RESEARCH NEWS & VIEWS
reconstruction have observed the tension
between the imperative to provide immediate relief and efforts to launch self-organizing
processes of change that sustain recovery
and equity5. Immediate disaster relief often
requires quick decisions, has only limited
opportunities for participatory interventions,
and is typically externally driven. Long-term
positive changes, on the other hand, require
local buy-in and activation of a community’s
capacities both through strategic interventions
and through disengagement at the appropriate
time. Supporting internal institutional change
that is structured by the interests of weaker
and poorer groups is a crucial precursor to
disengagement. Knowing when to restrict or
cease to provide external material support, and
instead facilitate a transition to educational
and institutional support mechanisms that
recognize local capacities and provide opportunities for the less powerful, is necessary for
disaster relief to be effective in the long run. ■
Arun Agrawal is in the School of Natural
Resources and Environment, University of
Michigan, Ann Arbor, Michigan 48103, USA.
e-mail: [email protected]
1. World Bank/United Nations. Natural Hazards,
UnNatural Disasters: The Economics of Effective
Prevention (World Bank, 2010).
2. World Bank. Building Resilient Communities: Risk
Management and Response to Natural Disasters
through Social Funds and Community-Driven
Development Operations (World Bank, 2009).
3. McSweeney, K. & Coomes, O. Proc. Natl Acad. Sci.
USA 108, 5203–5208 (2011).
4. Pelling, M. Natural Disasters and Development in a
Globalizing World (Routledge, 2003).
5. Arnold, M. & Burton, C. Protecting and Empowering
Vulnerable Groups in Recovery (World Bank, in the
press).
surface of the inner core6. In addition, heterogeneity in grain size and composition at that
surface may eventually be buried by innercore growth, possibly explaining the complex
structure detected inside the inner core7. All
in all, melting of the inner core provides a tidy
Melting and solidification of iron alloys in Earth’s core may explain structural
explanation for several observations, although
complexity in the solid inner core, and alter the way we think about the
a few details remain to be explored.
dynamics of the deep interior. See Letter p.361
The authors pay careful attention to several
problems that arise in applying their model
to Earth. Two additional points are worth
BRUCE BUFFETT
growth of the inner core, and is so intense that mentioning. The first is the perennial consolidification is required below narrow regions cern about the validity of numerical models,
extbooks depict Earth as having an of cold fluid in order to offset large areas of given that the physical parameters are very far
onion-layered structure with a solid melting in warmer regions.
from realistic values. More specifically, could
steel ball at the centre. The central
Fractionation of impurities in the liquid on small-scale turbulence, largely absent from the
body, known as the inner core, is thought to solidification is expected to enrich the solid current models, disperse cold plumes long
have formed by gradual cooling and solidifica- in iron5; therefore, melting should produce before they reach the inner-core boundary? As
tion of the surrounding liquid outer core1. On a dense liquid that pools on top of the inner the numerical models improve we can expect
page 361 of this issue, Gubbins and colleagues2 core. Gubbins et al.2 argue that such a melt to gain better insight into their reliability.
turn convention on its head by arguing that layer offers a simple explanation for unusual
A second question involves the role of
a large fraction of the inner core’s surface is values recorded for seismic velocities near the composition in the melting temperature.
melting. Our understanding of both
Impurities in the liquid core depress
the structure and the dynamics of the
the liquidus temperature relative
core may change as a consequence.
to that of pure iron by 600 kelvin or
The authors’ conclusion is based
more8. An iron-rich melt is expected
Warming
Liquidus
on a numerical model3 that simulates
to solidify before a liquid with the bulk
Geotherm
convection and magnetic-field generacomposition of the outer core. So what
tion in the liquid core. Cooling of the
happens after the inner core melts?
liquid core drives convection, but it is
We expect the inner-core boundthe more massive and sluggish mantle
ary to represent the top of a mushy
Inner-core boundary
surrounding it that regulates the rate
region where solid and liquid co­exist9.
of cooling. Spatial variations in heat
A temperature increase at the boundflux at the top of the core exert a strong
ary initially promotes melting.
influence on the pattern of fluid flow4.
However, a small amount of melt
In the authors’ simulations, cold fluid
enriches the surrounding liquid in
Melt
is focused into narrow plumes, which
iron, which elevates the local liquidus
descend to the inner-core boundary
temperature and brings the interface
and promote localized solidification.
back into equilibrium (Fig. 1). Given
Temperature
Elsewhere, a broad return flow is assothe magnitude of the melting-point
ciated with warm fluid that persistently Figure 1 | Melting at the inner-core boundary. Gubbins et al.
depression, a small amount of melt
exceeds the melting temperature at the suggest2 that warming in the liquid outer core produces widespread
should be sufficient to compensate
melting at the boundary with the solid inner core. In this phase
inner-core boundary.
for thermal fluctuations in the liquid
Temperatures in the fluid’s interior diagram, the initial position of the inner-core boundary is defined by outer core. Would small variations
can exceed the boundary temperature the intersection of the core temperature (geotherm; dotted blue line)
in melt volume be detectable in seisand the melting temperature (liquidus; dotted red line). An increase
because the core is mainly cooled from
mic observations? This question will
in the geotherm (solid blue line) promotes melting of the iron-rich
above rather than heated from below. solid, diluting the concentration of impurities in the liquid. That
require a better understanding of the
Fluid parcels become warmer relative raises the liquidus temperature (solid red line) until it intersects the
phase diagram.
