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| Two recent models for the formation of the moon, one that allows exchange through a silicate atmosphere (top), and another that creates a more thoroughly mixed sphere of a supercritical fluid (bottom), lead to different predictions for potassium isotope ratios in lunar and terrestrial rocks (right). |
Around 4.5 billion years ago, when the solar system was still in its
infancy, a Mars-sized planet collided with the proto-Earth and blasted
our baby planet to smithereens. Earth’s rocks did not just melt; they
vaporized, the very elements in those rocks turning into gas the way
boiling water turns into steam. Eventually, the remains of the original
Earth cooled and settled down, condensing to once again form a solid
planet. The leftovers formed the moon.
The moon, our first friend, is more than a familiar fixture in the
night sky; it dictates the slosh of tides, stabilizes Earth’s rotation,
and might contribute to earthquakes. It is also the best place we have,
other than Earth, for studying the formation of rocky worlds. Scientists
are still unsure how it formed, in part because it’s hard to reconcile
the moon’s chemical composition with the origin stories our computers
tell.
In the 1970s, scientists proposed the moon formed from a grazing
collision with a Mars-sized world. Earth’s surface would have liquified
and part of it would have sloughed off. The moon coalesced from that
debris and leftover pieces from the impactor, a scenario which came to
be called the giant impact hypothesis. But analysis of moon rocks
returned home after the Apollo missions suggested it can’t be that
simple. The isotopic compositions of elements in Earth and moon rocks
are the same. That would mean the Earth and the Mars-sized impactor were
the same, too, and that’s very unlikely.
To explain this similarity, scientists needed to come up with a way
to make the moon mostly from the Earth, and not the impactor, says Kun
Wang, a geochemist at Washington University in St. Louis. He was
intrigued by a new computer model that debuted this spring at the annual
Lunar and Planetary Science Conference. In this model, the Mars-sized
object hit Earth with such violence that the impactor and Earth’s mantle
vaporized.
“Maybe the core is still solid, but the mantle, and the Mars-size planet itself, all vaporized,” Wang told Popular Science. “Earth and the Mars-size planet, after the impact, will evaporate entirely.”
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| Earth's moon |
When the temperature starts to decrease with time, the vapor begins
to condense, forming liquid moonlet drops. The material would eventually
coalesce into a disk — something like the rings of Saturn, says Robin
Canup, a planetary scientist at the Southwest Research Institute. The
moon eventually formed from this material, and most of the rest fell
back to Earth.
Wang published a new analysis of moon rocks today in Nature Geoscience,
which he says supports this idea. The key is potassium, a volatile
element found in abundance in both Earth and lunar rocks. Wang and his
coauthor, Stein Jacobsen, examined lunar dust from several Apollo
missions and counted potassium isotopes. The element K has three stable
isotopes, but only two--potassium-39 and potassium-41--are abundant
enough to count with enough precision, Wang says. He spent a year
studying Earth rocks to fine-tune his measurements before he tried it
with moon dust.
He found the lunar samples had heavier potassium signatures. Wang
says this supports the idea that the moon formed from vaporized Earth
rock: the heavier stuff condensed first, forming the moon. He says the
isotope data can be used like a “paleo barometer” of sorts, revealing
the physical conditions during the Earth-shattering event that formed
the moon.
Canup says the result is a key piece of evidence that will drive new and improved moon-formation theories.
“The data is ahead of the models, which it should be. Now the burden
is going to be on those of us doing models of the disk and the impacts,
to argue that we can or can’t explain this new piece of data,” she says.
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