An ocean of liquid water is believed to lie beneath Pluto's icy surface
Pluto's lack of an internal heat source should have frozen all liquid water below its surface. Researchers at Hokkaido University therefore asked themselves the question of what prevents water from freezing.
According to theories established in 2016, Pluto would present an ocean of liquid water under its mantle of ice, without having frozen. Researchers may have just figured out why.
Formed 4.6 billion years ago, Pluto is composed of a rocky silicate core surrounded by a thick layer of water ice about 300 km long. In 2015, thanks to images taken by the New Horizons probe, researchers spotted on its surface what is called the plain of Sputnik Planitia, a very deep basin filled with nitrogen ice, caused by a giant impact. From this impact resulted what is called a positive mass abnormality, overweight. Pluto was then placed so that this anomaly is opposite the common center of gravity of the Pluto-Charon system, the moon of Pluto. One of the theories explaining this mass anomaly is the presence of an ocean of liquid water under the surface of the dwarf planet, but the convection of the ice should have frozen everything a very long time ago.A study published in the journal Nature Geoscience by scientists from Hokkaido University (Japan) offers an answer to this intriguing anomaly.
2016, FIRST SPECULATIONS
It was in 2016 that Francis Nimmo , one of the authors of the study, theorized the presence of an ocean of liquid water under the mantle of Pluto to explain the positive anomaly detected at the level of the Sputnik Planitia basin. Indeed, how to explain an addition of mass after an impact? “During the impact that hollowed out the basin, an upwelling of liquid water could have caused a thinning of the ice crust. The local addition of liquid water can explain overweight,” explains François Forget, CNRS researcher at the Dynamic Meteorology Laboratory (LMD) .
“Everything indicated the presence of liquid water under the surface of Pluto” adds the researcher. Much denser than ice, it appears to be the most plausible explanation. A mystery remained however; why did this water not freeze by the effect of convection and conduction [thermal transfer where the hot moves upwards and the cold moves downwards] of the ice? On Pluto, there is no longer any source of heat to justify the liquid state of water. “There are very few internal sources.
Pluto, at its formation, was a very hot body; it is the result of an accretion of many materials whose each impact heated the whole. Some materials are also radioactive and emit heat. But there is no other internal source. This lack of an internal heat source causes even the dwarf planet to gradually cool; as the ocean becomes thinner and thinner over time, Pluto's surface expands and its ice sheet cracks. “Its surface swells like a muffin,” explains François Forget. "The Sputnik anomaly shows us, however, that there could be liquid water under the surface."
To explain the longevity of this ocean, researchers from Hokkaido University have proposed a response model: an insulating layer, a mixture of water and methane, would have formed between the surface ice and the ocean. “We expect that in a world so rich in organic molecules, especially methane, a sort of protective layer will form. This is called a Clatrate” specifies the researcher.
PLUTO, THE ONLY ONE IN HIS CASE?
Pluto is not alone in this case. In our Solar System, many other icy worlds contain an ocean of liquid water within them. This is the case for Europa, one of the moons of Jupiter, or Enceladus, moon of Saturn, whose layers of ice are constantly renewed. “On all the moons of giant planets, we actually think that there is an ocean under the surface,” adds François Forget.
“Unlike Pluto, the explanation for these bodies is more obvious; their internal heat source is related to gravitational tidal forces. These different moons disturb each other and their orbits are not very circular,” says the researcher. “The gravitational forces exerted by Jupiter, for example, therefore vary, which causes deformations. It's like taking a small spoon. If you twist it in all directions, it heats up. For these bodies, the same mechanism occurs; a deposit of gravitational energy in the form of heat is created inside, keeping the water in a liquid state.
For Ganymede, orbiting Jupiter, it was shown that there was indeed an ocean inside thanks to its magnetic field, the explanation of which is the ocean circulation of salt water.
“We could have demonstrated the presence of this ocean in the subsoil of Pluto, as for Ganymede. This is also one of the great regrets of the New Horizons team; there was no magnetometer on board the mission for reasons of space and mass. We could have directly observed the signature of a liquid ocean” concludes the researcher.
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