Pluto's hidden ocean: astronomers find liquid water in the confines of the Solar System

On July 14, 2015, the New Horizons spacecraft made a historic flight over Pluto, the space probe approached the mysterious dwarf planet like never before. What secrets did he reveal? Find out here!

Pluto seen from the New Horizons spacecraft in 2015
Pluto seen from the New Horizons spacecraft in 2015. This is the last and most detailed image sent to Earth before the spacecraft's maximum approach to Pluto on July 14. Credit: NASA

The fleeting encounter with Pluto by the New Horizons probe in the first and only overflight of the probe, left a lasting impression. The amazing images and volumes of data revealed a surprisingly vibrant and dynamic world.

In addition to the known characteristics, such as floating ice hills, nitrogen icebergs and nitrogen winds, the data hinted at something even more intriguing and misterious, because due to the distance it was not expected to exist of an ocean under the icy crust of Pluto.

This discovery effectively turned Pluto and his largest moon, Charon, into members of the select "Club of Oceanic Worlds." Almost a decade after that historic encounter, the astronomical community continues to unravel Pluto's secrets from the data collected by New Horizons.

In a recent article published in the specialized journal Icarus, planetologists Alex Nguyen and Dr. Patrick McGovern used mathematical models and images to explore the possible ocean between the icy surface of Pluto and its metal and silicate core.

A doctorate out of this world

Nguyen, graduate student in Earth, Environmental and Planetary Sciences at the University of Washington in St. Louis (WUSTL), and Dr. McGovern, a senior scientist at the Lunar and Planetary Institute (LPI) in Houston, published their research in the journal Icarus.

The study is part of Nguyen's doctorate at the University of Washington, where he is a member of the Olin Chancellor's Fellow and the postgraduate research program of the National Science Foundation, where he has worked with the data obtained from the NASA mission.

According to his analysis, Pluto's ocean is located under a surface layer that is between 40 and 80 km thick. This layer acts as an insulator, ensuring that the inland ocean remains in a liquid state.

For decades, planetary scientists assumed that Pluto was too cold to house an inland ocean. Orbiting far beyond the "Ice Line" of the Solar System, where volatile elements become solid, Pluton has an average surface temperature of -229 °C. Even nitrogen and methane solidify like rocks.

Clues to something mysterious

The high-resolution images, with a detail ten times greater than the previous ones, showed us the icy surface and the intriguing characteristics of Pluto. What mysteries did this distant world hide? What secrets did his frozen heart keep? The answer was only a few kilometers away.

Thanks to New Horizons, astronomers found multiple indications that Pluto could have an inland ocean, including cryovolcanans similar to those observed in other "oceanic worlds" such as Ceres, Europa Ganymedes, Encelatus, Titan and Triton.

layers of fog on Pluto's limbo
This image of the layers of fog on Pluto's limbo was taken by NASA's New Horizons spacecraft. About 20 layers of fog can be seen. Credit: NASA

Although the existence of this ocean is still debated, the theory gains acceptance. For their study, Nguyen and McGovern modeled the cracks and bumps in the ice that covers Pluto's Sputnik Planitia basin.

The results suggest that an ocean could exist under an icy layer 40 to 80 km thick, enough to keep it liquid despite the extreme temperature and pressure conditions on the surface.

The characteristics of this ocean are vital contributions for various types of models that are carried out to illuminate the interior structure and evolution of Pluto, especially in light of the complementary relationship between the dimensions of the ocean and an outer water ice sheet.

New horizons

In addition, they calculated the probable density or salinity of the ocean, concluding that it could be up to 8% denser than the terrestrial oceans. This level of salinity would equate the Pluto ocean with the Great Salt Lake and the Dead Sea on Earth.

Any variation in this density would be evident from the cracks and fractures in the Sputnik Platinum basin.

We estimate a kind of Goldilocks area where the density and thickness of the layer are correct.

If the ocean were less dense, the ice sheet would collapse, causing many more fractures on the surface and if it were denser, the ice sheet would be more sustained, which would be evident with fewer fractures.

Unfortunately, it could be many decades before another spacecraft reaches Pluto to help confirm these findings. Meanwhile, the defense of Pluto's inland ocean becomes stronger!