Snow­ball Earths thaw faster with longer days

How long a frozen planet will take to de­frost could de­pend on how long its days are

Sky at Night Magazine - - BULLETIN -

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)

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