It is located in the icy reaches of our solar system, two peaks above the surface of the dwarf planet Pluto Planetary scientists have puzzled for years. Some have speculated that it could be an ice volcano, spewing not lava but massive amounts of glacial slash ice – yet the caldera can’t be seen like a cauldron.
Now, a full analysis of images and topographic data indicates that it is not a single ice volcano but the merging of many – some up to 7,000 meters high and about 10-150 kilometers wide. Their discovery sparked another controversy: What could keep Pluto hot enough to support volcanic activity?
Located at the southern edge of a broad, heart-shaped ice sheet, these unusual surface features were first observed when NASA’s New Horizons spacecraft flew by in July 2015, providing the first close-up images of the former ice planet and its moons.
“It immediately piqued our interest in this area because it was so different and amazing looking,” said Dr. Kelsey Singer, a New Horizons research associate and deputy project scientist at the Southwest Research Institute in Boulder, Colorado.
“There are these gigantic broad mounds, and then this undulating, hamocki-like texture superimposed on top of them; and even above that are a smaller kind of rock.”
At the time, an icy volcano appeared to be the least exotic explanation for these features—there were no impact craters from nearby asteroids or meteorites, suggesting that these features had been erased by relatively recent geological events; And there is no evidence for plate tectonics – a major contributor to mountain formation on Earth.
However, Singer and her colleagues were cautious about calling them volcanoes: “It’s kind of a big claim that there are ice volcanoes,” she said. “It’s theoretically possible, but there aren’t a lot of other examples in the solar system, and they all look really different, and they don’t look like the features on Pluto.”
Since those first images aired in 2015, more has arrived, along with compositional and topographical data. Taking all of this together, the team concluded that these unusual features are in fact volcanoes – although their appearance and behavior are much different than those on Earth.
said Singer, whose results were published in Nature Communications. “So we think it’s likely that the material is ejecting from the bottom, and the dome is growing from the top.”
As for the nature of this substance, the compositional data indicate that it is mainly water ice, but with some additional “anti-freeze” components mixed in, such as ammonia or methanol. “It’s still hard to think that it’ll be liquid, because it’s so cold – the average surface temperature of Pluto is about 40 K (-233 degrees Celsius),” Singer said. “So, it’s probably more, either a slush material, or it can mostly be in a solid state – like a solid glacier, but it can still flow.”
She added that even this is surprising, because due to the extremely low temperature, this material should not be mobile at all. Probably it indicates that the rocky core of Pluto is much warmer than expected, and that the heat energy released by radioactive decay is somehow trapped in some of its elements, for example by an insulating layer of material, and is released periodically, which leads to volcanic eruptions.
All this is just speculation. “I’ll freely admit that we don’t have a lot of information about what’s going on in the subsurface of Pluto,” Singer said. But this forces people to come up with some creative ideas on how to do it [ice volcanism] might happen.”
Whatever the explanation, the old idea of Pluto as just an inert ball of ice seems increasingly improbable.