HIROSHIMA, Japan, Aug. 10, 2016 — A laser shock wave technique has been used to create pressures and temperatures comparable to the extreme collisions between objects in space in order to measure the shock response of forsterite, a major planetary material and the most abundant constituent of Earth’s mantle. Previous studies, performed without the laser shock technique, only measured the properties of forsterite at shock pressures below 200 Giga Pascals (GPa). The experiments using laser shock compression put forsterite crystals under pressures between approximately 250 and 970 GPa. The pressure at the center of Earth, in comparison, is estimated to be 360 GPa.
The use of the laser shock technique will potentially enable researchers to better understand the development of planets too distant for satellites to explore.
Researchers observed the melting of forsterite, the most
common constituent of Earth's mantle, to understand how the cores of planets
form and develop. The laser is able to create pressures representative of the
extreme collisions between objects in space. The target is a 4 millimeter
square. Al is aluminum and Qz is quartz. Courtesy of Toshimori Sekine,
In a collaborative endeavor, researchers at Hiroshima University, Osaka University, Ehime University, University of Tokyo and the Chiba Institute of Technology used a high-powered laser to irradiate a block of forsterite to measure its behavior under extreme conditions comparable to planetary impact events. They found that energy from the irradiation caused an abrupt expansion of the forsterite's molecules. This expansion generated a shock wave with enough heat and light to melt the forstertite in a manner comparable to the intense conditions that turn minerals into magma.
Researchers measured the pressure, temperature, density and reflectivity of the laser-shocked forsterite and reported a shock response of forstertite above about 250 GPa. They simultaneously measured the Hugoniot and temperature of shocked forsterite and interpreted the results to suggest incongruent crystallization of magnesium oxide (MgO) at 271 to 285 GPa; phase transition of MgO at 285 to 344 GPa; and remelting above about 470 to 500 GPa.
The research results may lead to increased knowledge of the interior processes of large rocky planets, how material is transformed by impact and how planetary systems are formed. With new details of forsterite’s melting behavior, researchers may be able to predict how minerals separate into different layers of magma and which minerals may be close enough to react.
"Our results provide a better understanding of how impact-generated magmas evolve and allow us to model Earth-type planets' inner structures. Collisions at these extreme temperatures and pressures created our own Earth and may have also formed the mantles of other Super Earth planets, for example CoRoT-7b and Kepler-10b," said Toshimori Sekine, professor at Hiroshima University.
The research was published in Science Advances (doi: 10.1126/sciadv.1600157)