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EI2GYB > ASTRO 13.10.25 14:39l 93 Lines 5601 Bytes #999 (0) @ WW
BID : 46535_EI2GYB
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Subj: The Moon's Biggest Crater Tells a New Story
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Sent: 251013/1224Z @:EI2GYB.DGL.IRL.EURO #:46535 LinBPQ6.0.25
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The Moon's Biggest Crater Tells a New Story
The gravitational interaction between the Earth and Moon has led to one
hemisphere of the Moon being locked facing away from Earth. Don't be misled
though, the Moon does rotate, it just takes as long to rotate once on its axis
as it takes to complete an orbit of Earth. This is known as synchronous
rotation and on the far side there is a massive crater called the South
Pole-Aitken basin. Spanning over 1,930 km from north to south and 1,600 km east
to west. This ancient impact crater formed roughly 4.3 billion years ago when a
giant asteroid delivered a glancing blow to the young Moon. A new study from
the University of Arizona reveals that this colossal crater holds secrets about
the Moon's formation and early evolution.
Jeffrey Andrews-Hanna and his colleagues made the discovery by carefully
analysing the shape of the South Pole-Aitken basin. Giant impact basins across
the Solar System share a characteristic teardrop shape, narrowing in the down
range direction of the impact. Previous assumptions held that the asteroid
struck from the south, but the new analysis shows the basin actually narrows
toward the south, meaning the impact came from the north instead. This
seemingly subtle detail has profound implications for what Artemis astronauts
will find when they land near the site.
Impact craters don't distribute their excavated material evenly. The down range
end of a basin usually gets buried under a thick blanket of ejecta, material
blasted from deep within the Moon's interior during the collision. The up range
end receives far less of this debris. Since Artemis missions are targeting the
southern rim of the basin, the corrected impact direction means astronauts will
be landing in exactly the right spot to study material from the Moon's deep
interior, essentially getting a geological core sample without needing to drill.
What makes this discovery particularly exciting is what that excavated material
contains. Early in its history, the Moon was covered by a global magma ocean.
As this molten layer cooled and crystallised over millions of years, heavy
minerals sank to form the mantle while lighter minerals floated to create the
crust. However, certain elements resisted incorporation into solid rock and
instead became concentrated in the final dregs of liquid magma. These leftover
elements; potassium, rare earth elements, and phosphorus, collectively known as
KREEP, refused to solidify until the very end.
The mystery has always been why KREEP ended up concentrated almost entirely on
the Moon's near side, the hemisphere facing Earth. This radioactive material
generated heat that drove intense volcanism, creating the dark basaltic plains
that form the familiar "face" we see from Earth. Meanwhile, the far side
remained heavily cratered and largely volcanic-free. The new study offers an
explanation that the Moon's crust must be significantly thicker on its far
side, an asymmetry that scientists still don't fully understand. The team
proposes that, as the far side's crust thickened, it squeezed the remaining
magma ocean beneath it toward the thinner near side.
The South Pole-Aitken impact provides crucial evidence that supports this
model. The western flank of the basin shows high concentrations of radioactive
thorium, a signature element in KREEP rich material, while the eastern side
does not. This asymmetry suggests the impact sliced through the lunar crust
right at the boundary where a thin, patchy layer of KREEP enriched magma still
existed beneath parts of the far side. The collision essentially opened a
window into this transitional zone between the near side's KREEP rich region
and the far side's more typical crust.
When Artemis astronauts collect samples from this radioactive region and return
them to Earth, scientists will have the opportunity to test these models with
unprecedented detail. Those rocks may finally explain how our Moon evolved from
a molten sphere into the geologically diverse world we see today, with its
dramatically different hemispheres telling two very different stories of the
same past.
Source : Moon's biggest impact crater made a radioactive splash
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