Computer simulations have provided compelling evidence that an insulating layer of gas hydrates could keep a subsurface ocean from freezing beneath Pluto’s icy exterior. Similar gas hydrate insulating layers could also be maintaining longlived subsurface oceans in other relatively large but minimally heated icy moons and distant celestial objects.
“This could mean there are more oceans in the universe than previously thought, making the existence of extraterrestrial life more plausible,” says Hokkaido University planetary scientist Shunichi Kamata.
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 an unevenly surfaced basin roughly the size of Texas. Named Sputnik Planitia, the whitecoloured, ellipsoidal basin is located near the equator.
Because of its location and topography, scientists believe a subsurface ocean exists beneath Sputnik Planitia’s ice shell. However, these observations are contradictory to the age of the dwarf planet: an underground ocean should have frozen a long time ago.
Researchers in Japan and the USA considered what could keep a subsurface ocean warm on Pluto while keeping the ice shell’s inner surface frozen and uneven. The team hypothesized that an insulating layer of gas hydrates exists beneath the surface of Sputnik Planitia. Gas hydrates are crystalline, ice-like solids formed of gas trapped within molecular water cages. They are highly viscous, have low thermal conductivity, and could act as an insulator.
The researchers conducted computer simulations covering a timescale starting 4.6 billion years ago, 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 not.
The simulations showed that, without a gas hydrate insulating layer, the subsurface sea would have frozen completely 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 crust to completely form over the ocean, but with a gas hydrate insulating layer, it takes more than one billion years. The simulation’s results support the possibility of a long-lived liquid ocean existing beneath the icy crust of Sputnik Planitia.
The team believes that methane from Pluto’s rocky core is the most likely gas present inside the hypothesized insulating layer. This theory, in which methane is trapped as a gas hydrate, is consistent with the unusual composition of Pluto’s methane-poor and nitrogen-rich atmosphere.
Kamata and his colleagues plan to continue looking for other subsurface oceans.
“The idea that our research suggests is not only new, but also universal,” Kamata says. “I would like to solve many more mysteries of the universe by applying our versatile idea to other celestial objects.”
Associate Professor Shunichi Kamata | E-mail: [email protected]
Graduate School of Science