These items, with supplementary material, firstappeared at http://www.physicstoday.org.

W

hat kept the Moon’s dynamo alive? The Moon’s
magnetic field used to have both the strength and thedipolar structure of a vigorous, dynamo-generated field like
Earth’s. Now, however, the lunar field is weak and patchy.
Accounting for the field’s enfeeblement might seem straightforward. As it aged, the Moon’s molten core cooled andshrank to the point at which it could no longer sustain a dynamo. The trouble is,whereas an analysis ofMoon rocks publishedlast year put that transformation at 3.7 billion
years ago, models ofthermal convection in
the Moon’s core put it
at 4.1 billion years ago.
What kept the dynamo
alive for the intervening 400 million years? To find out, MIT’s Clément Suavet andhis collaborators recently subjected two Moon rocks, both3.56 billion years old, to magnetic, thermal, and other tests.
(The photo shows a 5-g sample of one of the rocks next to a1-cm3 cube.) The researchers deduced that the rocks had
been magnetized by a surface field of at least 13 μT, which isconsistent with a strong dynamo. Although the age difference is just 4% between the rocks in the 2012 study and theyounger rocks in the new study, one explanation for the prolonged life of the dipolar field beyond its expected span canbe ruled out: an off-center hit by an asteroid that set theMoon rocking back and forth in its tidally locked orbit. No
impacts big enough occurred that late. Another proposedmechanism remains in play: The chemically stratified layersthat formed when the mantle crystallized could have become dynamically unstable and triggered a second round ofconvection. (C. Suavet et al., Proc. Natl. Acad. Sci. USA, in press,
doi:10.1073/pnas.1300341110.)—CDP

roton beams from a nanotube accelerator. Carbon nanotubes (CNTs) are hardy and versatile, with remarkable
material and electronic properties. And they could be useful
in some extreme
conditions as well.
Two physicists inJapan, Masakatsu
Murakami ofOsaka Universityand Motohiko
Tanaka of Chubu
University, propose using a CNT
as a shotgun barrel to shoot abeam of protons.
Their scheme
nests two small hydrogen-rich fragments—which could bewater ice, paraffin, or some other low-Z material but weremodeled as hydrogen nanotubes—within a larger CNT thathas gold atoms chemically adsorbed in its wall. The assem-

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bled structure is then zapped from the side with an ultraintense femtosecond laser pulse (green in the schematic).
As shown in three-dimensional simulations, the laser partiallyionizes the gold and fully ionizes the hydrogen and carbon in
the assemblage; after a few swings of the laser’s electric field,
significant numbers of electrons (white) are blown off andform a cloud around the CNT. The now highly ionized coaxial
structure generates a Coulomb potential in which the protons(blue) from the low-Z shotgun shells are squeezed toward theaxis and accelerated out both ends of the CNT. The simulations indicate that even a non-optimized setup can produce
highly collimated beams of nearly monoenergetic protons—
1.5 MeV for the parameters used. Such beams are of greatinterest in fields as diverse as medicine, fusion energy, andmaterials engineering. (M. Murakami, M. Tanaka, Appl. Phys.
Lett. 102, 163101, 2013.)—SGBD

emystifying the ice giants’ puzzling poles. Like ourown planet, Jupiter and Saturn have magnetic and geographic poles that are closely, but not perfectly, aligned; eachplanet’s magnetic dipole is angled just a
few degrees off its
rotational axis. But themagnetic fields ofUranus and Neptune,
the so-called ice giants,
are dramaticallytilted—by 59° and 46°,respectively. New experiments by Eric King
(University of California, Berkeley) andJonathan Aurnou(UCLA) may help to explain why. Planetary magnetic fields arethought to be generated by dynamos—turbulent, convective
flows of electrically conducting fluid in the planet’s interior(see the article by Daniel Lathrop and Cary Forest in PHYSICSTODAY, July 2011, page 40). Coriolis forces due to planetary rotation can influence those convective flows and thereby orient the dynamo’s field. To probe that effect in the lab, Kingand Aurnou did what’s known as a rotating Rayleigh–Bénard
experiment: They heated a 20-cm-wide spinning drum of liquid gallium from below while simultaneously cooling it fromabove. Using conditions that, through appropriate scaling,
mimic those of planetary interiors, the researchers found thatthe rotation–convection coupling is especially pronounced in
fluids with high thermal conductivity. Therein may lie the secret of the ice giants’ skewed poles: Based on trends seen in
the experimental data, the researchers estimate that Earth,
Jupiter, and Saturn, whose dynamos comprise high-conductivity
molten metal, fall within the strongly coupled regime, whereas
Uranus and Neptune, whose dynamos comprise moderateconductivity aqueous solution, do not. (E. M. King, J. M. Aurnou,
Proc. Natl. Acad. Sci. USA 110, 6688, 2013.)
—AGS

NASA

physics updateHot fire, cool soil. Wildfires around the planet burn anaverage of 3.7 million km2 of vegetation annually and canleave landscapes scorched, barren, and vulnerable to erosion
and flooding. According to the literature, the more vegetative
fuel, the more intense the fire, the hotter the soil, and themore severe the damage to fragile roots, seeds, and microbes.

