Earth-like planets may be locked in winter
Even active volcanism may not be enough to prevent a world from falling into a deep freeze
For planets such as Earth, it’s thought that there’s a natural thermostat control on the global climate. This is known as the carbonate-silicate cycle, as it involves the relative erosion and production rates of carbonate and silicate rocks. As silicate rocks are weathered by rainwater they effectively suck carbon dioxide out of the atmosphere to form carbonates (such as limestone), which build up on the seafloor. The carbon would remain locked here, were it not for the grinding conveyor belt of plate tectonics; the carbonates are gradually dragged into the interior of the planet by subduction where they break down in the heat. The carbon dioxide is then belched back out into the atmosphere through erupting volcanoes, completing the cycle.
It’s this global thermostat that is believed to have saved our world several times from becoming locked in a ‘Snowball Earth’ state of global, thick ice-cover – most recently about 650 million years ago. Smothered in thick ice, the rate of weathering of silicate rocks drops, while volcanoes continue to erupt and so carbon dioxide builds up in the atmosphere until the greenhouse effect becomes so powerful it rapidly thaws the planet out again.
But many other Earth-like planets may not be quite so lucky, argues Martin Turbet and his colleagues. They used 3D global climate models of Earth-like planets to see if volcanoes could always save them from world-wide glaciation. The problem, they discovered, is that a terrestrial planet’s north and south poles can become so frigid that the carbon dioxide gas in its atmosphere begins to freeze. The poles serve as cold traps and as well as water ice caps, such planets also accumulate a thick layer of dry ice. This means that carbon dioxide is being pulled out of the atmosphere as quickly as volcanoes can pump it back in, so it can’t accumulate and the planet remains locked in the deep-freeze forevermore.
Turbet calculates that for a replica of Earth there would be no volcanic escape from permanent freeze-out for orbital distances greater than 1.27 AU. But he also realised that this critical limit could be much lower if certain features of the planet were slightly different to those on Earth – a faster rotation, less axial tilt or more reflective water ice on the surface, for example. The worst case that Turbet found is for an Earth-like planet that already has thick water ice caps at the poles. As heavier carbon dioxide ice begins to build up on top of the polar cap it sinks through and becomes buried by a cover of water ice. Here it remains trapped and volcanoes have little chance of ever accumulating enough of a greenhouse atmosphere to rescue the planet from global glaciation.
What this means is that even if our telescopes detect a seemingly Earth-like planet (in terms of its mass, orbital distance and so on) orbiting a Sun-like star, the story on the ground may be very different. The world may be locked in a Narnia-like eternal winter; its volcanoes unable to save it from this frozen fate.
“A terrestrial planet’s north and south poles can become so frigid that the carbon dioxide in its atmosphere begins to freeze”
From afar a planet can bear all the hallmarks of being like our own, but we’ve no idea what conditions are like on the ground
LEWIS DARTNELL is an astrobiology researcher at the University of Westminster and the author of The Knowledge: How to Rebuild our World from Scratch (www.theknowledge.org)