Computer simulations conducted by Japanese scientists provide convincing evidence for the existence of a liquid subsurface ocean on Pluto and similar worlds, and suggests that there are many more exoplanets and exomoons in the Universe, under the icy shells of which the environment can potentially live.
Our work showed that if there is an insulating layer of gas hydrates on Pluto between the subsurface ocean and the ice shell, it protects the ocean from freezing, and the water in it can remain liquid for billions of years.
In July 2015, the NASA spacecraft New Horizons made a historic flight through the Pluto system, providing the world’s first images of this distant dwarf planet and its satellites. In these photographs, Pluto appeared unexpectedly as a living world with interesting landscapes and surface features, among which the ellipsoidal snow-white Sputnik Planitia plain stands out, located near the equator and having a diameter of almost 1.5 kilometers.
Because of its location and topography, scientists believe that under the ice shell of Pluto, which is thinning in Sputnik Planitia, there is an underground ocean. However, these observations contradict the age of the dwarf planet, because any ocean on it had to freeze for a long time, and the inner surface of the ice shell, facing the ocean, evened out.
To resolve the inconsistency of models and observations, the researchers tried to find out what could keep warm in the subsurface ocean and not flatten the inner surface of the ice shell, leaving it frozen and uneven.
We hypothesized that there is an“ insulating layer ”of gas hydrates under the ice surface of Sputnik Planitia. Gas hydrates are crystalline, ice-like solids that are formed from water and gas under certain conditions. They have high viscosity, low thermal conductivity and can provide insulating properties.
In order to test their assumption, scientists conducted a computer simulation covering 4.6 billion years of the evolution of Pluto since the inception of the solar system, considering two scenarios: the first, where an insulating layer of gas hydrates existed between the ocean and the ice shell, and the second without it. The simulation revealed the thermal and structural evolution of the depths of the dwarf planet and the time required for freezing the hidden ocean and for the formation of a homogeneous thick ice shell.
Modeling showed that without an insulating layer the ocean would completely freeze hundreds of millions of years ago, while it would take only one million years to form a uniform thick ice crust. However, when the gas hydrates layer was included in the simulation, the ocean did not freeze at all, and a billion years were spent on the formation of an even layer.
-Shunichi Kamata, lead author of the study from Hokkaido University
The team believes that the most likely gas in the proposed insulating layer is methane, which originates from the stone core of Pluto. This theory, according to which methane is captured as a gas hydrate, is consistent with the unusual composition of the atmosphere of a dwarf planet, which is almost devoid of methane and rich in nitrogen.
Our results may mean that there are many more worlds with oceans in the Universe than previously thought. This makes the existence of extraterrestrial life more likely.
The conditions for liquid ocean existence on Pluto and similar worlds
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