RESEARCH NEWS & VIEWS
describing the elasticity of a crystal of arbitrarily low structural symmetry. Introduction of
this technique to the Earth sciences allowed
the single-crystal elasticity of high-pressure
silicate minerals to be determined for the first
time — from Brillouin spectra measured on
100-micrometre-sized crystals3.
Brillouin spectroscopy is also ideally suited
to measurements of microcrystals confined at
very high pressures within a diamond-anvil
high-pressure apparatus4,5. Murakami and
colleagues’ application of the technique to
polycrystalline samples of minerals with
strongly direction-dependent (anisotropic)
elasticity, under conditions of extreme pressure
and high temperature, breaks new ground.
Analyses of the elasticity, chemical composition and temperature of the lower mantle date
from the classic work of Birch6, who boldly
suggested that “dense high-pressure modifications of the ferro-magnesian silicates, probably close-packed oxides … are required to
explain the high elasticity of the deeper part
of the mantle”. This hypothesis was confirmed
by subsequent demonstration of the transformation of upper-mantle silicates first to spinel,
garnet and related crystal structures7 at pressures corresponding to mantle depths of 250–
500 kilometres, and, ultimately — at depths
greater than 660 kilometres — to a mixture of
silicate perovskites based on SiO6 octahedral
building blocks, and coexisting ferro­periclase8.
However, the question of whether or not there
is also a change in chemical composition
between the upper and lower mantle has not,
until now, been answered definitively.
Increasingly detailed knowledge of the
pressure–temperature conditions required
for the stability of the relevant minerals, and
of their densities and elastic properties, has
provided the basis for numerous analyses of
the elasticity, composition and temperature of
the lower mantle. The result has been a series
of conflicting claims over whether a contrast
in chemical composition between the upper
and lower mantle is required to match the
seismological models. Diverse inferences9–12
have been drawn concerning the need for
lower-mantle enrichment or depletion in silica, iron oxide and other components, and/or
for substantial departures from the adiabatic
temperature–depth gradient —
­­ expected to
develop in a fluid subject to thermal convection without exchange of heat between ascending and descending parcels. Such contrasting
conclusions are attributable to differences in
methodology, trade-offs between chemical
composition and temperature, and to residual
uncertainties in the thermo-physical database
for the high-pressure minerals.
It is gradually becoming accepted that an
internally consistent framework13 should be
used for the evaluation and assimilation of
thermoelastic data from laboratory experiments and computer models of mineral
behaviour, and for extrapolation to higher
pressures and temperatures. Application of
such a framework to diverse experimental data
for periclase (MgO) highlighted and resolved14
minor tensions between different data sets.
The result was a robust compromise model of
the material’s thermoelastic behaviour, including a well-constrained value for the pressure
derivative — at zero pressure — of the shear
modulus, or rigidity, from which the shearwave speed is calculated.
A markedly lower value (by about 20%)
of the zero-pressure pressure derivative was
deduced by Murakami and colleagues from
ultra-high-pressure Brillouin spectroscopy
on polycrystalline MgO (ref. 15). Possible
explanations for this discrepancy include a
systematic error in pressure calibration in the
ultra-high-pressure diamond-anvil experiments and uncertainty concerning the average wave speed that is determined by Brillouin
scattering of incident light by the individual
crystallites within a polycrystalline specimen.
Given the systematically low values of the
pressure derivatives of shear modulus obtained
from all of the recent ultra-high-pressure
Brillouin spectroscopic measurements, and
the consequences for inferred lower-mantle
composition, it is vital that the technical issues
surrounding pressure calibration and Brillouin
scattering from polycrystalline material
— ideally of both shear and compressional
waves — be resolved. More precise ab initio calculations of elastic properties and laboratory
measurements independent of an empirical
pressure scale16 may help to explain and to
eliminate the discrepancy.
Notwithstanding the impressive experiments of Murakami et al., we are probably still
awaiting the final word on the chemical composition and thermal regime of Earth’s lower
mantle. ■
Ian Jackson is at the Research School of Earth
Sciences, Australian National University,
Canberra, ACT 0200, Australia.
e-mail: [email protected]
1. Murakami, M., Ohishi, Y., Hirao, N. & Hirose, K.
Nature 485, 90–94 (2012).
2. Dziewonski, A. M. & Anderson, D. L. Phys. Earth
Planet. Inter. 25, 297–356 (1981).
3. Sawamoto, H., Weidner, D. J., Sasaki, S. &
Kumazawa, M. Science 224, 749–751 (1984).
