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

Sky at Night Magazine - - BULLETIN - LEWIS DARTNELL was read­ing… CO2 con­den­sa­tion is a se­ri­ous limit to the deglacia­tion of Earth-like plan­ets by Martin Turbet et al. Read it on­line at https://arxiv.org/abs/1703.04624

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)

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