New evidence for Pluto’s subsurface ocean

Large round space body, mostly tan with darker areas.
A natural color view of Pluto, as seen by NASA’s New Horizons spacecraft in 2015. New research adds to the evidence for a subsurface ocean beneath Pluto’s ice crust. Sputnik Planitia is the region of smoother-looking nitrogen ice in the middle right of the image. Image via NASA/JHUAPL/SwRI/Alex Parker.

At least several moons in the outer solar system are now known or suspected to have subsurface oceans beneath their cold, icy surfaces. Scientists also think that the dwarf planet Pluto may have one as well, based on data from the 2015 flyby of NASA’s New Horizons mission. How can this little, frozen, rocky ball much farther out than Neptune have an ocean?

Now, new research by scientists in Japan and the U.S. adds more evidence for this intriguing possibility. The findings were announced in a joint press release from Hokkaido University, Earth-Life Science Institute at Tokyo Institute of Technology, Tokushima University, Osaka University, Kobe University and the University of California, Santa Cruz. The researchers published their findings in the peer-reviewed journal Nature Geoscience on May 20, 2019.

In the new study, computer simulations suggest that a layer of gas hydrates (clathrate hydrates) – crystalline water-based solids physically resembling ice – between the ocean and the outer ice crust could keep the subsurface ocean insulated and liquid. Without that insulating layer, any liquid water would probably have frozen solid millions or billions of years ago. From the paper’s abstract:

Many icy solar system bodies possess subsurface oceans. On Pluto, Sputnik Planitia’s location near the equator suggests the presence of a subsurface ocean and a locally thinned ice shell. To maintain an ocean, Pluto needs to retain heat inside.

Triangular diagram of geological layers with liquid ocean halfway down.
The current proposed interior structure of Pluto. The clathrate (gas) hydrate layer – most likely methane – would help keep Pluto’s interior ocean liquid, while the outer ice crust remains frozen. Image via Kamata S. et al., Pluto’s ocean is capped and insulated by gas hydrates. Nature Geosciences, May 20, 2019.

Similar oceans on moons such as Europa and Enceladus are thought to be kept liquid by gravitational tugging from their host planets, Jupiter and Saturn. Ammonia and salts in the water itself may also help with this. But Pluto has no larger planet pulling on it, so there must be a different mechanism at work.

Data from New Horizons suggested that Pluto has an ocean below the outer ice crust, and that the ice shell is thinner in the region called Sputnik Planitia. But according to standard theory, the inner surface of that ice shell should have been flattened, and it isn’t. Sputnik Planitia is a vast region of smooth nitrogen ice glaciers unlike anything on Earth.

The researchers now theorize that an insulating layer of gas hydrates – most likely composed of methane – lies beneath Sputnik Planitia. This insulating layer helps to keep the water below warm enough to remain liquid, while the ice crust remains frozen. This layer would be composed of gas hydrates, crystalline ice-like solids formed from gas trapped within molecular water cages. These hydrates are highly viscous and have low thermal conductivity, so therefore could provide ideal insulating properties.

Icy flat areas divided by very thin lines and lines of tiny dark pits.
Sputnik Planitia in more detail. The smooth icy surface (nitrogen ice) is divided into irregularly shaped “cells” that are ringed by narrow troughs, some of which contain darker materials. There are also groups of mounds and fields of unusual small pits. The outer ice shell is thinner in this region than elsewhere on Pluto. Image via NASA/JHUAPL/SwRI.

From the abstract:

Clathrate hydrates act as a thermal insulator, preventing the ocean from completely freezing while keeping the ice shell cold and immobile. The most likely clathrate guest gas is methane, derived from precursor bodies and/or cracking of organic materials in the hot rocky core. Nitrogen molecules initially contained and/or produced later in the core would probably not be trapped as clathrate hydrates, instead supplying the nitrogen-rich surface and atmosphere. The formation of a thin clathrate hydrate layer cap to a subsurface ocean may be an important generic mechanism to maintain long-lived subsurface oceans in relatively large but minimally heated icy satellites and Kuiper Belt objects.

The researchers tested two scenarios, one with the insulating methane gas layer, and one without. With no such layer, the ocean should have frozen hundreds of millions of years ago. But with that added layer, the ocean would have remained liquid without having frozen much, if at all. In that scenario, it would take the ocean over one billion years to completely freeze.

Three Japanese scientists in suits, one holding a basketball-sized model of Pluto.
Left to right: researchers Atsushi Tani of Kobe University, and Shunichi Kamata and Kiyoshi Kuramoto of Hokkaido University. Image via Yu Kikuchi.

The methane most likely originates from Pluto’s core. Pluto’s very tenuous atmosphere, which contains abundant hydrogen but very little methane, also supports this conclusion, according to the researchers.

These new findings not only show how Pluto’s ocean could still be liquid today even in the most seemingly unlikely of circumstances, but also suggest that there could be many more similar worlds in the universe. It’s already thought that ocean moons like Europa and Enceladus, which orbit large planets, could be common. But if even a remote, cold dwarf planet like Pluto can have an ocean, then that could mean that many more such worlds exist in our galaxy and beyond. That, of course, could have implications for the possibility of life elsewhere. According to Shunichi Kamata of Hokkaido University, who led the research team:

This could mean there are more oceans in the universe than previously thought, making the existence of extraterrestrial life more plausible.

Whitish round space body with network of brown cracks.
Jupiter’s moon Europa is famous for its subsurface ocean, which could possibly support life. Now it seems that Pluto has one too. How many more might there be out there? Image via NASA/JPL-Caltech/SETI Institute.

New Horizons revealed Pluto to be a more complex and active world than had been generally anticipated, with nitrogen ice glaciers, water ice mountains, methane “snow” on the mountains and unusual pitted terrain within Sputnik Planitia. Now it seems that Pluto also probably has a water ocean deep down below the surface. It sounds absurd at first thought, but the evidence so far says it’s there. And if Pluto of all places can maintain an ocean, then how many more ocean planets and moons might there be out there? Perhaps many more than ever thought possible.

Bottom line: At its great distance from the sun, Pluto might be the last place you’d expect to find an ocean. But this new study adds to the growing evidence that Pluto really does have an ocean beneath its outer ice crust. The new findings show how, against expectations, Pluto’s ocean might remain liquid for hundreds of millions of years, and – according to these scientists – probably still is today.

Source: Pluto’s ocean is capped and insulated by gas hydrates

Via Hokkaido University

May 23, 2019

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