4. Duffy, T. S., Zha, C. S., Downs, R. T., Mao, H. K. &
Hemley, R. J. Nature 378, 170–173 (1995).
5. Sinogeikin, S. V & Bass, J. D. Phys. Earth Planet. Inter.
120, 43–62 (2000).
6. Birch, F. J. Geophys. Res. 57, 227–286 (1952).
7. Ringwood, A. E. & Major, A. Phys. Earth Planet. Inter.
3, 89–108 (1970).
8. Liu, L. Earth Planet. Sci. Lett. 31, 200–208 (1976).
9. Li, B. & Zhang, J. Phys. Earth Planet. Inter. 151,
143–154 (2005).
10.Khan, A., Connolly, J. A. D. & Taylor, S. R. J. Geophys.
Res. 113, B09308 (2008).
11.Matas, J., Bass, J., Ricard, Y., Mattern, E. &
Bukowinski, M. S. T. Geophys. J. Int. 170, 764–780
(2007).
12.Cobden, L. et al. J. Geophys. Res. 114, B11309
(2009).
13.Stixrude, L. & Lithgow-Bertelloni, C. Geophys. J. Int.
162, 610–632 (2005).
14.Kennett, B. L. N. & Jackson, I. Phys. Earth Planet.
Inter. 176, 98–108 (2009).
15.Murakami, M., Ohishi, Y., Hirao, N. & Hirose, K. Earth
Planet. Sci. Lett. 277, 123–129 (2009).
16.Li, B., Woody, K. & Kung, J. J. Geophys. Res. 111,
B11206 (2006).
PL A N E TA RY S CI E N CE
Mercury’s mysteries
start to unfold
The origin of the planet Mercury has been a continuing puzzle. Data from NASA’s
MESSENGER space probe, combined with ground-based observations, are
delivering information on the planet’s structure and evolution.
D AV I D J . S T E V E N S O N
W
riting in Science, Smith et al.1 and
Zuber et al.2 report analyses of data
obtained by NASA’s MESSENGER
space mission that advance our understanding
of Mercury, the innermost of the eight planets in the Solar System. Smith and colleagues
determine Mercury’s gravity and internal
structure, whereas Zuber and co-workers
map the topography of the planet’s northern
hemisphere. Taken together with earlier results
on Mercury’s magnetic field3 and variable
spin4, the new studies suggest that the planet’s
structure and evolution are unlike those of any
other terrestrial planet or moon in the Solar
System.
Because Mercury lies deep within the Sun’s
gravity potential well, it is hard to get to, and
even harder to orbit. MESSENGER is only the
second spacecraft to make the journey (after
Mariner 10, which arrived in 1974) and the
first to enter orbit. It has long been known that
Mercury is unusually dense for its size, which
means that it must have a large component of
iron. Ground-based radar data4 confirmed
the presence of a liquid-iron-rich outer core,
suggesting a fully differentiated structure,
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NEWS & VIEWS RESEARCH
Earth
Crust
Upper mantle
Mercury
Cr
us
So
t
lid
M
an
lay
tle
er
of
iro
n
su
Lower mantle
Liq
uid
Liquid outer
core
So
lid
co inn
re er
m
id
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hi
lp
qualitatively like that of Earth. Its magnetic
field, known since Mariner 10 but poorly
characterized until the advent of data from
MESSENGER3, suggests an active dynamo,
also qualitatively like Earth’s.
However, the similarities quickly run out:
Mercury’s core makes up a far larger fraction
of the planet than does Earth’s core, and yet the
surface magnetic field is much smaller. It has
been shown5 that Mercury’s rotational properties, combined with gravity data obtained
from precise spacecraft tracking, allow the
planet’s total moment of inertia to be determined, as well as the moment of inertia of just
the outermost solid part, or outer shell. The
moment of inertia measures an object’s ability
to resist changes in its rotation and depends on
the object’s internal structure and density. The
moment-of-inertia values obtained5 indicate
a thin mantle, but also suggest an unusually
dense one.
Smith and colleagues’ analysis of the
MESSENGER data for Mercury’s gravity field
indicates that the planet’s unusually dense
outer shell might include a layer of iron sulphide at the base of the mantle (Fig. 1). This
solid layer would be part of the planet’s outer
shell from the point of view of rotational
properties, because the material immediately
below it is liquid and thus partially decoupled
from the libration (pendulum-like motion)
that the outer shell undergoes owing to the
torque exerted by the Sun on Mercury’s distorted shape. It should nonetheless be counted
as part of the core because it forms from the
core as upward-floating ‘snow’ as Mercury’s
interior cools with the ageing Solar System.
