Pluto was warm with large underground ocean according to new study

Space Science

MYSTERY WIRE — It’s hard to imagine just how cold the temperature is on dwarf planet Pluto. It averages a temperature of negative 378 – 396 degrees Fahrenheit, which is well over negative 230 Celsius.

But new research is showing Pluto may not have always been this way. Using geologic observations of Pluto’s surface, the research suggests that Pluto actually started out in a hot formation scenario. Researchers modeled and compared hot versus cold formation scenarios and found that the surface features on Pluto best match hot.

In this scenario, the liquid ocean would slowly freeze over time, although not completely, and cause the extensional faults seen by New Horizons in Pluto’s icy crust.

Arrows mark the location of extensional faults on the surface of Pluto that indicate expansion of its crust, which scientists think is due to the freezing of a subsurface ocean. (Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker)

The researchers analyzed so-called “extensional features” on Pluto’s surface. Water expands as it freezes, so as Pluto’s interior cooled, Pluto’s surface stretched, generating recognizable structures.

“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. “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.”

The study was published Monday in the journal Nature Geoscience.

Researchers had long thought that Pluto began as an icy sphere when it formed in the Kuiper Belt, a home to cold, dark objects on the edge of our solar system.

Some scientists also believe there is a liquid ocean beneath Pluto’s ice shell that is estimated to be 249 miles thick. But they thought it formed later in Pluto’s history as radioactive elements were heated near the dwarf planet’s rocky core and decayed. This heat could have been enough to melt ice and form a subsurface ocean.

“The most exciting part of this research to me is that we can use the geology we observe on Pluto today to peer back in time and learn about what Pluto was like shortly after its formation,” said Carver Bierson, first study author and recent PhD graduate from the University of California, Santa Cruz.

Pluto, as seen by NASA’s New Horizons spacecraft during its historic flyby of the dwarf planet in July 2015. (Image: © NASA/JHUAPL/SwRI)

The long, slow freeze could also explain the mix of features on Pluto’s surface because expansion would occur throughout the history of the dwarf planet. “The oldest surface features on Pluto are harder to figure out, but it looks like there was both ancient and modern extension of the surface,” said Francis Nimmo, study co author and professor of Earth and planetary sciences at the University of California, Santa Cruz.

What led to Pluto forming in the first place is also being questioned, “As Pluto was forming, material was repeatedly coming in and impacting the surface,” Bierson said. “Each of those impacts is like an explosion that will warm the surface where it hit. If Pluto formed slowly the surface can cool off before the next impact. However if Pluto formed quickly you have impact on top of impact, repeatedly warming the surface until you get warm enough that an ocean can form. We calculate that Pluto would have had to form in less than about 30,000 years for this to form an ocean.”

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