Electrified Deep Earth Changing Length of Day

Tiny shifts that make our days milliseconds longer may be due to forces under our feet, a new study has found.

It has long been known that natural phenomena on Earth's surface, such as tides and winds, affect its rotation speed. Now scientists are investigating how events in a mineral layer at the core-mantle boundary, 1,615 miles (2,600 kilometers) deep, similarly affect the planet's spin.

"The length of a day … is changing due to the interaction between the mantle and the core in the very deep Earth," said study co-author Kei Hirose, a geoscientist at the Tokyo Institute of Technology in Japan.

"This is basically because the bottom of the mantle has very high electrical conductivity."

(Related: "Earth's Core Spins Faster Than Surface, Study Confirms" [August 25, 2005].)

The research appears tomorrow in the journal Science.

Electric Deep Earth

Hirose and his colleagues simulated the physical properties of the deep mantle in their lab to learn more about how minerals in Earth's lower mantle behave.
They squeezed a mineral called post-perovskite between the points of two 0.2-carat diamonds under high pressure.

The researchers then heated the mineral sample with a laser to 4,900 degrees. Under these conditions, the mineral conducted electricity at high rates.

"This means that we have lots of electricity at the bottom of the mantle, which is coming from [Earth's] core," Hirose said. Raymond Jeanloz, an Earth and planetary scientist at the University of California, Berkeley, was not part of the study.

"What this means is that the magnetic field in the core can grab onto, or lock into, the lowermost mantle," he said.

"And so one of the influences that this can have is in altering the length of day, or the rotation rate of the Earth, depending on when and where the core is grabbing onto the mantle."

Not So Minuscule

This interaction accounts for several milliseconds of increase in day length over the past 150 years, co-author Hirose said.

Such minuscule time periods might seem negligible, but they do matter, he added.

Quentin Williams, an Earth and planetary scientist at University of California, Santa Cruz, agreed.

"We do care about Earth's rotation, because you really want to know, at any given time, where a spot on the surface of the Earth is relative to its orbit," he said.

"That's why agencies like NASA have cared a lot about the Earth's rotation over the years." - natgeo