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increase in viscosity. This region
marks the transition from upper
to lower mantle and is critical
to the mixing between the up-
per mantle [from which we have
hand samples ( 3 )] and the lower
mantle [from which we have little
but a few diamond inclusions ( 4 ,
5 )]. The consensus ( 6 ) is that 8 0 %
or more of the lower mantle is the
high-viscosity bridgmanite phase,
with around 17 % of the weaker
magnesium-iron oxide and the
remainder as calcium perovskite.
We rely on the reverberations
of seismic waves to detect Earth’s
layers and their properties. Ev-
ery earthquake with a magnitude
greater than about 5.0 floods the
planet with seismic energy that
sloshes back and forth for hours
afterward in the form of body
waves [those that travel through
the planet, divided into compres-
sional (P) waves and shear (S)
waves] and surface waves (those
that roll along the surface). These
movements amount to only micrometers
at Earth’s surface, requiring sensitive seis-
mometers to measure and record these
motions. Seismic waves radiate at wave-
lengths from a few meters to thousands
of kilometers. But the lowest-noise seis-
mic records occur at long (~1 00 km) and
short (~ 5 km) wavelengths. Therefore, seis-
mologists typically examine Earth at short
scales or long scales.
The dramatic change in mineralogy from
the upper to the lower mantle results in
a small increase in wave speed
and density ( 7 ), so that each time
a wave passes through Earth, a
small amount of energy is re-
flected or converted from shear
to compression and vice versa. By
combining many seismic records,
scientists have shown that these
reflections and conversions reveal
the boundary between the upper and lower
mantle to occur at around 660 km depth,
with topography that undulates by 30 to 40
km over scales of hundreds to thousands of
kilometers. It has not been possible to map
the 660 -km seismic discontinuity at scales
of a few kilometers owing to a lack of coher-
ent signals at these wavelengths.
Wu et al. now obtain data suggesting
such topography by exploiting that in places
where seismometers or seismic arrays are
within ~4000 km of strong earthquakes
(around magnitude 7. 0 ), it is possible to
record P waves that have traveled through
the core, to the other side of the planet,
and back again. These waves are known inbody wave seismology as PKPPKP, which is
abbreviated to P 9 P 9. Any reflected energy
along the way at a depth d is then known
as P 9 dP 9. If the boundary is smooth, all re-
flected energy will arrive at the same time,
creating a peak in the stacked seismic data.
However, if the surface is rough, the energy
will be spread out, instead creating a pla-
teau. Using carefully selected earthquakes
and stations, Wu et al. have gleamed a pla-
teau of energy from P 9 660P 9 , indicating
rough topography of 1 to 3 km.The results may help to answer funda-
mental questions about Earth’s evolution.
Geologists estimate Earth’s composition on
the basis of what we can see from the crust
and small samples of the upper mantle and
what we can infer, such as a mostly iron
core. However, comparing Earth with the
Sun and other bodies in the solar system,
there are discrepancies in bulk element
abundance and isotopic ratios. Whether
these discrepancies are due to sequestra-
tion of rock early in Earth’s history or to
heterogeneities in the protoplanetary
disk remains a fundamental question that
connects planet formation to the current
Earth structure.Wu et al.’s findings suggest that answers
may come from the lower mantle. The
lower-mantle topography at long (thou-
sands of kilometers) and short (a few ki-
lometers) wavelengths is analogous to the
type of surface topography we observe on
Earth (see the figure) and other planets,
suggesting that in some regions, the lower
mantle is chemically distinct from the up-
per mantle above it. Thus, these distinct
regions differ in the extent to which they
have mixed with the rest of the mantle
since Earth’s formation, presum-
ably with the sluggish lower
mantle resisting convection.
Three-dimensional rendering of
seismically fast and slow regions
reveals the subduction of oceanic
crust and tectonic plates into the
lower mantle and warm upwelling
regions returning material back
to the upper mantle ( 8 ). But it appears that
the lower mantle is also a vault-preserving
relic of the time when Earth emerged from
dust to become a card-carrying planet. jREFERENCES
1. W. Wu, S. Ni, J. C. E. Irving, Science 363 , 736 ( 201 9).
2. O. Tschauner et al., Science 346 , 1100 (2 014 ).
3. S. Demouchy, N. Bolfan-Casanova, Lithos 240 – 243 , 4 02
(2 01 6).
4. F. Nestola et al., Nature 555 , 237 (2 018 ).
5. O. Tschauner et al., Science 359 , 1136 ( 2018 ).
6. E. Mattern, J. Matas, Y. Ricard, J. Bass, Geophys. J. Int. 160 ,
973 ( 2005 ).
7. P. M. Shearer, M. P. Flanagan, Science 285 , 1545 ( 1 999).
8. K. Hosseini et al., Geochem. Geophys. Geosyst. 19 , 1464
(2 018 ).10 .1 12 6/science.aaw 46010 500 1000 km0 500 1000 km1 to 3 km
scale ripples0 km670 kmCrust-atmosphere boundary
Highest elevation:
Himalaya, ~ 9 kmCrust-mantle boundaryUpper mantle–lower
Lowest point: Mariana mantle boundary
Trench ~11 kmGraphic not to scaleCrustUpper
mantleLower mantle40 km0
–5
–1 050–5–1 01040 km“The transport...between the upper and
lower mantle largely determines the
evolution of our planet, but little is known
about this boundary at small scales.”
15 FEBRUARY 2019 • VOL 363 ISSUE 6428 697The topography of Earth’s boundaries
Seismic data are beginning to reveal the large-scale topography of the boundaries between Earth’s crust and mantle and
between the upper and the lower mantle. The height of these large-scale topographies resembles that at Earth’s
surface. Wu et al. now report evidence for smaller, kilometer-scale topography at the upper–lower mantle boundary.Published by AAASon February 14, 2019^http://science.sciencemag.org/Downloaded from