.001
millitesla
Minimum magnetic
field strength in
moon rocks
5
millitesla
Magnetic field
strength of a
refrigerator magnet
How the moon kept its magnetism
Outside forces may have maintained the ancient lunar field
NEWS BRIEFS
scientist Francis Nimmo of UC Santa
Cruz. The wobble is greater when the
moon is closer to Earth. As the moon
moves farther away — as it has over the
last 4 billion years — the wobble decreases.
The magnetic field decreases, too,
disappearing completely about
2. 7 billion years ago. If this
model is right, says Nimmo,
“younger [lunar] rocks ought
to be magnetized, but they
ought to be magnetized less
strongly than older rocks.”
The second team proposes that large
asteroids smacking into the moon messed
up its rotation rate and perturbed the
liquid core. Six large, ancient lunar impact
basins with magnetized rocks at their
centers support this idea, says study coau-
thor and fluid dynamicist Michael Le Bars
of the French national research agency
and the University of Aix-Marseille. “A
reasonably large impact can generate a
magnetic field for about 10,000 years,” he
says. If so, then lunar rocks might show
spike after spike of magnetism as impacts
pummeled the early moon.
Though different, the two theories are
not mutually exclusive; together they
add up to “a major, major advance in our
understanding,” says planetary scientist
Benjamin Weiss of MIT. Studying the
magnetic record preserved in lunar rocks
would be a good way to test the theories,
he notes, since tracing magnetic intensities over time should produce a pattern
that might match one or both predictions.
Some lunar rocks already on Earth
have revealed magnetic fields, though the
measurements are decades old. “It would
be nice if somebody goes back and redoes
those measurements with the modern
techniques,” Nimmo says.
Ian Garrick-Bethell, a planetary scientist at UC Santa Cruz, says both papers are
“clever and elegant,” but that more mathematical simulations need to be done.
By Nadia Drake
External forces beating up the ancient
moon may explain how it once
maintained a magnetic field for more
than 400 million years — longer
than scientists had thought
such a small object could be
magnetized.
Either wobbly rotation
produced by Earth’s gravitational tug or asteroids
smacking into the lunar surface
may have triggered enough turbulence
in the moon’s molten core to generate a
long-lasting magnetic field, report two
teams of scientists in the Nov. 10 Nature.
“This has been a very fundamen-
tal question for 40 years,” says study
coauthor Christina Dwyer, a graduate
student at the University of California,
Santa Cruz. Though absent today, this
ancient field is recorded in rocks retrieved
from the moon’s surface and in magne-
tized patches of crust spied by orbiting
spacecraft. “The moon was magnetized,”
Dwyer says. “We don’t know how.”
Normally, heat escaping from a plan-
et’s interior causes fluid in the core to
slosh around, creating a magnetic field.
But the moon would have cooled off
too quickly for the sloshing to be main-
tained. Instead, each research team
points to a different spoon that would
have mechanically stirred the early
moon’s innards to create magnetism.
Dwyer’s team suggests that a slight,
Earth-driven wobble in the moon’s axis
of rotation mixed the liquid core. “The
Earth is tugging on the moon and that
tug — even though it’s quite small — keeps
the moon wobbling,” says planetary
Duck and cover
It’s likely that Russia’s Phobos-
Grunt spacecraft, launched
November 8 from the Baikonur
Cosmodrome in Kazakhstan and
aiming for the Mars moon Phobos,
will fall back to Earth—but where
and when are still unknown. after
launch, the unmanned craft’s
engines failed to fire, leaving it
stuck in low-Earth orbit. Efforts
to rescue the mission have been
unsuccessful, and a dwindling
power supply leaves the Russian
space agency a limited amount of
time to save the probe. This mis-
sion is Russia’s first to Mars since
1996, when the Mars-96 space-
craft failed. The Phobos-Grunt craft
had been intended to rendezvous
with Phobos, scoop up some
samples and return them to Earth.
— Nadia Drake
Earth’s gravitational tug or asteroid
impacts may have helped the young
moon sustain its magnetic field for
more than 400 million years.
Faster-than-light neutrinos,
the sequel
Neutrinos have been spotted
traveling faster than light —again.
This time, physicists on the
OPERa collaboration used shorter
3.5-nanosecond pulses of particles
that allowed individual particles to
be tagged instead of the spread-
out bunches in the original experi-
ment, which had raised concerns
that some particles started their
journey early. again, neutrinos
made the 730-kilometer trip from
the European laboratory CERN
near Geneva to Italy’s Gran sasso
National Laboratory 60 nano-
seconds faster than light would
have. This additional evidence,
which rules out one possible
source of error in the experiment,
was reported online November 17
at arXiv.org. — Devin Powell
www.sciencenews.org
December 17, 2011 | SCIENCE NEWS | 17
