A solar system's habitable zone is the region of space that is warm enough to allow water to exist in a liquid state on a planet. But new geological research suggests that, even if an exoplanet has water, it is doomed to be a dying world unless its rocky mantle is soft enough to allow the flow of heat.
Earth's mantle, which sits between the crust and the core, plays a big role in ensuring the presence of liquid water on our planet's surface. That's because the mantle can store hydrogen, a component of water, for extended periods of time and then reintroduce it into the Earth's oceans.
As water evaporates, radiation in the atmosphere can split it apart into hydrogen and oxygen. Hydrogen, the lightest element in the universe, then floats away into space. Without the hydrogen stored in the mantle, the Earth would have lost all its water long ago.
"The mantle kind of acts like a savings account for water," explains Ohio State University's Cayman Unterborn, who, with a group of colleagues, developed computer models to predict the structure of planets with different amounts of magnesium and silicon—which are crucial components of the Earth's mantle.
According to a report by the American Geophysical Union:
If [a planet's] mantle is too stiff, this water recycling can't happen. The mantle needs to be soft enough to allow heat inside the Earth to drive convection, which is ultimately responsible for the way pieces of the Earth's crust, called plates, move over time. It's the movement of these plates that traps hydrogen in mantle.
In the team's model, the amounts of magnesium and silicon affected the abundance of two minerals that make up much of the Earth's mantle and determine its stiffness. More magnesium led to more ferropericlase, a softer mineral, while more silicon led to more bridgmanite, which is stiffer. These results show that even exoplanets that could support liquid water by virtue of being the right distance from their own sun might not be habitable due to their composition.