Pluto’s Surface was Shaped by Ice Volcanoes

For all of Earth’s geological diversity and its long history, the planet has never had ice volcanoes. But Pluto has. And that cryovolcanism has shaped some of the ice dwarf’s surface features.

The resulting structures are unique in the Solar System.

When the New Horizons spacecraft visited Pluto in 2015, it revealed more complexity than planetary scientists imagined. Images from the spacecraft’s cameras showed a much more geologically active and complex surface than thought. In 2016, one year after the spacecraft’s flyby of Pluto, New Horizons Project Scientist Hal Weaver said, “We’ve been astounded by the beauty and complexity of Pluto and its moons, and we’re excited about the discoveries still to come.”

Pluto’s most visible landmark is the heart-shaped feature named “Tombaugh Regio” in honour of astronomer Clyde Tombaugh, who discovered the dwarf planet. The bright expanse of the western lobe of Pluto’s “heart” is called Sputnik Planitia. The new study focuses on the southwest of Sputnik Planitia, shown with the yellow rectangle. (Note: False Colour Image.) Credit: Courtesy NASA / JHUAPL / SwRI

A new paper published in the journal Nature Communications presents one of these discoveries, and Weaver is one of the authors. The paper is “Large-scale cryovolcanic resurfacing on Pluto.” The lead author is Kelsi Singer from the Southwest Research Institute in Boulder, CO. Singer is a planetary scientist and a Deputy Project Scientist with the New Horizons mission.

“This newly published work is truly landmark, showing once again how much geologic personality Pluto has for such a small planet, and how it has been incredibly active over long periods,” said New Horizons Principal Investigator Alan Stern of the Southwest Research Institute. “Even years after the flybyy, these new results by Singer and co-workers show that there’s much more to learn about the marvels of Pluto than we imagined before it was explored up close.”

The New Horizons mission left Pluto behind years ago and went to the Kuiper Belt to visit 486958 Arrokoth (Ultima Thule) and other Kuiper Belt Objects. But scientists are still working their way through the more than six gigabytes of data provided by the spacecraft’s seven different science instruments during itsflybyy. Even getting the data took a long time because the vast distance and other mission limitations restricted data transfer from the spacecraft to 1 kbit/s per transmitter.

This new study results from some of that hard-won data, and it shows that Pluto underwent multiple periods of cryovolcanism that altered its surface. That activity shaped the surface in a way not seen anywhere else in our Solar System. According to a press release announcing the findings, material from below Pluto’s surface created “… a region of large domes and rises flanked by hills, mounds and depressions.”

“The particular structures we studied are unique to Pluto, at least so far,” said lead author Singer. “Rather than erosion or other geologic processes, cryovolcanic activity appears to have extruded large amounts of material onto Pluto’s exterior and resurfaced an entire region of the hemisphere New Horizons saw up close.”

The region studied lies southwest of Pluto’s “heart,” Sputnik Planitia, and contains multiple large domes and rises up to 7 kilometres tall and 30 to 100 kilometres across, with interconnected hills, mounds, and depressions covering the sides and tops of many of the larger structures. Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Isaac Herrera/Kelsi Singer

The cryovolcanic region contains multiple large domes, ranging from 1 to 7 kilometres (about one-half to 4 miles) tall and 30 to 100 or more kilometres (about 18 to 60 miles) across, that sometimes merge to form more complex structures. The tallest structures are almost as tall as Hawaii’s Mauna Loa. Hummocky terrain consisting of irregular interconnected hills, mounds and depressions covers the sides and tops of some of the larger structures. There are no impact craters in the area, evidence that the region is geologically young.

This image from the study shows some of the features with labels. The dotted red line is the boundary between sun-lit terrain and haze-lit terrain. Image Credit: Singer et al. 2022.

The lack of impact craters in the region tells scientists something about Pluto’s history. The region’s geological youth combined with the sheer mass of the cryovolcanic features suggest that Pluto’s interior was warm in the recent past. The interior convection allowed materials rich in water-ice to be deposited on the surface.

Pluto’s surface is far too cold for water-ice to flow across it. Typical surface temperatures are about -240 C to -215 C (35–60 K; -400 to -350 F). “At these low temperatures, pure water ice should generally form an immobile bedrock…” the study says. Ammonia and salts in the ice mixture can delay the freezing, “…but the surface temperatures on Pluto are so cold and the atmospheric pressure so low that freezing of a fluid on the surface would still occur on relatively short geologic timescales.”

New Horizons’ instruments detected ammonia or ammoniated compounds near fractures on Pluto’s surface where cryofluid could’ve flowed to the surface. But the region in the study showed no clear ammonia signature. But a thin layer of seasonally frozen methane covers low altitude areas on Pluto, and the ammonia “…could be obscured by the methane signature,” the authors write.

Because of the anti-freeze properties of ammonia, the researchers think the cryomaterial likely flowed with the consistency of toothpaste. It would’ve moved across the surface as glaciers do on Earth. Or, a frozen solid top capped the flowing material, and the material underneath continued to flow. Eventually, it all froze into the forms New Horizons saw in 2015.

This image is a topographical image of the region in the study. Piccard Mons and Wright Mons are visible. Image Credit: Singer et al. 2022.

