Sky and Telescope - October 2018 - 53

M AG NE TIC FIELD: JG R PL A NE TS / HIDEO TSUN A K AWA E T A L;
G RE AT M AG NE TIC LOW: N ASA / LUN A R RECONN AISSA NCE ORBITER; REINER G A M M A: SE A N WA LK ER / S&T

ciation with Imbrium suggested that
its very low ﬁeld strength was due to
shock waves generated by the collision
that formed that impact basin. These
shock waves can randomize the magnetic alignments in atoms previously
organized by external magnetic ﬁelds.
According to this theory, the event
that formed Imbrium also erased any
magnetic ﬁeld that had been present
in the target rocks. It's an elegant and
very plausible explanation. But the GML
extends beyond the Imbrium basin, and
other impact basins lack similarly weak
magnetic ﬁelds.
Wieczorek and colleagues now
propose an alternative interpretation
that resolves these conﬂicts. They note
that the GML and its spatial extensions ﬁt within the Procellarum KREEP
Terrane (PKT). This is a province of
the Moon distinguished by high levels
of the radioactive elements thorium,
potassium, and uranium that release
heat as they decay. KREEP, by the way,
is an acronym built from the letters
K (the atomic symbol for potassium),
REE (rare-earth elements) and P (for
phosphorus).
For some still poorly understood
reason, radioactive elements are concentrated in the nearside's northwest
quadrant, under the lavas of Procellarum, Imbrium, Nubium, and western
Mare Serenitatis. The extra heat they
released could explain why most of the
Moon's nearside lava ﬂows are within
the PKT boundaries.
But what does heat have to do with
magnetism? It turns out to be plenty.
Pierre Curie long ago discovered that
as he progressively heated magnetized
materials in his laboratory, they lose
their magnetization at a certain temperature, now known as the Curie point.
For iron, which carries magnetism in
lavas, the Curie point occurs at 770°C
(1418°F). Terrestrial basaltic lavas,
similar to ﬂows found on the Moon,
typically erupt at about 1100°C and
thus don't carry a magnetic ﬁeld. But
as lavas cool below 770°C, they acquire
the ambient magnetic ﬁeld and, once
solid, freeze in the ﬁeld's intensity and
direction.

The Moon's abundant lava ﬂows presumably locked in the ambient magnetic
ﬁeld generated by an early lunar dynamo
as they cooled. All maria except those
in the GML seem to have done this. So
what was special about the GML?
Wieczorek's group proposes that
excess radioactivity kept the lavas
within the PKT crust above the Curie
point much longer than those elsewhere. That extra time is crucial,
because measurements of lunar lava
samples of various ages show that the
average magnetic ﬁeld strength has
decreased by a factor of 10 over the
past 3½ billion years. The researchers'
mathematical models of the decline in
PKT radioactive heating show that cooling to the Curie point could have been
delayed by as much as a billion years,
by which time the lunar dynamo had
nearly turned off. Such slowly cooled
lavas would only retain a weak magnetic ﬁeld.
This model, while plausible, still
has a few problems. One is that the
GML covers an area considerably
smaller than the PKT's. Perhaps since

the impact and PKT models are both
plausible, both happened. The Imbriumforming impact surely must have heated
the crust to great depth. This pulse of
extra heat could have extended the
time until the Imbrium basin's crust
and lavas cooled to the Curie point,
accounting for the nearly complete lack
of magnetic ﬁeld there. In this case, the
slightly stronger magnetic ﬁeld exhibited by the rest of the GLM would have
resulted from the PKT's radioactive
heating alone.
Unfortunately, you can't observe
magnetic ﬁelds even with the best
telescope. But your experience will be
enriched when observing Oceanus
Procellarum, Mare Imbrium, Mare
Nubium, and western Mare Serenitatis by recognizing that these vast lava
plains owe their existence to the PKT,
which in turn might also have contributed to the lack of a magnetic ﬁeld in
the Imbrium region of the GML.
¢ When observing, Contributing Editor
CHUCK WOOD envisions the Moon both
inside and out.

p Bright swirls such as Reiner Gamma coincide with localized areas of high magnetism, which
perhaps has shielded the lunar surface at these locations from the darkening effects of prolonged
solar radiation.
s k y a nsdktyeal ensdct oe p
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Sky and Telescope - October 2018

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