Ask Ethan: How Can Worlds That Never Get Above Freezing Have…

Ask Ethan: How Can Worlds That Never Get Above Freezing Have…

Ask Ethan: How Can Worlds That Never Get Above Freezing Have Liquid Water?

“I was reading about Saturn’s moon Enceladus, and how scientists believe it has liquid water oceans beneath its water-ice crust. And yet I also read that the warmest surface temperatures are -90 celsius. How can this moon have liquid water? […] At such cold temperatures and low pressures it seems Enceladus can have water ice and water gas but not liquid. What am I missing?”

Here on Earth, water can easily exist in all three phases of matter: solid, liquid, and gas. The reason for this is simple: Earth has the right range of temperatures and pressures to experience not just the common solid and gas phases, but the liquid water phase, too. In the outer Solar System, worlds like Europa, Enceladus, and Pluto are too far from the Sun to ever reach surface temperatures high enough to create a liquid phase; it seems that water is a no-go. But there must be subsurface oceans on these worlds! Not only is there geological evidence of an ocean beneath a thick layer of ice, but on some worlds, like Enceladus, we can actually see large plumes of liquid water ejected hundreds of kilometers above the surface, like some sort of planet-scale geyser. While the increased pressure from the ice plays a role, it isn’t enough on its own; there must be other factors, too.

How do worlds that never get above freezing actually come to have liquid water on their surfaces? Find out on this week’s Ask Ethan!