According to the researchers, no geological process other than cryovolcanism can explain these features. “These geologic features do not appear to be formed predominantly by erosion nor do they appear to be constructed primarily of volatile ices,” the authors write in their study. “We propose a large volume of material has erupted from multiple sources (and likely in more than one episode over time) to form the many large domes and rises found in this region.”

Some of the details are still obscure. If cryovolcanism formed these features, there should be some evidence of the source. There should also be some evidence of directional flow. “The lack of indications of source vent regions or directionality of material movement makes it difficult to positively determine the mechanism of material emplacement on the surface.,” the study states.

This image from the study compares Pluto’s Wright Mons with terrestrial and Martian volcanoes. Image Credit: Singer et al. 2022.

But this is Pluto, not Earth, and much of our initial understanding stems from what happens on Earth. “The scenarios described above illustrate how canonical models of emplacement (derived primarily from terrestrial studies) may not be directly applicable to Pluto,” the authors explain. “The geologic features in the Wright Mons region are morphologically unlike any other regions on Pluto and also have very few similarities to most terrains on other bodies in the solar system.”

The authors say their examination of New Horizons data and especially the Wright Mons feature provide clues to their formation. The size and morphological complexity of the cryovolcanic constructs point to “… multiple subsurface sources where the sources are below the constructs.”

“This scenario allows for a consistent formation mechanism for all of the large rises and depressions—where some are domical or annular, and others are complex shapes—through the merging of different rises,” the authors explain.

This image from the study shows the morphological complexity that distinguishes the region from other surface regions on Pluto. Wright Mons is about 150 km (90 miles) across and 4 km (2.5 miles) high, making it the largest known cryovolcano in the Solar System. The central depression is about 40–50 km (25-31 miles) across and extends down to approximately the level of the surrounding terrain or slightly below, making it about 4 km (2.5 miles) deep on average. Image Credit: Singer et al. 2022.

The existence of these cryovolcanic surface features is problematic for planetary scientists studying Pluto. In the current scientific understanding, heat flow from Pluto’s interior is minimal. “Given the low expected heat fluxes from Pluto’s interior, and Pluto’s cold surface temperatures, mobilizing material primarily made up of water ice is thermally challenging,” the authors point out.  

But these features are there, and with no unambiguous impact craters in the region, the cryovolcanic eruptions must have occurred in recent geological times. However, looking at these features as problematic is only part of it. They’re also pieces of the Pluto puzzle. “Multiple, massive water-ice cryovolcanic constructs present new pieces of information towards understanding Pluto’s thermal history,” the paper states.

Here’s where it gets really interesting. Previous research has shown that Pluto’s heat puzzle might involve a clathrate layer.

Earlier in its life, Pluto’s rocky core would’ve contained radioactive elements that produced heat through decay, like other rocky Solar System bodies. That heat would’ve kept the subsurface ocean in liquid form. But if that’s all that was involved, Pluto’s surface would look different. Significant variations in Pluto’s ice shell thickness suggest the heat doesn’t reach the surface.

A 2019 study showed that a clathrate layer between the ocean and the ice shell surface could insulate the ocean from the shell. If some of that stored heat from the ocean was released through the clathrate layer, it could’ve caused the cryovolcanic flows that created Wright Mons, Piccard Mons, and all the associated and interconnected features. The 2019 study said that “The formation of a thin clathrate hydrate layer cap to a subsurface ocean may be an important generic mechanism to maintain long-lived subsurface oceans in relatively large but minimally heated icy satellites and Kuiper belt objects.”

The geologically young cryovolcanic features add weight to the idea that Pluto has a subsurface ocean, similar to some moons outside the Solar System’s frost line. “… modelling suggests a subsurface water-rich ocean could potentially persist into the present on Pluto,” the study says. “Any ocean is generally predicted to exist 100–200 km or more below the surface of Pluto, at the base of the icy shell,” the authors explain. Convective upwellings in the ocean could explain the eruption of cryomaterial onto Pluto’s surface.

This cutaway image of Pluto shows a section through the area of Sputnik Planitia, with dark blue representing a subsurface ocean and light blue for the frozen crust. Artwflyby Pam Engebretson, courtesy of UC Santa Cruz.

If there were ever any doubts about the value of sending a spacecraft to Pluto, studies like this one have dispelled them. Each time we send a spacecraft to one of the Solar System’s distant destinations, we’re surprised by the variety of what we learn.

The next step in our effort to understand Pluto is probably an orbiter. An orbiter would allow us to completely map the surface, which New Horizons couldn’t do during its single fly-by. Not only could it map the surface, but an orbiter should also be able to confirm the presence of a sub-surface ocean.

But an actual lander would be best. The problem is that Pluto’s low gravity and thin atmosphere make it difficult for a lander to slow down. Any lander would have to carry engines and fuel to slow down and make a safe landing. That’s complicated and expensive for such a distant destination. One proposed solution was the Fusion-Enabled Pluto Orbiter and Lander. As things stand now, there are no planned missions to Pluto.

But that’s okay. There’s still lots of New Horizons data to keep scientists busy. And that data is revealing a lot of surprising things about icy worlds like Pluto.

“One of the benefits of exploring new places in the solar system is that we find things we weren’t expecting,” said Singer. “These giant, strange-looking cryovolcanoes observed by New Horizons are a great example of how we are expanding our knowledge of volcanic processes and geologic activity on icy worlds.”

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Evan Gough

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