Planet or not, Pluto started out hot

Pluto appears to have formed quickly and with such heat that from birth it had a large ocean beneath a layer of insulating ice, scientists say.

Extensional faults (arrows) on Pluto’s surface indicate expansion of its icy crust, attributed to freezing of a subsurface ocean. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker

It’s a finding that not only alters our understanding of the most famous dwarf planet, but also adds to the growing scientific consensus that once planets began forming, they continued at speed – practically in the blink of a cosmic eye.

One of the most startling discoveries of NASA’s New Horizons spacecraft, which flew by Pluto in 2015, was that such an ocean still persists, even though Pluto averages about 40 times farther from the Sun than the Earth.

The leading theory had been that the ocean formed over the course of a billion or more years, as heat from the decay of long-lived radioisotopes in Pluto’s rocky core slowly melted its icy mantle.

Under this “cold-start” theory, Pluto would have started out cold and frozen, with the ocean gradually growing as its interior heat mounted. Then, as the radioisotopes that powered its internal furnace slowly gave out, it would have begun cooling, allowing the ocean to gradually start to refreeze – a process that is continuing to the present day.

If Pluto formed hot, however, the ocean would have formed as Pluto itself was forming, then steadily cooled and frozen.

The distinction is critical, says Francis Nimmo, a planetary scientist at the University of California, Santa Cruz, because water expands as it freezes and contracts as it melts.

Under the cold-start theory, Nimmo says, Pluto would therefore have initially contracted and wrinkled “like an apple left too long in the fridge”, as more and more ice melted. When the internal heat started to dissipate and the ocean began to re-freeze, the process would have reversed and it would have begun expanded, forming cracks on its surface, “like water pipes during a cold winter”.

If it formed hot, however, it would have steadily expanded through its entire history, as it then steadily cooled and more and more of the ocean froze. This would mean that there would be no wrinkles on even the oldest parts of its surface, only expansional cracks, Nimmo says.

To figure out which was the case, his team carefully reviewed New Horizons’ best images of Pluto’s surface, looking for signs of old wrinkles.

They found none. “We see terrains that look to be as old as the Solar System,” says Nimmo’s colleague Carver Bierson, also of UC Santa Cruz. “In those terrains we don’t see any evidence of compression [wrinkles].”

New Horizons captured this high-resolution enhanced colour view of Pluto, combining blue, red and infrared images, in 2015. Credit: NASA/JHUAPL/SwRI

“The oldest features on Pluto look more like cracks,” Nimmo says. “This suggests that Pluto had an ocean from the beginning and that the ocean has been slowly freezing.”

If so, the next question is how Pluto could have formed hot enough to have a subsurface ocean. “This is a cold place,” says Bierson. “The average surface temperature is 45 degrees K ( minus 228 degrees Celsius).”

The answer, he says, is that Pluto must have formed quite quickly, building heat from the impact energy of the smaller objects that collided to form it.

“Each impact is like an explosion that is going to heat up the nearby region,” he says. “If Pluto formed slowly, that [region] could cool between impacts. But if it formed quickly, you would have impact on top of impact on top of impact.”

For this to produce enough heat to form an ocean, he says, “we calculate that it would have to have formed in less than 30,000 years” – a lot less than the millions of years hypothesised by other planet-formation models.

It’s a finding, he and Nimmo add, that’s consistent with new models showing that planetesimals – the building blocks of planets – can form very quickly due to the sudden “collapse” of swarms of smaller particles.

How big these smaller particles may have been is unknown, he says. Perhaps there were a myriad of them the size of ping pong balls… or perhaps they were tens of kilometres in diameter. “We don’t know,” he says.

But ultimately, the size of the impactors doesn’t matter, so much as the rate at which they crashed into the growing world’s surface. “It’s all about how quickly you deliver the energy,” Bierson says.

The finding isn’t only relevant for Pluto. “It means that other large objects in the Kuiper Belt, like Eris [about the same size as Pluto] and Quaoar [about half as big] probably also formed fast and developed oceans early in their history,” Nimmo says.

And that’s a really stunning thought, because Quaoar never comes as close to the Sun as Pluto currently is, and Eris spends much of its time even further out, in a cold, dark realm in which, until recently, nobody had even dreamed there might be liquid water… and with it, the possibility of life.

Bierson’s and Nimmo’s work is published in the journal Nature Geoscience.

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