to a decreasing background tempera- warmer geotherm, re-establishing thermodynamic equilibrium. The
The work of Gubbins and colture if the parcels are not cooled at the dense melt resides within the porous solid near the top of the inner
leagues 2 opens a door onto new
average rate. Cooling produces net core, as shown in the inset.
enquiries. Melting and solidification
E ART H SCIENCE
A deep foundry
Solid
Pressure
Liquid
T
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NEWS & VIEWS RESEARCH
of the inner core allows greater interaction
with the surrounding liquid core, and raises
the possibility that surprising phenomena are
yet to be discovered. Recent speculations10,11
about a steady translational motion of the
inner core demonstrate that strange things are
possible. The final chapter of this story is yet
to be written. ■
Bruce Buffett is in the Department of Earth
& Planetary Science, University of California,
Berkeley, Berkeley, California 94720-4767, USA.
e-mail: [email protected]
1. Jacobs, J. A. Nature 172, 297–298 (1953).
2. Gubbins, D., Sreenivasan, B., Mound, J. & Rost, S.
Nature 473, 361–363 (2011).
3. Sreenivasan, B. & Jones, C. A. Geophys. J. Int. 164,
467–476 (2006).
4. Olson, P. & Christensen, U. R. Geophys. J. Int. 151,
809–823 (2002).
5. Alfè, D., Gillan, M. J. & Price, G. D. Contemp. Phys.
48, 63–80 (2007).
6. Souriau, A. & Poupinet, G. Geophys. Res. Lett. 18,
2023–2026 (1991).
7. Sun, X. & Song, X. Earth Planet. Sci. Lett. 269, 56–65
(2008).
8. Gillan, M. J., Alfè, D., Brodholt, J., Vočadlo, L. & Price,
G. D. Rep. Prog. Phys. 69, 2365–2441 (2006).
9. Fearn, D. R., Loper, D. E. & Roberts, P. H. Nature 292,
232–233 (1981).
10.Monnereau, M. et al. Science 328, 1014–1017 (2010).
11.Alboussière, T., Deguen, R. & Melzani, M. Nature
466, 744–747 (2010).
E P IG ENETICS
Tet proteins in the
limelight
Tet proteins mediate the hydroxymethylation of DNA. New work reveals their
function in gene regulation and the extent of their activity throughout the genome
of embryonic stem cells. See Article p.343 & Letters p.389, p.394 & p.398
N AT H A L I E V É R O N &
A N T O I N E H . F. M . P E T E R S
D
uring mammalian development, the
one-cell zygote gives rise to a multitude of cell types. This remarkable
process is controlled by protein machines
that interpret the genetic code and regulate
the expression of genes, in part by chemically
modifying chromatin (DNA–protein complexes). One such modification is the addition
of a methyl group at the 5-position of the cytosine base in DNA (5mC) — an alteration that
serves a crucial role in the epigenetic (or cellto-cell) inheritance of gene expression during
development. However, proteins of the Tet
enzyme family can modify this DNA mark further by hydroxylating the methyl group to form
5-hydroxymethyl­c ytosine (5hmC)1–3. Five
papers4–8, including four in this issue, report
on the extent of 5hmC modification across
the genome of mouse embryonic stem cells
and on the role of Tet proteins in regulating
gene expression.
The 5mC modification is required for
genome stability and thus the embryo’s viability. It is also needed for the repression of
genes and repetitive genomic sequences; for
X-chromosome inactivation; and for genomic
imprinting (in which, for some genes, either
the maternal or the paternal copy is expressed).
Classical studies revealed that 5mC is erased
in primordial germ cells and during early
embryo development, and that this process
occurs independently of DNA replication as
the cells divide9. What’s more, genes containing 5mC can become active in differentiated
cells, supporting the notion of active demethyl­
ation10,11. This idea, along with researchers’
ability to epi­genetically reprogram cells (either
by the technique of somatic-cell nuclear transfer or by induced pluripotency experiments),
inspired the search for factors that mediate
5mC demethylation, although initially there
was limited success12.
Because proteins of the Tet family (Tet1–3)
can convert 5mC to 5hmC, they have been
considered promising candidates for mediating DNA demethylation. But this novel
enzymatic means of demethylation leads to
obvious questions. Where in the genome do
Tet proteins bind? How do they affect the stability/turnover of 5mC? Does this influence
gene expression during the cell cycle and in
development? Do Tet proteins alleviate gene
silencing by converting 5mC to 5hmC, or do
they protect against aberrant de novo methylation, thereby preventing silencing? And finally,
how is 5hmC processed further? The latest
studies4–8 shed light on these issues.
Williams et al. (page 343)4, Wu et al.8 and Ficz
et al. (page 398) 7 localized 5hmC in the
genome of mouse embryonic stem (ES) cells
using methods that predominantly recognize DNA sequences bearing multiple 5hmC
marks. Pastor and colleagues (page 394)6
developed two alternative methods that possess increased sensitivity for single 5hmCs.
The general finding is that 5hmC levels across
the genome are low. Nonetheless, the mark is
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