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both Hayakawa and Hillebrandt et al. 1O Earth sciences
have considered giant explosive events in
the centre. As mentioned above, Webber
et al. 2 have proposed an elegant solution,
based on the idea of associating the only
Composition of the inner core
two gamma-ray astronomy line measure- Raymond leanloz
ments from that general region of the sky.
According to these authors, the previous- THE inner core makes up only 0.7 per cent of the Earth.
ly observed 0.511-MeV line could come of the volume of the Earth. Despite its The compositional separation between
from the annihilation of positrons emitted small size, this region is very important to the inner core and outer core is also of
in the 26AI_26Mg decay, a process that is our understanding of the planetary in- interest to those modelling the dynamo
immediately followed by the emission of terior. In particular, current estimates of that produces the geomagnetic field. One
the 1.809-MeV photon bringing the 26Mg the temperature at the centre of the Earth; of the leading models for driving this
to its ground state. In this model a signifi- of the energy sources required to sustain dynamo is a compositionally induced con-
cant part at least of the observed 26AI the geomagnetic field; and of the core heat vection of the outer core, whereby denser
would not be created from continuous that is available to drive convection in the (more iron-rich) crystals freeze out of
diffuse nucleosynthesis in the disk, but overlying mantle all rely on models of the the less dense (more alloyed) outer-core
rather from the special environment in the chemical and physical state of the inner liquid. The crystals thus sink towards the
galactic centre. core. Thus, A. Jephcoat and P. Olson's re- inner core, and in doing so induce the
What all these results clearly call for are analysis elsewhere in this issue (Nature liquid motions that sustain the magnetic
better data, providing good spectral reso- 325, 332-335; 1987) of the geophysical field. According to Jephcoat and Olson,
lution as well as serious imaging for the constraints on the composition of the in-
gamma-ray sky. Cooled germanium de- ner core is of broad interest.
tectors already provide a spectral resolu- By comparing the seismologically de-
tion approaching 0.1 per cent around termined density of the inner core with the -7,600 K
1 Me V, as shown at least in part by the latest measurements of iron and iron-
sulphide densities at elevated pressures,
~
HEAO-3 experiments, and are currently
improving. Imaging in gamma-ray astron- Jephcoat and Olson conclude that some
omy is trickier, because of the physics of alloying constituent must be present in the
high-energy photon detection. True imag- solid inner core, as well as in the liquid
ing (at the arc minute level, a minimum outer core. This result is somewhat tenta-
requirement for astronomical work) is tive because seismological constraints on
possible with coded masks or 'multiple the inner-core density are less certain than Alloying
pinhole' collimators, but this has so far for other regions of the Earth. Also, the Eutectic constituents
never been coupled with the high-resolu- combined effects of temperature and
Iron Composition
tion spectroscopy possible with german- pressure on the experimental measure-
ium detectors. A mission combining the ments are still imperfectly known. Never-
two, and with good sensitivity in a wide theless, Jephcoat and Olson's conclusion Possible phase diagram for the Earth's core
range, is currently being considered by the is supported by the most recent shock- at a pressure of 330 GPa. IC, inner core;
European Space Agency under the name wave study on iron reported by J. M. OC, outer core.
of GRASP (Gamma-Ray Astronomy with Brown and R. G. McQueen (J. geophys.
Spectroscopy and Positioning) for launch Res. 91, 7485-7494; 1986). Although however, the compositional and hence den-
on whatever carrier may be available in Brown and McQueen suggest that the sity difference between inner- and outer-
the early-to-mid 1990s. inner core is pure iron, their analysis in core materials is substantially less than
Looking beyond nucleosynthesis, much fact shows that both the density and bulk previously thought. In the light of their
new astronomical data would be revealed sound velocity of iron are about 5 per cent results, they suggest that the dynamo is
to a mission looking for the first time at higher than those of the inner core, when sustained mainly by the alternative energy
gamma-ray sources with the same reso- compared at the same pressures and tem- source of radioactive decay. Models of
lution as the historical imaging pro- peratures (see Fig. 6 of their paper). radioactive heating lead to the prediction
portional counter on the Einstein Observ- A possible phase diagram for the core of considerably higher fluxes from the
atory: from the active galactic nucleus alloy at the pressure of the boundary core into the mantle than for the composi-
gamma-ray luminosity function, to new between the inner and outer core is shown tional dynamo. Hence, Jephcoat and
red shift measurements in the 0.511-MeV in the figure. The coexisting solid (inner Olson favour core heating as a significant
line, to a wealth of galactic problems such core) and liquid (outer core) compositions source of energy for mantle convection
as accurate mapping of the 26Alline. 0 are shown in accord with Jephcoat and and the resultant motion of crustal plates
Olson's estimate that about half the observed at the surface.
1. Mahoney, W.A., Ling, J.e., Wheaton, W.A. & Jacobson,
A.S. Astrophys 1. 286,578 (1984). alloying of iron is indicated by the inner- Regardless of whether one fully accepts
2. Share, G.H. etal. Astrophys. 1.292, L61 (1985). core density relative to that of the outer Jephcoat and Olson's conclusions, their
3. von Ballmoos, P., Diehl, R. & Shonfelder, V. Astrophysl.
(submitted).
core. If this analysis is correct, it will be timing is appropriate not only because it is
4. Webber, W.R., Schonfelder, V. & Diehl, R. Nature 233, possible to combine further refinements the 50th anniversary of the discovery of
692 (1986). of the seismological densities with high- the inner core, but also because very re-
5. Burbridge, E.M .• Burbridge, G.R., Fowler, W.A. &
Hoyle, F. Rev. Mod. Phys. 29,547 (1957). pressure measurements (now in progress) cent advances in seismology and in high-
6. Clayton, D.O. Astrophys. 1.280, 144 (1984). on the melting relations of iron-rich alloys pressure experimentation have led to a
7. Norgaard, H. Astrophys. 1.236,895 (1980).
8. Cameron, A.G.W. Icarus 60, 416 (1984). to identify the temperature at which the resurgence of studies on the deepest
9. Hayakawa, S. Prog. Theor. Phys. (in the press). inner core and outer core compositions region of our planet (see, for example, the
10. Hillebrandt, W., Mair, G. & Ziegert, W. Proc. 2nd lAP
Rencontu Nuc!. Astrophys. (in the press).
coexist. Because the inner core is suffi- December issue of Geophysical Research
ciently small to be essentially isothermal, Letters). 0
Giovanni F. Bignami is at the Istituto di Fisica
such a determination of the temperature Raymond feanloz is in the Department of Geo-
Cosmica - CNR, 15 Via Bassini, 20133 Milano, at the inner-core boundary is equivalent to logy and Geophysics, University of California,
Italy. measuring the temperature at the centre Berkeley, California 94720, USA.