June 2013

Physics Today

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search and discovery

But that understanding is basedon laboratory studies and prescribedburns in small fields.
An experiment led
by Cornell University’s Cathelijne
Stoof, who at thetime was working at
Wageningen University in theNetherlands, now provides evidence that in more heterogeneous conditions the opposite may occur—the hotter the fire,the cooler the soil. To study the effects of landscape and fire
dynamics on soil temperature, the group mapped a 0.1-km2
shrub-covered watershed in Portugal, installed 52 thermocouples throughout the region, and then set it ablaze. Although
the most thickly vegetated areas burned at temperatures ashigh as 800 °C, topsoil in those areas remained below 100 °C.
The soil temperature remained low, the researchers argue, because large air-temperature gradients increased the upward
transport of heat and dense vegetation contained the mostmoisture. Dry, sparsely vegetated areas that burned less intensely, in contrast, suffered the greatest damage; their soil
reached more than 300 °C in places. Managers of fire-prone
ecosystems could use the results to decide how, where, andwhen to set off controlled burns. (C. R. Stoof et al., Geophys.
Res. Lett., in press, doi:10.1002/grl.50299.)—RMW
irst results from the Planck microwave telescope. The
European Space Agency’s Planck satellite, launched in
2009, surveys the entire sky at microwave and submillimeter
wavelengths with much better sensitivity, angular resolution,and spectral coverage than was available to earlier generations of microwave orbiters. Planck’s main objective is tomeasure the parts-per-million spatial temperature fluctuation of the cosmic microwave background—the light fromthe first moments of cosmic transparency, 3.7 × 105 years
after the Big Bang. Precision measurements of its tiny, random departures from thermal isotropy on all angular scales
inform and constrain cosmological models. Now the Planck
collaboration has presented the results of its first 16 monthsof observation in 28 simultaneously released papers; theteam’s overview paper is cited below. The principal finding isthat cosmology’s widely accepted concordance model isalive and healthier than ever. Some of its fundamental parameters have suffered interesting tweaks, but none thatclearly require new physics or additional parameters in themodel’s scenario of cosmic birth, inflation, and structure formation. For example, the cosmic inventory of matter anddark energy has shifted by a few percent toward more matter, with a consequent slight reduction of the expansion rate
and a mere hundred million years added to the previous best
estimate (13.7 × 109 years) of the age of the universe. (P. A. R.
Ade et al., Planck collaboration, Astron. Astrophys., in press,http://arxiv.org/abs/1303.5062.)—BMSFI

deal point source for modeling room acoustics. When
analyzing the characteristics of sound—be it in a concert
hall, a doctor’s office, or a city street—acousticians can’t
always have unfettered access to the soundscape. So they18

June 2013

Physics Todaybuild scale models and adjust the sounds’ frequencies andamplitudes accordingly. A broadband, omnidirectionalsource of sound is very useful to modelers, and electricalsparks have been used to that end for many years. But thewaveforms that emanate from electrodes are not only directional but vary from spark to spark in unpredictable ways. Inaddition, the electrodes’ presence can complicate the soundpropagation. So a group of acousticians at Aalto Universityand Helsinki University, both in Finland, came up with a solution that has been effective for studying shock wave propagation: They used a laser-induced pressure pulse (LIPP).
When a point in space is heated to thousands of degrees by afocused laser, a local dielectric breakdown in air gives rise toan electrodeless spark that sends out a pressure wave—theLIPP. In their version, the acousticians focus a pulsed laserand send it into the scale model through an acousticallyopaque glass window. Projected into the enclosed space, theLIPP has all the hallmarks of an ideal acoustical point source:
It produces a lot of sound while being small, massless, omnidirectional, and broadband. (J. G. Bolaños et al., J. Acoust. Soc.
Am. 133, EL221, 2013.)—SGBT

he crystal structure of a lower-mantle mineral. Just
above Earth’s liquid outer core lies a 200-km-thick region,
dubbed D″, whose properties differ from those of the rest ofthe mantle. Long a mystery, the origin of D″ became clear in
2004 when the most common mineral in Earth’s mantle, magnesium silicate (MgSiO3), was found to change its crystalstructure at conditions that prevail in D″—that is, at pressures
above 120 gigapascals and temperatures above 2500 kelvin.
A new study by Ho-Kwang Mao of the Carnegie Institution ofWashington and his collaborators sheds further light on thetransition. Like their predecessors, Mao and his team used adiamond anvil cell to apply pressure, a laser to apply heat,and x-ray crystallography to determine structure. As the transition neared, a hundred or so randomly
oriented crystallites of the highpressure phase nucleatedwithin the micron-scale
sample. To gather
enough structural information about theensemble, Mao andteam rotated thesample through 51°
in 0.2-degree steps,
yielding 256 sets ofoverlapping diffraction patterns (seefigure for an example).
A computer algorithm
sorted through thousands ofdiscrete spots to determine thecrystallites’ orientations and structures. Besides confirming previous results, Mao’s study reveals that
replacing 10% of the magnesium with iron barely alters thestructure. An admixture of iron is expected in the lower mantle. Its lack of structural influence is encouraging, because it
suggests that inhomogeneities in seismic data could bemapped to inhomogeneities in temperature and pressure,
such as plumes and hot spots. (L. Zhang et al., Proc. Natl. Acad.
Sci. USA 110, 6292, 2013.)—CD ■www.physicstoday.org

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