Washington, DC, 14 April 2015— The cores of terrestrial planets and satellite bodies, including our own Moon, all contain large quantities of iron. As a result, understanding the physical properties of iron at high temperatures, and under extreme pressures, is crucial to studying planetary interiors and interpreting the observed seismic data. The Moon is the only other terrestrial body besides Earth on which multiple seismic observations have been made--from the Apollo missions. New work from a team including the Geophysical Laboratory's Yingwei Fei provides new measurements of iron at lunar core conditions that help us to build a direct compositional and velocity model of the Moon’s core in conjunction with limited lunar seismic data.

The terrestrial planets all share a similar layered nature: a central metallic core, composed mostly of iron, which is surrounded by a silicate mantle, and then a thin, chemically differentiated crust.  However, differences in planetary masses mean that they will have different pressure and temperature conditions in their cores.

In the Earth’s core, iron is stable with a closely packed hexagon structure, called hcp iron (for hexagonal close-packed), under extreme pressure. As a result, most studies have focused on physical properties of hcp iron in order to understand the structure and composition of the core. However, the stable iron in the cores of smaller planets and satellites, such as Mars, Mercury, and the Moon, has a cubic structure called face-centered cubic, or fcc, instead of the hexagonal form found in Earth’s core. Lack of accurate sound velocity data for fcc iron prevent a reliable compositional and velocity model of the Moon’s core.

The team took measurements of the sound velocity through fcc iron under high pressures and temperatures, as well as its density. Their measurements ranged from no atmospheric pressure to about 188,000 times normal atmospheric pressure (0 to 19 gigapascals) and temperatures ranging from 80 to about 1,600 degrees Fahrenheit (300 to 1,150 kelvin). The data can be directly used to model the velocity and density profile of the Moon’s core and compared with lunar seismic observations.

“Our experimental measurements on relevant phase of iron will help to develop better models of the cores of these smaller bodies and planets and guide future planetary missions for which seismic study is planned,” Fei said.

Of particular interest, the new velocity model of the Moon’s core shows significantly higher compressional and shear sound velocities than previous estimates for the solid inner core. Their results also provide estimates of the inner core and outer core sizes that are consistent with the observed seismic travel times from the Apollo missions. The integration of mineral physics data and planetary seismic observation has proven to be a powerful way to interpret limited planetary seismic data and build comprehensive models of planetary cores.

This is a highly collaborative research project with expertise in mineral physics, planetary seismology, and experimental petrology. Fei has collaborated with French researchers including Daniele Antonangeli (lead author), Guillaume Morard, Guillaume Fiquet, and Michael Kirsch, and former Carnegie Fellows Nicholas Schmerr  of University of Maryland and Tetsuya Komabayashi of Tokyo Institute of Technology.

Their work is published by Proceedings of the National Academy of Sciences.


Michelle Scholtes, 14 April 2015

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