11/2/2016 Earth science: Another energy source for the geodynamo : Nature : Nature Publishing Group http://www.nature.com/nature/journal/v529/n7586/full/529288a.html 1/3 nature.com Publications AZ index Browse by subject Access provided to Universite de Montreal by BibliothequesAcquisitions (Periodiques) Cart Login Register NATURE | NEWS & VIEWS Subject terms: Solid Earth sciences Biogeochemistry Geophysics Planetary science 日本語要約 Earth science: Another energy source for the geodynamo Bruce Buffett Nature 529, 288–289 (21 January 2016) doi:10.1038/529288a Published online 20 January 2016 Magnesium is not usually considered to be a constituent of Earth's core, but its presence there has now been proposed to explain an ongoing enigma — the identity of the energy sources that drive our planet's magnetic field. See Letter p.387 Turbulent flow in Earth's liquidiron core generates the planet's magnetic field through a process known as the geodynamo. This process is sustained by energy drawn from the core as it slowly cools 1 . Thermal convection is thought to be crucial, but revised estimates 2, 3 of thermal conductivity in liquid iron at high pressure have called into question the adequacy of the commonly cited energy sources 1 . On page 387 of this issue, O'Rourke and Stevenson 4 propose a solution to this energy crisis. They argue that if magnesium had dissolved in the liquid iron at high temperature when the core formed, then subsequent precipitation of magnesiumbearing minerals on cooling would be an important source of energy. The authors' theory warrants a serious reassessment of magneticfield generation in other rocky (terrestrial) planets. Sustaining a magnetic field is difficult for a terrestrial planet. Creeping flow of the planet's rocky shell (mantle) restricts heat loss from the underlying core. By comparison, the liquidiron core is an efficient thermal conductor. Thermal convection in the core ceases when heat flow into the mantle falls below the core's capacity to deliver this heat by conduction alone, so high thermal conductivity may push the threshold for thermal convection beyond reach. In this scenario, turbulent flow in the core is driven mainly by buoyancy effects due to variations in the abundance of core constituents — as the core cools, some of the iron solidifies and accumulates on the solid inner core, leaving lighter elements in the liquid outer core and thus causing convection 1 . A problem with this conventional view of the geodynamo's energy sources emerges when we extrapolate back in time. Before the inner core formed (possibly less than 1 billion years ago 1 ), the only energy source was thermal convection. But current estimates of iron's thermal conductivity suggest that the heat flow required to sustain such convection at that time was extremely high. Even if this heat flow was feasible, an implausibly high core temperature would be needed to sustain it over geological time. Despite these difficulties, Earth has somehow maintained a magnetic field for at least the past 3.4 billion years 5 . O'Rourke and Stevenson address this quandary by proposing a new energy source. They suggest that magnesium can enter the core to form an iron alloy, even though it is normally considered to be nearly insoluble in liquid iron. Other alloying elements are more commonly proposed 6 to explain why estimates of the core's density, based on seismic data, are less than that of pure iron. But theoretical predictions 7 and some experiments (see ref. 8, for example) suggest that magnesium can dissolve in liquid iron at sufficiently high temperatures. The authors argue that, because of its insolubility in iron, magnesium would probably become supersaturated as the core cools. The subsequent precipitation of magnesiumbearing minerals would leave behind a residual liquid enriched in iron, providing a compositional buoyancy that would drive fluid flow (Fig. 1). Figure 1: Possible processes at the boundary between Earth's liquid core and rocky mantle. Article News & Views Issue 7586 Volume 529 Archive