Pluto Likely Had Liquid Oceans After Quick and Violent Evolution

(NASA/New Horizons)

(CN) — New research suggests that as Pluto formed billions of years ago, a series of natural materials impacting on the planetary surface may have created liquid oceans beneath the dwarf planet’s icy crust — oceans that may still exist to this day.

Ever since astronomer Clyde Tombaugh discovered Pluto in the 1930s, scientists have never shied from rigorously exploring and debating the mysteries surrounding it. For decades researchers have sought to better understand the complex gasses that make up Pluto’s atmosphere, discover what may lie beneath its icy surface and have fiercely debated whether Pluto should even be called a planet at all. 

These research efforts intensified further when in 2015 the New Horizons spacecraft, first launched by NASA in 2006, made the first-ever flyby over Pluto’s surface — the only spacecraft to achieve such an up-close look at the dwarf planet.

Ever since the historic New Horizons mission, researchers have pored over the data and images captured by the spacecraft to help uncover new and crucial discoveries related to the dwarf planet’s structure and composition.

Now, research published Monday in Nature Geoscience proposes a theory that may represent another scientific breakthrough.

Researchers from the University of California, Santa Cruz, and Southwest Research Institute have presented a new scenario on Pluto’s earliest formation dubbed the “hot start” scenario. When Pluto began to form roughly 4.5 billion years ago, the planet’s surface was consistently bombarded with new large chunks of ice and rock that generated massive amounts of heat and warmed the impact areas.

While this early bombardment of Pluto is widely accepted, what remains unclear is how quickly or slowly Pluto formed as these materials struck the dwarf planet. Enter the hot start scenario.

Researchers say that while these materials hit Pluto’s surface, the formation of the planet may have happened so quickly — possible in as few as 30,000 years — that the dwarf planet never properly had the time to cool between each material impact. This process resulted in the melting of Pluto’s ice, creating a liquid ocean beneath its surface. 

This scenario stands in contrast with the “cold start” scenario which paints Pluto as a completely frozen sphere of ice.

To support the hot start scenario, the researchers note water reacts differently to freezing and thawing. Because water expands when it freezes and contracts when it melts, the manner in which Pluto experienced its earliest water and ice formations would leave the surface of the dwarf planet scarred.

Carver Bierson, first author of the study and UCSC graduate student, said that an understanding on how water reacts to freezing and thawing, coupled with the data from the New Horizons mission, helped researchers determine that a hot-start Pluto is the most likely.

“If it started cold and the ice melted internally, Pluto would have contracted and we should see compression features on its surface, whereas if it started hot it should have expanded as the ocean froze and we should see extension features on the surface,” Bierson said with the release of the study. “We see lots of evidence of expansion, but we don’t see any evidence of compression, so the observations are more consistent with Pluto starting with a liquid ocean.”

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

The likelihood of the hot start theory also ties to Pluto’s evolutionary timeline. According to researchers’ calculations, a hot start scenario for Pluto is most likely if the dwarf planet formed quickly. If it formed over the course of millions of years, however, then a hot start would only be possible if the objects that struck Pluto during its early years buried themselves so deeply that their energy continued to radiate underneath the planetary surface.

Researchers say that these possibilities are not exclusive to Pluto. Many of the dwarf planets close to Pluto, found in the fringes of our solar system in the stellar disk known as the Kuiper belt, likely formed in a way similar to Pluto.

Bierson said that exploring the Kuiper belt objects could potentially help us better understand how they unfolded throughout the ages. 

 “Pluto’s large neighbors (Eris, Makemake, Haumea) likely had a similar formation history and so may have also formed with oceans buried under ice sheets,” Bierson said in an email. “We don’t know if those oceans would still be around today or if they totally refroze. By going to those worlds, we could get a more complete understanding about how these icy worlds evolved and their unique stories.”

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