“Using chemistry as a detective’s tool, we are able to trace certain features we see on Pluto today to formation processes from long ago. This leads to a new appreciation of the richness of Pluto’s ‘life story,’ which we are only starting to grasp,” said Christopher Glein with the Southwest Research Institute in 2018.
“When NASA’s New Horizons spacecraft zipped past Pluto in 2015,” reported Shannon Hall for Nature, “it showed a world that was much more dynamic than anyone had imagined. The dwarf planet hosts icy nitrogen cliffs that resemble the rugged coast of Norway, and giant shards of methane ice that soar to the height of skyscrapers. Cracks deeper than the Grand Canyon scar the surface, while icy volcanoes rise taller than Mount Everest. In one part of the distant orb, the spacecraft’s cameras captured a giant heart-shaped feature that caused a collective swoon among countless fans on Earth.”
That heart-shaped feature is the ice-shrouded impact basin, Sputnik Planitia, believed to harbor answers to Pluto’s formation and the possibility of a hidden ocean.
Prior to the heated debate that surfaced about renewing Pluto’s status following the decision of the International Astronomical Union (IAU) in August 2006 to downgrade the then-ninth planet, Southwest Research Institute scientists integrated NASA’s New Horizons discoveries with data from ESA’s Rosetta mission to develop a new theory about how Pluto may have formed at the edge of our solar system.
Formed by a billion comets or other Kuiper Belt objects
“We’ve developed what we call ‘the giant comet’ cosmochemical model of Pluto formation,” said Glein. At the heart of the research is the nitrogen-rich ice in Sputnik Planitia, a large glacier that forms the left lobe of the bright Tombaugh Regio feature on Pluto’s surface. “We found an intriguing consistency between the estimated amount of nitrogen inside the glacier and the amount that would be expected if Pluto was formed by the agglomeration of roughly a billion comets or other Kuiper Belt objects similar in chemical composition to 67P, the comet explored by Rosetta.”
“Sputnik Planitia is one of Pluto’s crown jewels, and understanding its origin is a puzzle,” said New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute, Boulder, Colorado. “Whatever caused Sputnik to form, nothing like it exists anywhere else in the solar system. Work to understand it will continue, but whatever that origin is, one thing is for certain—the exploration of Pluto has created new puzzles for 21st century planetary science.”
In addition to the comet model, scientists also investigated a solar model, with Pluto forming from very cold ices that would have had a chemical composition that more closely matches that of the Sun.
Scientists needed to understand not only the nitrogen present at Pluto now — in its atmosphere and in glaciers — but also how much of the volatile element potentially could have leaked out of the atmosphere and into space over the eons. They then needed to reconcile the proportion of carbon monoxide to nitrogen to get a more complete picture. Ultimately, the low abundance of carbon monoxide at Pluto points to burial in surface ices or to destruction from liquid water.
“Our research suggests that Pluto’s initial chemical makeup, inherited from cometary building blocks, was chemically modified by liquid water, perhaps even in a subsurface ocean,” Glein said. However, the solar model also satisfies some constraints. While the research pointed to some interesting possibilities, many questions remain to be answered.
“Life can tolerate a lot of stuff: It can tolerate a lot of salt, extreme cold, extreme heat”, says William McKinnon, professor of earth and planetary sciences at Washington University, “but I don’t think it can tolerate the amount of ammonia Pluto needs to prevent its ocean from freezing — ammonia is a superb antifreeze. Not that ammonia is all bad.”
Exotic, Pre-Cellular, Life?
On Earth, microorganisms in the soil fix nitrogen to ammonia, which is important for making DNA and proteins and such. The idea that bodies of Pluto’s scale, of which there are more than one out there in the Kuiper Belt, they could all have these kinds of oceans. But they’d be very exotic compared to what we think of as an ocean,” McKinnon adds.
“If you’re going to talk about life in an ocean that’s completely covered with an ice shell, it seems most likely that the best you could hope for is some extremely primitive kind of organism. It might even be pre-cellular, like we think the earliest life on Earth was.
Deep winding surface fractures that stretch hundreds of kilometers cover Pluto’s surface, likely due to tectonic activity, which is the result of global expansion, which is caused by a slowly freezing subsurface ocean, said scientists at Brown University.
Since NASA’s New Horizons spacecraft flew by Pluto in 2015, evidence has been mounting reinforcing that the dwarf planet may have a liquid ocean beneath its icy shell. By modeling the impact dynamics that created a massive crater on Pluto’s surface, the researchers at Brown University made a new estimate of how thick that liquid layer might be.
The study, led by geologist Brandon Johnson and published in Geophysical Research Letters, found a high likelihood that there’s more than 100 kilometers of liquid water beneath Pluto’s surface. The research also offers a clue about the composition of that ocean, suggesting that it likely has a salt content similar to that of the Dead Sea.
McKinnon, and a co-author of two of four Pluto studies published in Nature, also argue that beneath the heart-shaped Sputnik Planitia there lies an ocean laden with ammonia. The presence of the pungent, colorless liquid helps to explain not only Pluto’s orientation in space but also the persistence of the massive, ice-capped ocean that other researchers call “slushy” — but McKinnon prefers to depict as syrupy.
Using computer models along with topographical and compositional data culled from the New Horizon spacecraft’s July 2015 flyby of Pluto, McKinnon led a 2016 study on Sputnik Planitia’s churning nitrogen ice surface that appeared in Nature. He is also an author on the study regarding the orientation and gravity of Pluto caused by this subsurface ocean some 600 miles wide and more than 50 miles thick.
A Noxious Ocean
“In fact, New Horizons has detected ammonia as a compound on Pluto’s big moon, Charon, and on one of Pluto’s small moons. So it’s almost certainly inside Pluto,” McKinnon said. “What I think is down there in the ocean is rather noxious, very cold, salty and very ammonia-rich — almost a syrup.
“It’s no place for germs, much less fish or squid, or any life as we know it,” he added. “But as with the methane seas on Titan — Saturn’s main moon — it raises the question of whether some truly novel life forms could exist in these exotic, cold liquids.”
As humankind explores deeper into the Kuiper Belt and farther from Earth, this means to McKinnon the possible discovery of more such subsurface seas and more potential for exotic life.
The research delves into the creation — likely by a 125-mile-wide Kuiper Belt object striking Pluto more than 4 billion years ago — of the basin that includes Sputnik Planitia.
The collapse of the huge crater lifts Pluto’s subsurface ocean, and the dense water — combined with dense surface nitrogen ice that fills in the hole — forms a huge mass excess that causes Pluto to tip over, reorienting itself with respect to its big moon.
Chilled Pancake Syrup
But the ocean uplift won’t last if warm water ice at the base of the covering ice shell can flow and adjust in the manner of glaciers on Earth. Add enough ammonia to the water, and it can chill to incredibly cold temperatures (down to minus 145 Fahrenheit) and still be liquid, even if quite viscous, like chilled pancake syrup. At these temperatures, water ice is rigid, and the uplifted surface ocean becomes permanent.
“All of these ideas about an ocean inside Pluto are credible, but they are inferences, not direct detections,” McKinnon said. “If we want to confirm that such an ocean exists, we will need gravity measurements or subsurface radar sounding, all of which could be accomplished by a future orbiter mission to Pluto. It’s up to the next generation to pick up where New Horizons left off!”