Solid inner, inner core may be relic of Earth’s earliest days

SAN FRANCISCO — Earth’s deepest realm may be billions of years older than previously thought. New simulations of the planet’s formation suggest that the innermost part of the inner core solidified shortly after Earth’s assembly, rather than roughly 3 billion years later alongside the rest of the inner core.

Though not all scientists are convinced, the new proposal offers insights into the early days of Earth and other rocky planets such as Mars, said George Helffrich, who presented the findings December 17 at the American Geophysical Union’s fall meeting.

“This early inner core might actually be one of the most ancient solid objects we have in the whole entire planet,” said Helffrich, a geophysicist at the Tokyo Institute of Technology.

Earth’s innards are divided into layers: a solid iron-rich inner core, a molten iron outer core, a gooey mantle and a rigid outer crust. Earthquake waves traveling through the planet ricochet off the boundaries between these layers, allowing scientists to glimpse the planet’s structure.

In 2002, scientists discovered that the 2,440-kilometer-wide inner core had layers of its own. The orientation of the iron in the core depends on what conditions were like when the iron solidified. Scientists can measure that orientation by seeing how quickly earthquake waves move at different angles: They move faster when they’re aligned with the iron’s orientation. Earthquake waves revealed that the center of the inner core, a region about 1,200 kilometers across, has a different orientation compared with that of the rest of the inner core. The cause of this off-kilter core became an unsolved mystery in the geophysics community.

Helffrich and Ramon Brasser, also a geophysicist at the Tokyo Institute of Technology, didn’t intend to address that mystery when they put together their simulation. They were instead studying how Earth’s early surface formed. The planet was initially a growing ball of molten rock. The team simulated large rocks that slammed into this early Earth roughly 4.5 billion years ago, adding to its mass. These rocks contributed the material that would become the innermost inner core: Metal from these impactors dropped into the planet’s interior, and energy released during this descent turned into heat. As the planet grew in size, metal from additional impactors also sunk deep into the planet. Because this metal had farther to sink, it got hotter than the old metal from those first rocks already in the core. This created a relatively cool center surrounded by hotter material.

Around 100,000 years after Earth’s accretion began, rising pressures inside the growing planet caused the cooler innermost core to solidify, the simulations showed. The work predicts that this core would be around 300 to 2,000 kilometers across, a relatively good fit for the actual size of the innermost inner core. More than 3 billion years later, the rest of the solid inner core grew around the ancient layer as heat dissipated (SN: 9/19/15, p. 18), the team proposes.

This process should apply to other rocky planets such as Mercury, Venus and Mars, Helffrich and Brasser suggest. NASA’s InSight rover, scheduled to depart for the Red Planet in March, may be able to use the planet’s seismicity to detect the presence of a similar innermost core inside Mars, Helffrich said.

While interesting, the simulations ignore important processes that shaped Earth’s evolution, says Peter Driscoll, a geophysicist at the Carnegie Institution for Science in Washington, D.C. Hot material surrounding the early Earth’s center should have kept temperatures hot enough to prevent solidification at the time, he said. Massive impactors such as the one that formed the moon could have also mixed Earth’s interior, redistributing the planet’s heat and removing the cold center. “I’m not sure [this explanation] works for the Earth,” he says.