A computer simulation of the earth’s magnetic field, which is generated by heat transfer in the earth’s core. Photo credit: NASA / Gary A. Glatzmaier
By creating conditions similar to the center of the earth in a laboratory chamber, researchers have improved the age estimate of the solid inner core of our planet to 1 to 1.3 billion years.
The results represent the nucleus at the younger end of an age spectrum that is typically between 1.3 and 4.5 billion years, but they also make it well older than a current estimate of just 565 million years.
In addition, the experiments and accompanying theories help pinpoint the amount of heat conduction of the core and the sources of energy that power the planet’s geodynamo – the mechanism that maintains the Earth’s magnetic field, pointing the compass north, and harming life Cosmic rays protect damage.
“People are very curious and excited to know about the origin of geodynamos and the strength of the magnetic field, as they all contribute to the habitability of a planet,” said Jung-Fu Lin, a professor at the University of Texas at Jackson School in Austin of Earth Sciences who led the research.
The results were published in the journal on August 13, 2020 Physical Examination Letters.
The earth’s core consists mainly of iron, with the inner core being solid and the outer being liquid. The effectiveness of iron in transferring heat by conduction – known as thermal conductivity – is key to determining a number of other properties of the core, including the formation of the inner core.
Over the years, estimates for core age and conductivity have increased from very old and relatively low to very young and relatively high. However, these more recent estimates have also created a paradox where the core would have had to reach unrealistically high temperatures to maintain geodynamo billions of years before the inner core was formed.
The new research solves this paradox by finding a solution that will keep the temperature of the core within realistic parameters. Finding this solution depended on directly measuring the conductivity of iron under nuclear-like conditions – where pressures are greater than 1 million atmospheres and temperatures can compete with those on the solar surface.
The researchers achieved these conditions by pressing laser-heated iron samples between two diamond anvils. It wasn’t an easy task. It took two years to get suitable results.
“We faced many problems and failed multiple times, which frustrated us and we almost gave up,” said Youjun Zhang, an associate professor at Sichuan University in China, said article co-author. “With the constructive comments and encouragement from Professor Jung-Fu Lin, we finally made it after several test runs.”
The newly measured conductivity is 30% to 50% below the conductivity of the young core estimate and suggests that the geodynamo was sustained by two different energy sources and mechanisms: thermal convection and composite convection. At first, the geodynamo was maintained solely by thermal convection. Now every mechanism plays an equally important role.
Lin said that with this improved information about conductivity and heat transfer over time, the researchers could make a more accurate estimate of the age of the inner core.
“Once you actually know how much of that heat flux is going from the outer core to the lower mantle, you can actually start thinking about when the earth has cooled down enough that the inner core starts to crystallize,” he said.
This revised age of the inner core could correlate with an increase in the strength of the Earth’s magnetic field recorded by the arrangement of magnetic materials in rocks that were formed around that time. Taken together, the evidence suggests that the formation of the inner core was an integral part of creating today’s robust magnetic fields.
Reference: “Reconciliation of experiments and theory on transport properties of iron and geodynamo” by Youjun Zhang, Mingqiang Hou, Guangtao Liu, Chengwei Zhang, Vitali B. Prakapenka, Eran Greenberg, Yingwei Fei, RE Cohen and Jung-Fu Lin, August 13, 2020 , Physical Examination Letters.
DOI: 10.1103 / PhysRevLett.125.078501
The National Science Foundation and the National Natural Science Foundation of China supported the research.
The research team also included Mingqiang Hou, Guangtao Liu, and Chengwei Zhang from the Center for High Pressure Research and Technology in Shanghai; Vitali Prakapenka and Eran Greenberg from the University of Chicago;; and Yingwei Fei and RE Cohen of the Carnegie Institution for Science.
