Icy moons may not be habitable after Sun becomes red giant

The simulation suggests these worlds will go directly from melting to runaway greenhouse effects.
By Laurel Kornfeld | Aug 02, 2017
Jupiter's moon Europa and Saturn's moon Enceladus, both icy worlds with subsurface oceans, may not be habitable refugees for humanity once the Sun expands into a red giant several billion years from now.

Sun-like stars evolve into red giants at the end of their lives when they start running out of hydrogen in their cores. When this happens to our Sun, it will expand and engulf the inner planets, possibly including Earth and Mars.

Some have suggested that if humans are still around at that time, they could flee to worlds such as Europa and Enceladus, where the expanding Sun will melt surface ice, making those worlds habitable for life as we know it.

However, a recent study by scientists using global 3D climate models suggests icy planets and moons orbiting F- and G-type Sun-like stars might go directly from their current frozen state to a runaway greenhouse effect when their parent star becomes a red giant, completely skipping a habitable period.

According to the simulation, melting the surfaces of icy worlds will require an extremely high amount of energy, but once the melting happens, a runaway greenhouse effect will quickly take over, causing the oceans to completely evaporate.

Ramses Ramirez, a research assistant at Cornell University's Carl Sagan Institute and co-author of a study on the simulation published in Nature Geoscience, emphasized this series of events will occur with F- and G-type stars, which are as bright or slightly brighter than the Sun.

Icy worlds orbiting K- and M-type stars, which are cooler and dimmer, may become habitable and remain so for significant lengths of time once their surface ice melts, making them potential havens for future humans.

"If you had Europa and Enceladus analogs around those stars, they'd probably be safe," Ramirez said.

Whether the model used actually reflects future events in our solar system remains unclear due to the complicated processes involved in planets' transitions between cold and heat, stated Andrew P. Ingersoll of Caltech's Division of Geological and Planetary Sciences.

"Geological evidence shows that early Earth had both warm episodes and snowball episodes. And early Mars once had liquid water flowing on its surface even though Mars today is dry and frozen. The lesson for the search for habitable worlds beyond our Solar System is that our models, based on Earth experience, still have large uncertainties," Ingersoll wrote in an article about the study published in Nature News & Views.

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