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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