Pluto’s subsurface ocean could possibly support primitive life
Scientists have been pondering questions about Pluto‘s iconic frozen ‘heart’ feature: Was it formed by an ancient impact; was it closer to the dwarf planet’s north pole, and does it have a subsurface ocean? If a subsurface ocean exists beneath the western lobe of the heart-shaped region, informally named Sputnik Planitia (originally Sputnik Planum), it could possibly host primitive or exotic microbial life.
These questions regarding the origin and influence of Sputnik Planitia are discussed in four separate articles published on December 1 in the journal Nature.
During its 2015 flyby of the Pluto system, New Horizons detected ammonia existing as a compound on Pluto’s large moon Charon and on one of its smaller moons. This observation raised the likelihood of it being present on Pluto as well, noted William McKinnon of Washington University in St. Louis, co-author of one of the papers.
Ammonia is likely to be present in Pluto’s subsurface ocean, where it acts as an anti-freeze and prevents the ocean from freezing. If enough ammonia is present in the liquid water that makes up the ocean, that water can stay in a fluid state or, at least, a syrupy form at temperatures as low as -145 °F (-93 °C), McKinnon said.
He described the likely cold and salty ocean beneath Pluto as “noxious”.
While life can tolerate high salt content, extreme cold, and extreme heat, most life forms cannot tolerate the amount of ammonia necessary to keep Pluto’s ocean liquid.
However, even on Earth, there are soil microorganisms that combine nitrogen with ammonia to create proteins and DNA, so a high level of ammonia does not necessarily preclude any life.
“It’s no place for germs, much less fish or squid, or any life as we know it. But as with the methane seas of Titan – Saturn’s main moon – it raises the question of whether some truly novel life forms could exist in these exotic, cold liquids,” he said.
“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.”
NASA Jet Propulsion Laboratory (JPL) geologist and astrobiologist Steven Vance thinks Pluto’s ocean probably contains alcohols such as methanol and ethanol, hydrocarbons such as methane and ethane, as well as molecules of carbon, nitrogen, hydrogen, and oxygen, all of which are present on its surface.
These chemicals could also function as an anti-freeze; plus, they make up the chemical basis for life and consumable energy.
If Pluto has a subsurface ocean, other dwarf planets in the Kuiper Belt may also have them and the accompanying potential of hosting primitive life.
In one of his currently published Nature papers, McKinnon proposes a 600-mile (966 km) wide, 50-mile (80 km) thick ocean beneath Sputnik Planitia influenced Pluto’s gravity and orientation.
Kuiper Belt Object impact
Many, though not all, scientists believe the original basin that includes Sputnik Planitia was created when a 125-mile (201 km) wide Kuiper Belt Object hit Pluto about four billion years ago.
The subsequent collapse of the giant crater created by the impact lifted up both the underground ocean and surface nitrogen that had accumulated in the crater, causing Pluto to tip over and change its orientation toward Charon.
Warm water ice at the base of the ice shell on top flowed much like glaciers do on Earth. If enough ammonia were present in the ocean water, that water could have remained slushy even at extremely low temperatures.
Surface ice that flowed from the underground ocean to the top would have frozen, however, resulting in the uplifting of the ocean becoming permanent.
Francis Nimmo, the lead author of one of the Nature articles, also thinks Sputnik Planitia originated with an impact, which resulted in a scenario like that described by McKinnon.
His theory is that the basin did not form in its current location but migrated there once Pluto’s rotation began to slow down.
“The migration happens because of extra mass beneath Sputnik Planitia. An impact will excavate ice at the surface, letting any water underneath it approach closer to the surface. Because water is denser than ice, it provides a source of extra mass to help drive Sputnik’s migration.”
Heat produced by radioactive decay of rock in Pluto’s interior can keep a subsurface ocean from freezing for billions of years, Nimmo noted.
The slow refreezing of an underground ocean would explain the fractured terrain that New Horizons imaged on Pluto’s surface.
Sputnik Planitia is unique in the Solar System, and its formation remains a mystery, emphasized New Horizons Principal Investigator Alan Stern.
All notions of a subsurface ocean are inferences rather than direct detections, McKinnon stressed. To directly determine that an ocean is present, a follow-up orbiter mission capable of gravity measurements and subsurface radar sounding is necessary.
Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.