The Solar System might be a soggier place than we previously thought – even in the glacially cold reaches of the Kuiper Belt. There, dwarf planet Pluto could be harbouring liquid oceans under a shell of nitrogen ice.
It was thought that the temperature required to maintain a liquid ocean on Pluto was too high for the thick ice to remain unmelted, but Japanese astronomers have found a new possibility: A layer of gas under the ice and above the liquid, insulating the two from each other, and allowing them to coexist.
This could help solve the gravity anomaly detected by the New Horizons probe in the form of the Sputnik Planitia, with its equatorial location and low topography, suggesting a liquid ocean underneath.
A liquid ocean underneath Sputnik Planitia could also explain tectonic features on the planet. And yet, based on Pluto’s age and location, scientists expected that all liquid should have frozen solid.
“To maintain an ocean, Pluto needs to retain heat inside. On the other hand, to maintain large variations in its thickness, Pluto’s ice shell needs to be cold,” the researchers wrote in their paper.
“Here we show … that the presence of a thin layer of clathrate hydrates (gas hydrates) at the base of the ice shell can explain both the long-term survival of the ocean and the maintenance of shell thickness contrasts.”
The team hypothesised that a gas hydrate layer – a solid, ice-like form of water with gas trapped in a lattice made of its molecules, also called a clathrate – was responsible. But obviously we can’t just nip over to Pluto to check things out, so they relied on computer simulations.
Starting 4.6 billion years ago, the age of the Solar System, the researchers simulated the evolution of Pluto, both with and without a gas hydrate layer between the icy shell that covers the Planitia, and the ocean beneath.
They modelled the dwarf planet’s thermal evolution, and calculated how long it would take for subsurface oceans to freeze and form a uniformly thick shell of ice.
They saw that, without the gas hydrate layer, the ocean – as expected – would have completely frozen some 800 million years ago, as Earth’s first animals were starting to emerge.
But that wasn’t the case in the simulation that included a gas hydrate. It showed that, when insulated by a layer of gas, the ocean barely froze. Where a uniformly thick ice layer only took about 100 million years to freeze without a gas hydrate layer, adding such a layer increased that time to over a billion years.
Even though the ice that covers the Sputnik Planitia is mostly nitrogen ice, the most likely candidate for that gas clathrate would be methane from Pluto’s rocky core – making that insulating layer a vast reserve of what we know on Earth as ‘flammable ice’.
“Clathrate hydrates act as a thermal insulator, preventing the ocean from completely freezing while keeping the ice shell cold and immobile,” the researchers wrote.
“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 simulation doesn’t just support the observations made by New Horizons and the possibility of a liquid ocean on Pluto.
It also demonstrates a way in which liquid oceans can exist even on the iciest of planets and planemos. The Universe could be a far soggier place than we thought possible.
The research has been published in Nature Geoscience.