. Although much of the dwarf planet’s surface is covered with a variety of ices, very little of the surface matches what you might find in your freezer at home. Temperatures on Pluto are so cold that water ice serves as the bedrock for the dwarf planet, while other ices pile on top.
But in the neighborhood surrounding Kiladze, water ice stretches across the surface. The ice has traces of an unidentified ammoniated compound. “It’s difficult to determine the exact composition,” Emran said. In fact, that particular signature of ammonia is not seen anywhere else on Pluto.
Ammonia may be what allows the frigid ice to flow. Its addition lowers the freezing point of water, allowing it to remain liquid for longer periods. Beneath the surface, pockets of water and ammonia could have avoided freezing as Pluto’s bedrock solidified. Eventually, tectonic pressure could have driven the icy magma to the surface, spewing it across the landscape around Kiladze, Emran explained.
The mysterious traces of ammonia could also help to date the cryovolcano. Pure ammonia is quickly obliterated by the solar wind, ultraviolet particles and cosmic rays. Its faint presence suggests that the latest eruption occurred fairly recently in Pluto’s history, Emran said.
Pluto’s haze can help to nail down geological activity. The light shroud covers the dwarf planet — a result of methane and other gases jumping from solid to gas. As the particles grow, they eventually fall back to the surface and spread all over the dwarf planet. Grounded haze particles would cover the water ice like a blanket, obscuring signs of the cryolava, Emran explained. If such a layer had formed, New Horizons would have seen nitrogen-rich ice instead of signs of water-rich volcanism.
Burying the water ice requires at least 0.4 inches (10 millimeters) of haze particles falling to Pluto. That process takes at least 3 million years, Emran said. That could mean Pluto isn’t as frozen as previously thought.
“If Kiladze erupted as recently as 3 million years ago, it would indeed suggest that Pluto’s interior may still retain some residual warmth today,” Emran said. “This aligns with the idea that cryovolcanism on Pluto could be ongoing or episodic.”
