The planet has never had ice volcanoes. But it has. Some of the ice dwarf's surface features have been shaped by cryovolcanism.
The structures in the Solar System are unique.
The complexity of the New Horizons mission was more than planetary scientists imagined. The images from the cameras showed a much more complex surface than was thought. One year after the flyby of Pluto, Hal Weaver said that he was amazed by the beauty and complexity of the dwarf planet.
Weaver is one of the authors of a new paper published in the journal Nature Communications. The paper was written by Kelsi Singer from the Southwest Research Institute. Singer is a planetary scientist and a deputy project scientist.
The newly published work shows how much geologic personality Pluto has for a small planet, and how it has been active over long periods.
The New Horizons mission went to the Kuiper Belt to visit 486958 Arrokoth and other objects. Scientists are still working their way through the more than six gigabytes of data provided by the spacecraft. It took a long time to get the data because of the distances and mission limitations.
The new study shows that there were multiple periods of cryovolcanism that altered the surface of Pluto. The activity shaped the surface in a way not seen elsewhere in the Solar System. A region of large domes and rises flanked by hills, mounds and depressions was created by material from below the surface.
The structures we studied are unique to Pluto.
The cryovolcanic region has many domes ranging in size from 1 to 7 kilometres tall and up to 100 kilometres across. The tallest structures are almost as tall as Hawaii. The sides and tops of some of the larger structures can be seen in the hummocky terrain. The region is geologically young and there are no impact craters in the area.
The lack of impact craters in the region tells scientists something. The geological youth of the region combined with the mass of the cryovolcanic features suggest that the interior of Pluto was warm in the past. The materials rich in water-ice were deposited on the surface.
Water-ice can't flow across the surface of Pluto. The study says that the average surface temperature is about -235 C to -225 C. The ice mixture can be delayed by Ammonia and salts, 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.
The instruments detected ammonia or ammoniated compounds near the fractured surface of the dwarf planet. There was no clear ammonia signature in the region. The authors write that a thin layer of frozen methane covers low altitude areas on the dwarf planet.
The researchers think the cryomaterial flowed with the consistency of toothpaste because of the anti-freeze properties of ammonia. As glaciers do on Earth, it would have moved across the surface. The material underneath continued to flow after a frozen solid top capped it. It all froze into the forms New Hampshire saw in 2015.
According to the researchers, no geological process other than cryovolcanism can explain these features.
Some of the details are not obvious. There should be some evidence of the source of these features. The lack of evidence of source vent regions or directionality of material movement makes it difficult to determine the mechanism of material emplacement on the surface.
The scenarios described above show how models of emplacement derived from studies on Earth may not be applicable to Pluto.
The authors say that the Wright Mons feature provides clues to their formation. The size and complexity of the constructs point to multiple sources below them.
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.
The existence of these features is problematic for planetary scientists. The current understanding is that the heat flow from the interior is minimal.
The features are there, and with no impact craters in the region, the eruptions must have happened recently. The problematic features are only part of the problem. The paper states that they are pieces of the puzzle.
It gets really interesting here. A clathrate layer might be involved in the heat puzzle of Pluto.
Pluto's rocky core contained radioactive elements that produced heat through decay, like other rocky Solar System bodies. The heat would have kept the ocean in liquid form. If that was all that was involved, the surface of Pluto would look different. The thickness of the ice shell suggests that the heat doesn't reach the surface.
A clathrate layer between the ocean and ice shell could insulate the ocean. If some of the stored heat from the ocean was released through the clathrate layer, it could cause the cryovolcanic flows that created Wright Mons, and all the associated and interdependent features. The formation of a thin clathrate hydrate layer cap to a subsurface ocean may be an important generic mechanism to maintain long-lived 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 ocean. The ocean could explain the eruption of cryomaterial.
Studies like this one have dispelled any doubts about the value of sending a craft to the dwarf planet. We are surprised by the variety of what we learn when we send a spaceship to one of the Solar System's distant destinations.
The next step is probably an asteroid. During its single fly-by, New Horizons couldn't map the surface completely because of an obstruction. It could map the surface and confirm the presence of a sub- surface ocean.
An actual lander would be the best. Low gravity and thin atmosphere make it difficult for a lander to slow down. It would take engines and fuel to make a safe landing. It's complicated and expensive to go to a distant destination. The proposed solution was the fusion-enabled Pluto Orbiter. There are no planned missions to the dwarf planet.
That's okay. There is still a lot of data to be had. The data shows a lot of surprising things about icy worlds.
One of the benefits of exploring new places in the solar system is that we find things we weren't expecting.
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