The oceans of the outer solar system

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Icy Enceladus, just 504 kilometres across, hangs in the space between the spacecraft Cassini and the giant planet Saturn.

As new ideas emerge about the ocean beneath the icy surface of Saturn’s moon Enceladus, scientists say they believe there is also an ocean of water inside Pluto.

European researchers have proposed a new model for Enceladus, that suggests its ocean lies only a few kilometres below the moon’s ice crust at the south pole.

The team of scientists from the Laboratoire de Planétologie Géodynamique de Nantes, Charles University in Prague, and the Royal Observatory of Belgium proposed a model to reconcile conflicting data sets from the Cassini spacecraft.

Their model also suggests that there is a strong heat source in the interior of the moon, they say in the study published online in Geophysical Research Letters.

The existence of a heat source would support the possibility that there is life in Enceladus’s ocean.

At their first look at the when Cassini initially flew past Enceladus, scientists estimated that the thickness of its ice shell ranged from 30 to 40 kilometres at the south pole to 60 kilometres at the equator.

But that early data did not explain whether the ocean extended beneath the entire ice shell.

Then, in 2015, Cassini scientists observed an oscillation in Enceladus’s rotation linked to tidal effects, suggesting that the ocean was global and the ice crust much thinner than predicted – on average just 20 kilometres deep.

But even this thickness appeared to clash with other gravity and topography data.

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Image showing the thickness of Enceladus’s ice shell, which reaches 35 kilometres in the cratered equatorial regions (shown in yellow) and less than five kilometres in the active south polar region (shown in blue).
LPG-CNRS-U. Nantes/U. Charles, Prague.

Now the new model suggests Enceladus has a rocky core with a radius of 185 kilometres, and an internal ocean approximately 45 kilometres deep, isolated from the surface by an ice shell with a mean thickness of around 20 kilometres, except at the south pole where it is thought to be less than five kilometres.

The top 200 metres of the ice shell is elastic, under this model that also suggests the ocean beneath the ice makes up 40% of the total volume of the moon.

Its salt content is estimated to be similar to that of Earth’s oceans.

“All this implies a new energy budget for Enceladus,” the scientists write. “Since a thinner ice shell retains less heat, the tidal effects caused by Saturn on the large fractures in the ice at the south pole are no longer enough to explain the strong heat flow affecting this region.”

That suggests strong heat production in Enceladus’s deep interior that may power the hydrothermal vents on the ocean floor.

Meanwhile, another paper, also published in Geophysical Research Letters, says that geological activity on Pluto, observed during the fly-by by NASA’s New Horizons spacecraft, could be caused by the partial freezing of a subsurface ocean.

The ocean is likely to still exists today, the paper led by Noah Hammond of Brown University says.

“This lends support to the idea that oceans may be common among large Kuiper Belt objects, just as they are common among the satellites of the outer planets,” said co-author Amy Barr of the Planetary Science Institute.

“In our paper, we look at tectonic features on the surface of Pluto to understand the interior and we run thermal evolution models to help us understand how Pluto’s interior may have evolved over time,” Hammond said.

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A high-resolution colour view of one of two potential cryovolcanoes spotted on the surface of Pluto by the New Horizons spacecraft in July 2015.

The study supported the hypothesis by showing that if the ocean froze, a specific crystalline form of ice known as ice II would probably form, causing compressional tectonic features which are absent from the surface.

The formation of ice II would cause the dwarf planet to contract and compressional tectonic features to form on the surface.

“Since the tectonic features on Pluto’s surface are all extensional and there is no obvious compressional features, it suggests that ice II has not formed and that therefore, Pluto’s subsurface ocean has likely survived to present day.”

Ice II forms at high pressures and low temperatures and is a phase of ice that is 25% more dense than the ice on Earth, which floats on water.

“We have been waiting a long time to see the surface of Pluto, and it did not disappoint,” Barr said.

“Many people thought that Pluto would be geologically ‘dead,’ that it would be covered in craters and have an ancient surface.

“Our work shows how even Pluto, at the edge of the solar system, with very little energy, can have tectonics.”

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