Pluto may have had a ‘hot start’ harboring liquid oceans when it first formed

Although Pluto orbits the sun at an average distance of 3.7 billion miles (5.9 billion kilometers), some studies claim that the icy dwarf planet may have a liquid ocean under its surface. According to a new study, the accretion of new material during Pluto’s early geological history may have generated enough heat to sustain the formation of this ocean, which would go on to continuously freeze over billions of years.

Extensional faults (arrows) on the surface of Pluto may have been created by the freezing of a subsurface ocean. Credit: NASA/Alex Parker.

Pluto is so far away from the sun that it lies inside the Kuiper Belt, a group of rocks and ice left over from the formation of the solar system. It’s really no wonder that today the planet looks like a barren ball of ice and rock.

However, detailed images of Pluto’s surface taken during flybys by NASA’s New Horizons spacecraft have revealed intriguing geological features that can tell us many things about the dwarf planet’s past.

According to Francis Nimmo, a professor of Earth and planetary sciences at UC Santa Cruz, Pluto’s surface shows evidence of both ancient and modern extensions of its surface. This suggests that Pluto may have been ‘hot’ during its early days.

To understand why this matters, we have to make a brief tangent to talk about how water freezes. Most liquids shrink when they are cooled because molecules are moving slower, making them less able to overcome the attractive intermolecular forces that draw them closer together. When the freezing point is reached, the substance solidifies in a tightly packed crystalline matrix.

Water is an exception to this chemical behavior. Liquid water also contracts when cooled, but only until it is chilled to approximately 4 degrees Celsius. If you cool water past this threshold, it will actually expand slightly. Cool it further to its freezing point and the water will expand by about 9%.

So, water expands when it freezes and contracts when it melts — and this has important implications for Pluto’s history.

“If Pluto had a subsurface ocean shortly after it formed, that ocean would have been continuously freezing over solar system history. That would cause Pluto to be expanding and that would result in specific geologic features we can look for. In contrast, if Pluto started with a cold ice shell, that ice would have warmed and melted slowly from the heat of radioactive decay forming an ocean. That ocean would have been refreezing in more recent times as the heat from radioactive decay waned. This would lead to early compressional tectonics followed by later extension,” Carver Bierson, UCSC graduate student and co-author of the new study, told ZME Science.

If Pluto had a ‘cold start’, compression on its surface would have occurred early on, followed by more recent extensions. However, if it had a hot start, then extension would have occurred throughout Pluto’s history — and this seems to be the case, current observations suggest.

“When we look at the surface of Pluto we don’t see any clearly compressional features even on the oldest terrains. We do see many extensional features, most of which are recent. The oldest tectonic features we see look to be extensional, but are hard to interpret because they are so eroded. Taken together we think this favors Pluto’s ocean already being present very early in Pluto’s history,” Bierson added.

Pluto’s hot past

Where could all this energy have come from? Pluto is too far from the sun to be of consequential difference, which leaves us with only two main possible sources of energy.

One is the heat released through the decay of radioactive elements in the rock, the other is gravitational energy released during impact with asteroids and other cosmic bodies. Early in the solar system’s history, it was quite common for planets to be bombarded by asteroids and meteorites.

Calculations performed by the researchers suggest that if all of that gravitational energy was retained as heat, Pluto could have supported an initial liquid ocean.

In reality, however, we know that is simply not possible — some of that energy will escape into space.

“How Pluto was put together in the first place matters a lot for its thermal evolution,” Nimmo said in a press release. “If it builds up too slowly, the hot material at the surface radiates energy into space, but if it builds up fast enough the heat gets trapped inside.”

The researchers found that if Pluto formed over a period of less than 30,000 years, then it would have most certainly been hot. If it took a couple million years for accretion of proto-Pluto to take place, then a hot start would only be possible if a large impactor buried its energy deep beneath the surface.

“If Pluto’s ocean was there early on it raises another question, “What was the heat source to form that ocean?” Today Pluto sits in an extremely cold part of the solar system. Its surface temperature is about 45 Kelvin (-480 Fahrenheit). When Pluto was forming new material would have been coming in and impacting its surface. Each impact is like an explosion that will warm the nearby area. If Pluto formed slowly, the surface would cool between each impact and generally stay very cold. If however Pluto formed quickly you have impact on top of impact and the surface doesn’t have time to cool. We calculate that if Pluto formed in less than 30000 years the heat from these impacts could have been sufficient to lead to an early ocean,” Bierson said.

There are a lot of assumptions and ‘ifs’ in this study, but if the findings are confirmed, other large objects in the Kuiper belt likely started out hot, possibly harboring oceans billions of years ago.

What’s intriguing is that these oceans could persist to this day in the largest objects in the belt, such as the dwarf planets Eris and Makemake, shielded under a blanket of thick ice.

“Pluto is the first Kuiper Belt object we visited and what we found was amazing. There is no reason to think that Pluto’s neighbors (Eris, Makemake, Haumea) are any less interesting. From this work, we suggest that they also should have formed with oceans, but we don’t know if they would have completely refrozen over solar system history. Up to now, we haven’t been able to see those worlds as more than points of light in the night sky. This is hard because the Kuiper belt is very very far away. It took New Horizons 9 years from launch to Pluto and it was one of the fastest spacecraft ever launched. Still, I am optimistic that with new telescopes and maybe a future mission we can keep unlocking their secrets,” Bierson concluded.

The findings were reported in the journal Nature Geoscience.

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