Snowball Earths thaw faster with longer days
How long a frozen planet will take to defrost could depend on how long its days are
T here have been several periods in Earth’s history when the entire planet has frozen over. These so-called ‘Snowball Earth’ phases are triggered when the climate cools to such a degree that large ice sheets start extending far from the poles. The bright white surface of the ice reflects more sunlight back out into space than rock, causing the temperature to drop even further, until this literal snowball effect sees even the equator freezing over.
The planet would remain locked for evermore in this frosty state were it not for the saving grace of volcanism: the sealed surface experiences minimal erosion and so erupting carbon dioxide builds up in the atmosphere until the intensifying greenhouse effect suddenly lurches the planet back into a warmer, deglaciated state. It’s almost as if the planetary climate has two extreme states between which it can occasionally – but suddenly – switch, and this has had significant implications for the evolution of life on Earth.
But what about on Earth-like extrasolar planets? Recent modelling studies found that an exo-Earth which is tidally locked to its star, so that one side is always facing its sun, doesn’t experience these sudden switches into and out of snowball events; it shifts gradually between the two. Even a small increase in atmospheric carbon dioxide will cause the ice to thaw at the planet’s warmest point – the ‘subsolar point’ where the sun is permanently directly overhead – exposing the darker surface beneath, allowing the planet to absorb more sunlight. This paves the way for a smooth transition back to a deglaciated state.
The big question, though, is what happens for planets with a day length somewhere in between Earth’s rapid rotation and the very slow spin of tidally locked worlds? What about a planet that isn’t perfectly tidally locked, but still has a slow rotation period of, say, a hundred or more Earth days?
That’s exactly what Dorian Abbot and his colleagues at the University of Chicago have been using computer modelling of planetary climates to find out. They created a simple scenario of a planet without continents and an ocean of constant depth, as this allowed them to rapidly run huge numbers of different simulations. They found that exo-Earths rotating with a period of just a few tens of Earth days, but with similar levels of volcanic activity to us, would quickly be able to rescue themselves from a snowball state. The greater their day-length, the longer the period that the same patch of ice on the equator is heated by direct sunlight and so the less additional carbon dioxide is needed from other sources to initiate the deglaciation process. The Earth became stuck in its snowball phases for tens of millions of years, but such extrasolar planets could re-thaw themselves almost immediately.
Such snowball phases on Earth have been rare, last occurring 2.4 and 0.65 billion years ago. But what Abbot’s simulations show is another possible outcome for extrasolar planets. If an exo-Earth has long days but also reduced levels of volcanic activity with resultingly slow rates of carbon dioxide release, its climate would perpetually swing back and forth between snowball and deglaciated states. If you could watch such a world in fast forward you’d see it rhythmically flash all white.
LEWIS DARTNELL was reading… Decrease in hysteresis of planetary climate for planets with long solar days by Dorian S Abbot and colleagues Read it online at https://arxiv.org/abs/1801.10551
“Exo-Earths rotating with a period of just a few tens of Earth days would quickly rescue themselves from a snowball state”
Þ Slowly-rotating exoplanets may freeze and defrost with surprising regularity
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.the-knowledge.org)