Ever since New Horizons first beamed back its beauty shots of Pluto in 2015, we’ve been poring over the data. Now, planetary scientists have reported evidence confirming the presence of cryovolcanoes on Pluto. The cryovolcanic region borders the southwest aspect of Sputnik Planitia, a brightly reflective, heart-shaped plateau of nitrogen ice.
At up to 7km tall, these enormous mountains of ice are in the height class of the tallest peaks on Earth. Only the Himalayas and Karakoram reach higher. But unlike the craggy peaks of the Himalayas, these cryovolcanoes are like a frozen version of Hawaii’s shield volcanoes. Their shape and mass are like Mauna Loa, with the largest measuring up to 100km wide at the base.
Perspective view of the cryovolcanic region on Pluto. At center, Wright Mons and its massive caldera. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Isaac Herrera/Kelsi Singer
NASA had already been eyeing Wright Mons, one of the largest cryovolcanoes identified in the new research. If confirmed, the agency said, “it would be the largest such feature discovered in the outer solar system.”
The researchers identified the cryovolcanoes while looking at a region of Pluto’s surface that was remarkably free of impact craters. The ice there, according to the report, may be less than 200 million years old. Stranger still, scans indicate that it’s water ice.
“The combination of these features being geologically recent, covering a vast area and most likely being made of water ice is surprising because it requires more internal heat than we thought Pluto would have at this stage of its history,” said lead author Kelsi Singer. (Read the open-access report in Nature Communications.)
‘A horrible Slurpee’
For ice volcanoes to work, they need cryolava. But, the report observes, “At these low temperatures pure water ice should generally form an immobile bedrock.” A world as small as Pluto should have long since frozen solid.
Pluto, as seen by New Horizons in 2015
However, the composition of Pluto’s ice is as varied as the colors of its surface. Ammonia, salts and other antifreezes in the cryolava slush might have given it a consistency not unlike a horrible Slurpee. (Try our new hit flavor, “Demoted Planet” – Ed)
As for the shape of the volcanic domes, Pluto’s surface temperature hovers between 35 and 60 K. Instead of piling up into a cone with a caldera, the cryolava would slump across Pluto’s icy surface, says the report. At the same time, Dr. Singer believes, there are whole “colonies” of these cryovolcanoes. Any slush that spilled out onto the surface would have immediately hardened into a dome. A porous region of ice could allow an upwelling of cryolava through many different apertures, some larger, some smaller. That could explain the “hummocks” we see.
The presence of these cryovolcanoes, the researchers say, suggests that there is more activity on Pluto than meets the eye. “The existence of these massive features suggests Pluto’s interior structure and evolution allows for either enhanced retention of heat or more heat overall than was anticipated before New Horizons, which permitted mobilization of water-ice-rich materials late in Pluto’s history,” the researchers wrote in their report.
As for life in the frozen reaches of the outer solar system, the jury’s still out. “I think that it is a little more promising, and that there might be some heat and liquid, potentially liquid water closer to the surface,” Singer said. “But there’s still some big challenges for poor microbes that want to live on Pluto.”
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