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Ultra high pressure laser experiments illuminate Super Earth cores

Using powerful laser beams, researchers have simulated conditions within a planet three times the size of the Earth.

Scientists have identified more than 2,000 of these "super-earths," exoplanets larger than Earth but smaller than Neptune, the planet's closest planet to our solar system. By examining how iron and silicon alloys react to extraordinary pressures, scientists gain new insights into the nature of super-Earths and their cores.

"We now have a technique that allows us direct access to the extreme pressures of the deep interiors of exoplanets and measure important properties," said Thomas Duffy, professor of Earth Sciences at Princeton.

"Previously, scientists were either limited to theoretical calculations or long extrapolations of low-pressure data.The ability to perform direct experiments allows us to test theoretical results and provide a much higher level of confidence in our models of how materials can handle these extreme conditions Behavior. "

The work leading to the highest X-ray diffraction data ever recorded was led by June Wicks when she worked as a research associate at Princeton with Duffy and colleagues at the Lawrence Livermore National Laboratory and the University of Rochester. Their findings were published in the journal Science Advances.

Because super-earths have no direct analogues in our own solar system, scientists want to learn more about their possible structures and compositions, thereby gaining insights into the types of planetary architectures that can exist in our galaxy. But they have two main limitations: we have no direct measurements of our own planetary nucleus from which we can extrapolate, and the internal pressure in super-earths can reach more than ten times the pressure in the center of the earth, far beyond the reach of conventional systems. 1

9659002] The pressures achieved in this study – up to 1,314 gigapascals (GPa) – are about three times higher than previous experiments, making them directly usable to model the internal structure of large, rocky exoplanets, Duffy said. 19659002] "Most high-pressure experiments use diamond anvil cells that rarely reach more than 300 GPa" or 3 million times the pressure at the surface, he said. The pressure in the earth core reaches up to 360 GPa.

"Our approach is new and many people in the community are not yet as familiar, but we have shown in this (and earlier) work that we can routinely achieve pressures in excess of 1,000 GPa or more (albeit for a fraction Our ability to combine this very high pressure with X-ray diffraction to obtain structural information provides us with a novel tool for exploring planetary interiors, "he said [19659002] The researchers compressed two samples for just a few billionths of a second, just long enough to examine the atomic structure with a pulse of bright X-rays. The resulting diffraction pattern provided information on the density and crystal structure of the iron-silicon alloys and showed that the higher silicon crystal structure changed.

"The method of simultaneous X-ray diffraction and shock experiments is still in its infancy" So it's exciting to see a "real use" for the core of the earth and beyond, "said Kanani Lee, an adjunct professor of geology and geophysics at Yale University, which was not involved in this research

a "very significant" contribution in the field of exoplanet research, said Diana Valencia, a pioneer in this field and assistant professor of physics at the University of Toronto-Scarborough, at

"This is a good study because we not only extrapolate from low pressures and hope for the best. This actually gives us the "best" that this data gives us, and it therefore limits our models better. "[19659002] Wicks and her colleagues directed a short but intense laser beam at two iron samples: an alloy containing 7% by weight of silicon, similar to the modeled composition of the Earth's core, and another containing 15% by weight of silicon, a possible composition in exoplanetary cores. 19659002] The nucleus of a planet exerts control over its magnetic field, thermal evolution, and mass-radius relationship, Duffy said.

"We know that the Earth's core is alloyed with about 10 percent of a lighter element, and silicon is one of the best candidates for this light element for both the Earth and extrasolar planets. "

The researchers found that the silicon alloy at ultra-high pressures organized their crystal structure into a hexagonal dense structure, while the higher silicon alloy used cubic body-centered packing." This atomic difference has enormous implications, "said Wicks, who is now assistant professor at the Johns Hopkins University is.

"Knowledge of crystal structure is the most basic information about the material that makes up the interior of a planet other physical and chemical properties follow from the crystal structure," she said.

Wicks and her colleagues also measured the density of the iron-silicon alloys over a range of pressures, they found that the iron-silicon alloys reach 17 to 18 grams per cubic centimeter at the highest pressures – about 2.5 times more dense than on the earth's surface and comparable to the density of gold or platinum on the earth's surface Also, with similar studies conducted on pure iron, they discovered that even under extreme pressures, the silicon alloys are less dense than unalloyed iron.

"A pure iron core is not realistic," said Duffy. The study of planetary formation will inevitably lead to the inclusion of significant amounts of lighter elements. Our study is the first to consider these more realistic core compositions. "

The researchers first calculated the density and pressure distribution in super-Earth, considering the presence of silicon in the core, and found that incorporation of silicon increases the modeled size of a planetary core, but reduces its central pressure. Future research will examine how other light elements, such as carbon or sulfur, affect the structure and density of iron under ultra high pressure conditions, and researchers also hope to measure other important physical properties of iron alloys to further constrain exoplanet interiors.

"For a geologist, the discovery of so many extrasolar planets has opened the door to a new field of exploration," Duffy said.

"We now realize that the types of planets that are out there are far beyond the limited ones Examples in our own solar system go out, and there is a lot of breadth res field of pressure, temperature and composition space that needs to be explored.

"Understanding the internal structure and composition of these large, rocky bodies is necessary to explore fundamental issues such as the possible existence of plate tectonics, magnetic field generation, their thermal evolution and even whether they are potentially habitable."

Research report: "Crystal Structure and Equation of State of Fe-Si Alloys in Super-Earth-Core Conditions"

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