By The Financial District
Jun 14, 20212 min
Earth’s core is losing heat faster under Indonesia than it is under Brazil, and that's messing with the seismic waves passing through it, Brandon Specktor reported for Live Science.
Scientists can only see it when they study the seismic waves (subterranean tremors generated by earthquakes) passing through the planet's solid iron inner core. For some reason, waves move through the core significantly faster when they're traveling between the north and south poles than when they're traveling across the equator.
Researchers have known about this discrepancy — known as seismic anisotropy — for decades, but have been unable to come up with an explanation that's consistent with the available data.
Now, using computer simulations of the core's growth over the last billion years, a new study in the June 3 issue of Nature Geoscience offers a solution that finally seems to fit: Every year, little by little, Earth's inner core is growing in a "lopsided" pattern, with new iron crystals forming faster on the east side of the core than on the west side.
"The movement of liquid iron in the outer core carries heat away from the inner core, causing it to freeze," lead study author Daniel Frost, a seismologist at the University of California, Berkeley, told Live Science. "So this means the outer core has been taking more heat from the east side [under Indonesia] than the west [under Brazil]."
The team's model proposes that Earth’s inner core grows faster on its east side (left) than on its west. Gravity equalizes the asymmetric growth by pushing iron crystals toward the north and south poles (arrows). This tends to align the long axis of iron crystals along the planet’s rotation axis (dashed line), explaining the different travel times for seismic waves through the inner core.
On average, the inner core's radius grows evenly by about 0.04 inches (1 millimeter) every year.
Gravity corrects for the lopsided growth in the east by pushing new crystals toward the west. There, the crystals clump into lattice structures that stretch along the core's north-south axis.
These crystal structures, aligned parallel with Earth's poles, are seismic superhighways that enable earthquake waves to travel more quickly in that direction, according to the team's models.