Geology Reference
In-Depth Information
Not all of Theia was captured, however. Higher up in space, Earth became encircled by
avastaccumulationofrockycollisionaldebris,mostlyanintimatemixtureofthetwoplan-
etary mantles. Cooling rocky droplets stuck together, with bigger chunks sweeping up the
smaller. In a sort of instant replay of the gravitational clumping that originally formed the
planets, the Moon coalesced rapidly and may have achieved more or less its present size in
a few years.
The physics of planet formation dictate where the Moon could have formed. Every
massive object has an invisible surrounding sphere, called the Roche limit, inside of which
gravitational forces are too great for a satellite to form. That's why Saturn has immense
rings but no moons within about fifty thousand miles of its surface. Saturn's gravity pre-
vents those icy particles from coalescing to form a moon.
Calculatedfromthecenterofarotatingobject,Earth'sRochelimitisabout11,000miles,
or roughly 7,000 miles up from the surface. Accordingly, models of Moon formation loc-
ate the new satellite at a safe distance of about 15,000 miles up, where it could grow in an
orderly fashion by sweeping up most of the scattered bits and pieces from the Big Thwack.
And so, perhaps 4.5 billion years ago by most estimates, the Moon was born. Earth found
itself with a companion, formed in large part from pieces of itself.
Scientists quickly embraced the Big Thwack theory because it explains all the major
clues better than any other model. The Moon lacks an iron core because most of Theia's
iron wound up inside Earth. The Moon lacks volatiles because Theia's volatiles were blas-
ted away during the impact. One side of the Moon always faces Earth because the angular
momentum of Earth and Theia were coupled into one spinning system.
The Big Thwack also helps to explain Earth's anomalous axial tilt of about 23 de-
grees—a factor not well handled by any of the previous scenarios. The impact of Theia
literally tipped Earth onto its side. Indeed, the realization that a giant impact formed the
Moon has led to speculation about other planetary anomalies in our Solar System. Perhaps
late Big Thwack events of one kind or another are common, even necessary. Perhaps that
explains why Venus rotates the wrong way on its axis and why it lost so much of its water.
Perhaps a late giant impact caused Uranus to rotate on its side.
A Different Sky
The Moon's formation was a pivotal moment in Earth history, with far-reaching conse-
quences that are utterly amazing and only just now coming into focus. The Moon of 4.5
billion years ago was not the romantic, silvery disk we see today. Long ago it was a far
more looming, dominant, and unimaginably destructive influence on Earth's near-surface
environment.
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