Image: NASA |
New
evidence from an ancient lunar rock suggests that the moon once harbored a
long-lived dynamo—a molten, convecting core of liquid metal that generated a
strong magnetic field 3.7 billion years ago. The findings, published in Science,
point to a dynamo that lasted much longer than scientists previously thought,
and suggest that an alternative energy source may have powered the dynamo.
“The
moon has this protracted history that’s surprising,” says co-author Benjamin
Weiss, an associate professor of planetary science at MIT. “This provides
evidence of a fundamentally new way of making a magnetic field in a planet a
new power source.”
The
new paper is the latest piece in a puzzle that planetary scientists have been
working out for decades. In 1969, the Apollo 11 mission brought the first lunar
rocks back to Earth—souvenirs from Neil Armstrong and Buzz Aldrin’s historic
moonwalk. Since then, scientists have probed the rocky remnants for clues to
the moon’s history. They soon discovered that many rocks were magnetized, which
suggested that the moon was more than a cold, undifferentiated pile of space
rubble. Instead, it may have harbored a convecting metallic core that produced
a large magnetic field, recorded in the moon’s rocks.
Exactly
what powered the dynamo remains a mystery. One possibility is that the lunar
dynamo was self-sustaining, like Earth’s: As the planet has cooled, its liquid
core has moved in response, sustaining the dynamo and the magnetic field it
produces. In the absence of a long-lived heat supply, most planetary bodies
will cool within hundreds of millions of years of formation.
A
dynamo still exists within Earth because heat, produced by the radioactive
decay of elements within the planet, maintains the core’s convection. Models
have shown that if a lunar dynamo were powered solely by cooling of the moon’s
interior, it would have been able to sustain itself only for a few hundred
million years after the moon formed—dissipating by 4.2 billion years ago, at
the very latest.
Heavy metal rock
However, Weiss and his colleagues found some surprising evidence in a bit of
lunar basalt dubbed 10020. The Apollo 11 astronauts collected the rock at the southwestern
edge of the Sea
of Tranquility;
scientists believe it was likely ejected from deep within the moon 100 million
years ago, after a meteor impact. The group confirmed previous work dating the
rock at 3.7 billion years old, and found that it was magnetized—a finding that
clashes with current dynamo models.
Weiss
collaborated with researchers at the University
of California at Berkeley
and the Berkeley Geochronology Center,
who determined the rock’s age using radiometric dating. After a rock forms, a
radioactive potassium isotope decays to a stable argon isotope at a known rate.
The group measured the ratio of potassium to argon in a small piece of the
rock, using this information to ascertain that the rock cooled from magma 3.7
billion years ago.
Weiss
and graduate student Erin Shea then measured the rock’s magnetization, and
found that the rock was magnetized. However, this didn’t necessarily mean that
the rock, and the moon, had a dynamo-generated magnetic field 3.7 billion years
ago: Subsequent impacts may have heated the rock and reset its magnetization.
To
discard this possibility, the team examined whether the rock experienced any
significant heating since its ejection onto the moon’s surface. Again, they
looked to isotopes of potassium and argon, finding that the only heating the
rock had experienced since it was ejected onto the lunar surface came from
simple exposure to the sun’s rays.
“It’s
basically been in cold storage for 3.7 billion years, essentially undisturbed,”
Weiss says. “It retains a beautiful magnetization record.”
Stirring things up
Weiss says the rock’s evidence supports a new mechanism of dynamo generation
that was proposed last year by scientists at University of California
at Santa Cruz (UCSC). This hypothesis posits that the moon’s dynamo may have
been powered by Earth’s gravitational pull. Billions of years ago, the moon was
much closer to Earth than it is today; terrestrial gravity may have had a
stirring effect within the moon’s core, keeping the liquid metal moving even
after the lunar body had cooled.
Francis
Nimmo, a professor of earth and planetary sciences at UCSC and one of the
researchers who originally put forth the new dynamo theory, says Weiss’
evidence provides scientists with a new picture of the moon’s evolution.
“We
generally assume that cooling is the main mechanism for driving a dynamo
anywhere,” says Nimmo, who was not involved in the current study. “This lunar
data is telling us that other mechanisms may also play a role, not just at the
moon, but elsewhere, too.”
