Earth’s interior is cooling faster than expected

The Earth's core gives off heat to the mantle (or­ange to dark red),
which con­trib­utes to the slow cool­ing of the Earth
Source: ETH Zurich

Re­search­ers at ETH Zurich have demon­strated in the lab how well a min­eral com­mon at the bound­ary between the Earth’s core and mantle con­ducts heat. This leads them to sus­pect that the Earth’s heat may dis­sip­ate sooner than pre­vi­ously thought.

The evol­u­tion of our Earth is the story of its cool­ing: 4.5 bil­lion years ago, ex­treme tem­per­at­ures pre­vailed on the sur­face of the young Earth, and it was covered by a deep ocean of magma. Over mil­lions of years, the planet’s sur­face cooled to form a brittle crust. How­ever, the enorm­ous thermal en­ergy em­an­at­ing from the Earth’s in­terior set dy­namic pro­cesses in mo­tion, such as mantle con­vec­tion, plate tec­ton­ics and vol­can­ism.

Still un­answered, though, are the ques­tions of how fast the Earth cooled and how long it might take for this on­go­ing cool­ing to bring the afore­men­tioned heat-​driven pro­cesses to a halt.

One pos­sible an­swer may lie in the thermal con­duct­iv­ity of the min­er­als that form the bound­ary between the Earth’s core and mantle.

This bound­ary layer is rel­ev­ant be­cause it is here that the vis­cous rock of the Earth’s mantle is in dir­ect con­tact with the hot iron-​nickel melt of the planet’s outer core. The tem­per­at­ure gradi­ent between the two lay­ers is very steep, so there is po­ten­tially a lot of heat flow­ing here. The bound­ary layer is formed mainly of the min­eral bridg­man­ite. How­ever, re­search­ers have a hard time es­tim­at­ing how much heat this min­eral con­ducts from the Earth’s core to the mantle be­cause ex­per­i­mental veri­fic­a­tion is very dif­fi­cult.

Now, ETH Pro­fessor Mo­to­hiko Murakami and his col­leagues from Carne­gie In­sti­tu­tion for Sci­ence have de­veloped a soph­ist­ic­ated meas­ur­ing sys­tem that en­ables them to meas­ure the thermal con­duct­iv­ity of bridg­man­ite in the labor­at­ory, un­der the pres­sure and tem­per­at­ure con­di­tions that pre­vail in­side the Earth. For the meas­ure­ments, they used a re­cently de­veloped op­tical ab­sorp­tion meas­ure­ment sys­tem in a dia­mond unit heated with a pulsed laser.

Meas­ur­ing device for de­term­in­ing the thermal con­duct­iv­ity of bridg­man­ite un­der high pres­sure and ex­treme tem­per­at­ure.
from Murakami M, et al, 2021

“This meas­ure­ment sys­tem let us show that the thermal con­duct­iv­ity of bridg­man­ite is about 1.5 times higher than as­sumed,” Murakami says. This sug­gests that the heat flow from the core into the mantle is also higher than pre­vi­ously thought. Greater heat flow, in turn, in­creases mantle con­vec­tion and ac­cel­er­ates the cool­ing of the Earth. This may cause plate tec­ton­ics, which is kept go­ing by the con­vect­ive mo­tions of the mantle, to de­cel­er­ate faster than re­search­ers were ex­pect­ing based on pre­vi­ous heat con­duc­tion val­ues.

Murakami and his col­leagues have also shown that rapid cool­ing of the mantle will change the stable min­eral phases at the core-​mantle bound­ary. When it cools, bridg­man­ite turns into the min­eral post-​perovskite. But as soon as post-​perovskite ap­pears at the core-​mantle bound­ary and be­gins to dom­in­ate, the cool­ing of the mantle might in­deed ac­cel­er­ate even fur­ther, the re­search­ers es­tim­ate, since this min­eral con­ducts heat even more ef­fi­ciently than bridg­man­ite.

“Our res­ults could give us a new per­spect­ive on the evol­u­tion of the Earth’s dy­nam­ics. They sug­gest that Earth, like the other rocky plan­ets Mer­cury and Mars, is cool­ing and be­com­ing in­act­ive much faster than ex­pec­ted,” Murakami ex­plains.

How­ever, he can­not say how long it will take, for ex­ample, for con­vec­tion cur­rents in the mantle to stop. “We still don’t know enough about these kinds of events to pin down their tim­ing.” To do that calls first for a bet­ter un­der­stand­ing of how mantle con­vec­tion works in spa­tial and tem­poral terms. Moreover, sci­ent­ists need to cla­rify how the de­cay of ra­dio­act­ive ele­ments in the Earth’s in­terior – one of the main sources of heat – af­fects the dy­nam­ics of the mantle.

Ref­er­ence:
Murakami M, Gon­charov A, Miyajima N, Yamazaki D, Holt­grewe N.: Ra­di­at­ive thermal con­duct­iv­ity of single-​crystal bridg­man­ite at the core-​mantle bound­ary with im­plic­a­tions for thermal evol­u­tion of the Earth. Earth and Plan­et­ary Sci­ence Let­ters

Source/Credit: ETH Zurich / Peter Rüegg

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