Posted on May 21, 2019
New simulations of Pluto support the possibility of a long-lived liquid ocean existing beneath the icy crust of Sputnik Planitia located near the equator and roughly the size of Texas. The simulations showed that, without a gas hydrate insulating layer, the subsurface would have frozen hundreds of millions of years ago; But with one, it hardly freezes at all. Also, it takes about one million years for a uniformly thick ice to completely form over the ocean, but with a gas hydrate insulating layer, it takes more than one billion years.
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Researchers at Japan's Hokkaido University, the Tokyo Institute of Technology, Tokushima University, Osaka University, Kobe University, and at the University of California, Santa Cruz, consider what could keep the subsurface ocean warm while keeping the ice shell's inner surface frozen and uneven on Pluto. The team hypothesized that an "insulating layer" of gas hydrates exists beneath the icy surface of Sputnik Planitia.
View of Pluto is shown above with color-coded topography as measured by NASA's New Horizons spacecraft. Sputnik Planitia stands out at top as a broad, 1300 km (800 mile) wide, 2.5 km (1.5 mile) deep elliptical basin, most likely the site of ancient impact on Pluto. New Horizons data is deeply beneath this nitrogen-ice filled basin is an ocean of dense, salty, ammonia-rich water. (P.M. Schenk LPI / JHUAPL / SwRI / NASA)
The international team believes that the most likely gas within the hypothesized insulating layer is methane originating from Pluto's rocky core. This theory, in which methane is trapped as a gas hydrate, is consistent with the unusual composition of Pluto's atmosphere-methane-poor and nitrogen-rich.
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Similar gas hydrated insulating layers could be used for long-lived subsurface oceans in other relatively large but minimally heated icy moons and distant celestial objects, the researchers conclude. "There could be more oceans than previously thought, making the existence of extraterrestrial life more plausible," says Shunichi Kamata or Hokkaido University who led the team.
Computer simulations provide compelling evidence that an insulating layer of gas Hydrates could keep a subsurface ocean from freezing beneath Pluto's icy exterior, according to a study published in the journal Nature Geoscience.
In July 2015, NASA's New Horizons spacecraft flew through Pluto's system, providing the first ever close-up images of this distant dwarf planet and its moons. The images showed Pluto's unexpected topography, including a white-colored ellipsoidal basin named Sputnik Planitia.
Because of its location and topography, scientists believe a subsurface ocean exists beneath the ice shell which is thinned at Sputnik Planitia. However, these observations are contradictory to the age of the planet because the ocean should have frozen for a long time and the inner surface of the ice shell facing the ocean should also have been flattened.
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Gas hydrates are crystalline ice-like solids formed of gas trapped within molecular water cages. They are highly viscous, have low thermal conductivity, and therefore provide insulating properties.
The researchers conducted computer simulations covering a timescale of 4.6 billion years, when the solar system began to form. The simulations showed the thermal and structural evolution of Pluto's interior and the time required for a subsurface ocean to freeze and for the icy shell covering it to become uniformly thick. They simulated two scenarios: one where an insulating layer of gas hydrates existed between the ocean and the icy shell, and one where it did.
The Daily Galaxy via Hokkaido University