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News Notes
Planetary Geology
Core compositions

Scientists presume Earth and Mars formed at the same time and, based on studies of meteorites, probably from similar materials. Nonetheless, the planets developed remarkably different mantle compositions and core masses: Mars’ mantle contains 18 percent iron oxide — responsible for the well-known martian rust color at the surface — versus 8 percent in Earth’s mantle. And on Mars, the core accounts for only 18 percent of the planet’s mass versus Earth’s core, which is 28 percent. A new theory of planetary core formation could explain these differences.

Scientists are working to explain the differences in composition between Earth and Mars, shown here via the Mars rover Spirit. Courtesy of NASA/JPL/Cornell.

David C. Rubie and colleagues at the University of Bayreuth, Germany, propose in the May 6 Nature that interior pressures and temperatures related to a planet’s size strongly influence how iron behaves and, therefore, whether it will ultimately end up in the core or the mantle. Using high-temperature, high-pressure experiments, they confirmed that at extremely high temperatures, oxygen becomes more soluble in molten iron, thus converting iron oxide into oxygen-rich molten iron.

“For the first time, Rubie et al. successfully applied this effect to explain the difference in the composition and structure of Earth’s and Mars’ interiors,” says Jie Li, a mineralogist at the University of Illinois, Urbana-Champaign, who discovered a similar temperature effect in 2001. “It provides a unifying scheme to understand the accretion and differentiation of terrestrial planets.”

Early in both of the planets’ histories, scientists theorize, meteorite bombardment would have melted the surfaces entirely, forming deep magma oceans that separated into two types of magma: metal iron and silicate, which contained iron oxide-bearing minerals such as olivine and pyroxene. Trace elements in Earth’s mantle today suggest that its magma ocean could have been up to 1,200 kilometers deep, with temperatures reaching 3,500 degrees Kelvin and pressures of 50 gigapascals at the bottom.

Under these conditions of extreme pressure, and therefore extreme heat, the temperature effect described by Rubie and co-workers would cause iron oxide in the silicate magma to dissolve into molten metal iron. Droplets of oxygen-bearing molten iron would then sink through the less dense silicate magma, meld together into dense globs and sink even further through the solid mantle and into Earth’s core. The loss of iron from the mantle to the core would make the core more massive than expected, given the iron content of the parent material.

Mars, however, is only one-tenth as massive, and interior pressures reach only a third of those on Earth. A magma ocean there, even one of great depth, could not produce the pressure and therefore the heat needed for the temperature effect. Thus the proportions of iron metal and silicate iron oxide in the eventual mantle would remain closer to the original proportions found in meteorites — the conditions found on Mars today.

“Rubie and colleagues’ hypothesis is a neat explanation of how terrestrial planets of different sizes could end up with dramatically different core mass fractions and mantle iron-oxide compositions, despite being formed from the same primordial material,” writes Carl B. Agee, head of the Institute of Meteoritics at the University of New Mexico, in an accompanying Nature commentary.

Other researchers have put forth explanations for the differences that relied on the idea that the planets’ bulk compositions varied. For example, one hypothesis proposed that Earth might have had less oxygen than Mars. With less oxygen, less iron would stay in the mantle as iron oxide, and more unbound metallic iron would sink into the core.

Li says, however, that Rubie’s team was able to explain the differences between Earth’s and Mars’ cores without resorting to difference in their element composition. Nevertheless, given that much of what we know comes from only 25 martian meteorites, Agee cautions that the possibility still exists that Earth and Mars formed from different materials. But with new data from the Mars rovers, he says, “the prospect of a sample-return mission and further seismic measurements of the martian interior, our understanding of the red planet, and its similarities to Earth, can only improve.”

Sara Pratt
Geotimes contributing writer

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