The beating heart of our planet has remained a mystery for scientists searching for how Earth formed and what went into its creation. But a recent study was able to recreate the intense pressures approaching those found in the center of the Earth, giving researchers a glimpse into our planet's early days, and even what the core may look like now.
They announced their findings in a recent issue of the journal Science. “If we figure out which elements are in the core, we can better understand the conditions under which Earth formed, which will then inform us about early solar system history,” said lead study author Anat Shahar, a geochemist at the Carnegie Institution for Science in Washington, D.C. It could also give researchers a glimpse into how other rocky planets, both in our own solar system and beyond, came to be.
Earth formed some 4.6 billion years ago through countless collisions between rocky bodies ranging in size from Mars-sized objects down to asteroids. As the early Earth grew, its internal pressure and temperature also increased.
This had implications for how iron — which makes up most of Earth's core — interacted chemically with lighter elements such as hydrogen, oxygen and carbon as the heavier metal separated from the mantle and sank into the planet’s interior. The mantle is the layer right under Earth's crust, and the movement of molten rock through this region drives plate tectonics.
Scientists have long recognized that changing temperatures can influence the degree to which a version, or isotope, of an element such as iron becomes part of the core. This process is called isotope fractionation.
Before now, however, pressure wasn't considered a critical variable affecting this process. “In the '60s and '70s, experiments were conducted looking for these pressure effects and none were found,” Shahar said. “Now we know that the pressures they were testing at — about two gigapascals [GPa] — weren’t high enough.”
A 2009 paper by another team suggested that pressure could have influenced the elements that made it into our planet's core. So Shahar and her team decided to reinvestigate its effects, but using equipment that could achieve pressures of up to 40 GPa — much closer to the 60 GPa that scientists think was the average during Earth’s early core formation.
In experiments performed at the High Pressure Collaborative Access Team 16-ID-D x-ray beamline at the U.S. Department of Energy’s Advanced Photon Source, an Office of Science user facility at Argonne, the team placed small samples of iron mixed with hydrogen, carbon or oxygen between the points of two diamonds. The sides of this “diamond anvil cell” were then squeezed together to generate immense pressures.
Afterwards, the transformed iron samples were bombarded with high-powered -rays. “We use the x-rays to probe the vibrational properties of the iron phases,” Shahar said. The various vibration frequencies told her which versions of iron she had in her samples.
What the team found is that extreme pressure does affect isotope fractionation. In particular, the team discovered that reactions between iron and hydrogen or carbon—two elements considered to be present in the core—should have left behind a signature in mantle rocks. But that signature has never been found.
“Therefore, we don’t think that hydrogen and carbon are the main light elements in the core,” Shahar said.
In contrast, the combination of iron and oxygen wouldn't have left a trace behind in the mantle, according to the group’s experiments. So it's still possible that oxygen could be one of the lighter elements in Earth's core.
The findings support the hypothesis that oxygen and silicon make up the bulk of the light elements dissolved in the Earth’s core, says Joseph O’Rourke, a geophysicist at Caltech in Pasadena, California, who was not involved in the study.
“Oxygen and silicon are hugely abundant in the mantle, and we know they're soluble in iron at high temperature and pressures,” O’Rourke says. “Since oxygen and silicon are basically guaranteed to enter the core, there's not much room for other candidates like hydrogen and carbon.”
Shahar said her team plans to repeat their experiment with silicon and sulfur, other possible constituents of the core. Now that they’ve shown that pressure can affect fractionation, the group also plans to look at the effects of pressure and temperature together, which they predict will yield different results than either one alone. “Our experiments were all done with solid iron samples at room temperature. But during core formation, everything was melted,” Shahar said.
The findings from such experiments could have relevance for exoplanets, or planets beyond our own solar system, scientists say. “Because for exoplanets, you can only see their surfaces or atmospheres, “Shahar said. But how do their interiors affect what happens at the surface, she asked. "The answer to those questions will affect whether or not there's life on a planet.”
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See: A. Shahar1*, E. A. Schauble2, R. Caracas3, A.E. Gleason4, M.M. Reagan5, Y. Xiao1, J. Shu1, and W. Mao5, “Pressure-dependent isotopic composition of iron alloys,” Science 352(6285), 580 (29 APRIL 2016). DOI: 10.1126/science.aad9945
Author affiliations: 1Carnegie Institution for Science, 2University of California, Los Angeles, 3Ecole Normale Supérieure de Lyon, 4Los Alamos National Laboratory, 5Stanford University
Supported by a Stanford University Blaustein Fellowship, during which this project developed, and National Science Foundation (NSF) grant EAR1321858 (A.S.); NSF grant EAR1464008 (A.S. and W.M.); NSF grant EAR1530306 (E.A.S.); and CNRS PICS grant Carmelts and eDARI/CINES grant x2015106368 for computational resources (R.C.).