This unusual structure would be unlike that
of any other known planet, but it is permitted
by the thermodynamics of iron–sulphur alloys,
and might help to explain the planet’s small
magnetic field.
But is this structure required to explain the
data? Perhaps not. The authors acknowledge
that a more conventional, Earth-like structure
is also conceivable, because MESSENGER’s
data have an uncertainty in gravity field and
planetary obliquity (the tilt of a planet’s spin
axis relative to an axis perpendicular to its orbit)
that implies a rather large margin of error in the
value for the total moment of inertia. Even with
a more conventional structure, it is interesting
that the results suggest that Mercury may have
a substantial amount of sulphur relative to iron
— perhaps more than Earth. This is surprising, because sulphur is normally considered a
volatile element, and yet Mercury is the innermost planet and may therefore have formed at
high temperatures, so that the sulphur would
be expected never to have been incorporated
or to have subsequently been lost.
Meanwhile, Zuber and colleagues’ analysis of MESSENGER’s altimetry data provides
information on Mercury’s topography and
geological evolution. Flow in the mantle or
crust seems likely to have occurred, because
co
re
Relative sizes
0 km
Solid inner
core
0 km
6,378 km
2,440 km
Figure 1 | Internal structure of Earth and Mercury. Earth’s structure consists of a thin crust overlying
a mantle that makes up about two-thirds of the total planetary mass. The iron-rich core has a central,
solidified part that may also exist on all terrestrial planets, including Mercury. Smith and colleagues’
study1 suggests that Mercury has a very thin mantle overlying a solid layer of iron sulphide. This layer
might have arisen by upward flotation of iron sulphide crystals as the core has cooled, but as the layer
contributes to the planet’s rotational properties, it counts as part of its outer shell. (Adapted from a figure
by NASA/Johns Hopkins Univ. Appl. Phys. Lab./Carnegie Inst. Washington.)
the topography, gravity and images of the
planet suggest deformations in its surface over
distances of many thousands of kilometres.
Although these deformations may be ancient,
they post-date much of the planet’s impact history and require an explanation based on the
planet’s internal dynamics. Part of the explanation may lie in the cooling and resulting
contraction of Mercury over time.
Zuber et al. also show that Mercury has
some topographic highs that correlate with
gravity highs, but the former are not mere
thickenings of crust and are not yet understood. MESSENGER’s data allow good
regional gravity determination only in the
northern hemisphere, but the combination of gravity and topography has allowed
the authors to identify large (many tens of
kilometres) regional variations in crustal
thickness, suggesting a complex volcanic and
cooling history.
What does all this say about the origin of
Mercury? That is unclear. Two ideas, both of
long standing, are in play. One is that Mercury
is an outlier in the pattern of terrestrial-planet
formation, carrying a memory of thermodynamic equilibrium under conditions different from those thought to have existed when
Earth formed: very high temperatures and
conditions possibly especially favourable for
chemical reduction. This model is well enough
defined as to be at least testable, although
whether it can be made consistent with the
ideas currently favoured for planet formation,
in which a planet’s orbital radius is thought to
change over time, is unclear.
The alternative theory involves a giant
impact in which silicate material is blasted
from a proto-Mercury. This is an attractive model because it seems likely that giant
impacts are a natural part of planet formation.
But it has been insufficiently explored for
researchers to judge whether it can meet the
challenges of the structure and composition
inferred by Smith and colleagues. The European–Japanese mission to Mercury, called
BepiColombo, which will launch in 2015 and
arrive at the planet in 2022, may help to resolve
some of these issues, particularly through its
high-quality gravity measurements. ■
David J. Stevenson is in the Division of
Geological and Planetary Sciences, California
Institute of Technology, Pasadena, California
91125, USA.
e-mail: [email protected]
1. Smith, D. E. et al. Science 336, 214–217 (2012).
2. Zuber, M. T. et al. Science 336, 217–220 (2012).
3. Anderson, B. J. et al. Science 333, 1859–1862
(2011).
4. Margot, J. L., Peale, S. J., Jurgens, R. F., Slade, M. A.
& Holin, I. V. Science 316, 710–714 (2007).
5. Peale, S. J., Phillips, R. J., Solomon, S. C., Smith, D. E.
& Zuber, M. T. Meteorit. Planet. Sci. 37, 1269–1283
(2002).
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