Density deficit

In addition to the observed density deficits, there are indications for the presence of light elements in the core from cosmochemical and geochemical studies as summarized in Chapter 2.15. Based on these arguments, the most likely candidates for the light elements in the core include silicon, sulfur, and carbon. 

Presently EOS data are limited to crystalline phases at relative low pressures and/or low temperatures (Eigure 7). These data demonstrate that all the proposed light elements are capable of reducing the density of iron as expected. The efiftciency of density reduction (or the amount of light element needed to account for the core density deficits) depends on the stmcture and EOS of the alloys or compounds. Assuming that the... 

In comparison, ab initio calculations of the electronic stmcture of Fe-S and Fe-Si compounds by Sherman (1997) indicate that due to large excess volumes of mixing as little as 2-8 wt.% of sulfur is enough to account for the density deficit in the core. Alfe et al. (1999b) estimated that 9-11 wt.% oxygen is required to explain the density deficit. 

Sulfur is a prime candidate for the principal light element in the core. It has strong affinity for iron, reduces density and surface tension of iron, preferentially partitions into the liquid phase upon freezing, and dissolves into solid iron under high pressure and temperature. Until recently, the only strong objection for sulfur came from geochemical considerations (Dreibus and Palme, 1995 see Chapter 2.15). Theoretical studies indicate that the sulfur contents in liquid and solid iron under core pressure may be too similar to satisfy the density deficits in both reservoirs. This issue can be resolved by experimental studies in the near future. 

That iron is the dominant element in the core has been well established. Recent experimental and theoretical work has substantially expanded and refined our knowledge on the physics and chemistry of iron under the pressure-temperature regime that is relevant to the core. New data on high-pressure, high-temperature phase transitions and thermal EOS of iron promise to provide a better estimate on the density deficit in the core. 

Anderson O. L. and Isaak D. G. (2002) Another look at the core density deficit of Earth s outer core. Phys. Earth Planet. Inter. 131, 19-27. 

The solid inner core, which has a radius of 1,220 + 10 km (Masters and Shearer, 1995), represents 5% of the core s mass and <5% of its volume. It is estimated to have a slightly lower density than solid iron and, thus, it too would have a small amount of a light element component (Jephcoat and Olson, 1987). Birch (1952) may have recognized this when he said that it is a crystalline phase, mainly iron. Like the outer core, uncertainties in the amount of this light element component is a function of seismically derived density models for the inner core and identifying the appropriate temperature and pressure derivatives for the EOS of candidate materials. Hemley and Mao (2001) have provided an estimate of the density deficit of the inner core of 4-5%. 

An estimate of the density deficit in the core is —5-10% (Boehler, 2000 Anderson and Isaak, 2002) the uncertainty in this estimate is dominantly a function of uncertainties in the pressure and temperature derivatives of EOS data for candidate core materials and knowledge of the temperatures conditions in the core. A tighter constraint on this number will greatly help to refine chemical and petrological models of the core. A density deficit estimate for the inner core is 4-5% (Hemley and Mao, 2001). 

Geophysical evidence shows that the Earth has a liquid Fe-Ni-S-alloy outer core with a thickness of 2,260 km, and a solid Fe-Ni-alloy inner core, with a radius of 1,215 km. Temperatures at the top of the core are thought to be in the range 3,500-4,500 K and rise to 5,000-6,000 K at the center of the Earth. There is a mismatch of a few percent between the measured density of the Earth s outer core and that predicted for an iron-nickel alloy at high pressure. This "core density deficit," as it is called, is thought to indicate the presence of other elements as impurities in the core in addition to iron and nickel, and this could be the principal reason why the outer core is molten. The impurities act as a form of "antifreeze" in the liquid metal (Stevenson, 1990). 

Impurities in the Earth s core Birch (1952) demonstrated that the Earth s liquid outer core is 10% less dense than expected if it were made up of an iron-nickel alloy. For this reason one or more light elements are thought to be present in the core. Although a core density deficit of 10% has been widely used in many geochemical studies this value has recently been revised downwards by Anderson and Isaak (2002) and is now thought to be 5.4%. 

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