The Earth's magnetic field alerted scientists long ago to the fact our planet has iron at its core. A new experiment has shown, however, that at pressures and temperatures like those beneath the mantle, pure iron would be more dense than the outer core. The findings demonstrate that whatever other elements exist within the outer core, they lower on average its density, rather than raising it like in the solid inner core.

The way seismic waves from earthquakes pass through and bounce off the Earth's core has revealed the existence of an inner and outer core and allowed us to estimate the density of each. Unfortunately, drilling deep enough to collect samples remains completely unrealistic, so to learn more of the core's composition requires reconstructions in the lab.

"Recreating conditions found at the center of the Earth up here on the surface is not easy," Professor Yasuhiro Kuwayama of the University of Tokyo said in a statement, which also ranks as quite an understatement. "We used a diamond anvil to compress a sample of liquid iron subject to intense heat. But more than just creating the conditions, we needed to maintain them long enough to take our measurements. This was the real challenge."

Kuwayama succeeded, allowing the team to probe the sample with X-rays from Japan's Spring-8 synchrotron at pressures of 116 gigapascals (around a million atmospheres) and 4,450 Kelvin (three-quarters of the surface of the Sun). At these pressures, pure liquid iron has a density of 10 tons per cubic meter – 10 times water under room conditions. The speed with which shock waves travel through iron was measured to 45 Gpa and 2,700 K – 8 times previous record pressure measurements. The findings are reported in Physical Review Letters.

If the outer core was pure iron, then seismic waves would cross it around 4 percent more slowly than they do, Kuwayama found, indicating its density is 8 percent less than iron's under the same conditions. Although impurities were expected, the finding they tend to lower rather than raise the average density is significant.

Nickel is thought to also be abundant in the Earth's core but has a density slightly higher than iron in both liquid and solid form. We don't yet know what other elements are present in the core, but Kuwayama's work suggests that, in contrast to the inner core, the lighter ones must be abundant enough that their presence more than offsets that of nickel and all other denser elements.

"It's important to investigate these things to understand more, not only about the Earth's core, but about the composition, and thus behavior, of other planets as well," Kuwayama added. Mercury and Mars also have primarily iron cores, but can be expected to have some other elements mixed in.