Published online 3 April 2008 | Nature | doi:10.1038/news.2008.717


Conductive mineral could change day length

The electrical properties of a new-found mineral could help to explain our planet's wiggle.

As the world turns: do conductive materials in the mantle interfere with its spin?NASA

If a day seems to go by faster now than it did when you were younger, it might not just be your imagination. The speed of the Earth’s rotation is known to fluctuate slightly over decades. Now researchers have found a piece of the puzzle to explain this drift: a highly conductive mineral that could change the way the Earth spins.

A full day is almost never exactly 24 hours long. Early on in the Earth’s history, a day may once have been as short as five hours; tidal friction with the Moon has since made each successive day slightly longer. In any given year, earthquakes and seasonal ice melting change the Earth’s rotational speed by exerting subtle influences on how the planet’s mass is distributed. On the scale of decades, geophysicists see irregular millisecond-scale fluctuations in day length.

The exact cause of this decadal-scale variation is a bit of a mystery, but many have speculated that some process 2,900 kilometres down, at the boundary between the Earth’s core and mantle, may cause it. The Earth's core and mantle spin somewhat independently; models show that if the bottom of the mantle contains a conductive layer, it may interact with the magnetic field coming from the core, putting a wiggle in the Earth's spin that affects day length on this time frame.

But what could cause this conductance? A team led by Kei Hirose at the Tokyo Institute of Technology took a close look at a recently discovered mineral and say it could be at the root of the issue.

Under pressure

The team focused on post-perovskite, a high-pressure phase of a magnesium silicate mineral that was discovered in 2004 and is thought to exist in the mantle.

To investigate post-perovskite’s properties, the team recreated the conditions found at the bottom of the mantle, some 2,700 kilometres down, in the laboratory. They heated samples of post-perovskite to more than 2,700 °C and squeezed them to more than 1 million times surface pressure. “These are really challenging experiments,” says Raymond Jeanloz of the University of California, Berkeley.

The mineral is up to 100 times more conductive under these conditions than it is near the surface, the team reports in Science1.

The team estimates that a 300-kilometre-thick layer of post-perovskite would create an electromagnetic interaction "strong enough to account for millisecond-order level change in length of a day," says Hirose. Unfortunately no one yet knows if such a layer actually exists.

Alternative theories

The team also found the conductivity of post-perovskite was sensitive to the amount of iron present in the mineral. This sensitivity might explain geological measurements showing that the electrical conductivity of the mantle is higher at the bottom of the mantle underneath Africa and the Pacific than it is elsewhere. "People thought that the temperature is higher, but our data shows it's more likely that the chemical composition is different, more iron-rich," says Hirose.

But other structures might also be responsible for decadal-scale fluctuations in the length of day. A scant 200-meter layer of iron, for example, could also create the same sort of electromagnetic interaction as 300 kilometres of post-perovskite. "Iron is kind of the magic ingredient here," says Quentin Williams of the University of California, Santa Cruz. "We’re not exactly sure what the detailed iron content is at the core-mantle boundary."

Alternatively, it might not be down to interactions in electromagnetic fields at all: some propose that a rough boundary on the underside of the mantle might be what's interacting with the Earth’s liquid outer core, creating a periodic sloshing that changes the Earth’s rotation. 

  • References

    1. Ohta, K., et al. Science. 320, 89-91. ([year]2008[/year]).
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