Geology Reference
In-Depth Information
rotate ever more slowly on its axis to conserve the total angular momentum of the Earth-
Moon system—again, think of the figure skater, stretching out her arms once more and
slowing her spin. Over the span of 4.5 billion years, Earth's rotation has slowed from once
every five hours to once every twenty-four hours, while the Moon has moved farther away
and picked up a lot of angular momentum in the process.
Noteveryplanet-moonsystemhastofollowthisstoryline.Iftheplanetrotatesonitsaxis
moreslowlythanitsmoonorbits,thenaninexorablebrakingeffectensues.Tidalbulgeson
the planet will trail behind; the moon will slow down with each orbit and fall ever closer
to its doom. Eventually the moon will spiral into the planet and be swallowed up, in yet
anothervariation ontheBigThwack theme. Perhapsthat'swhyVenus,withitswrong-way
retrograde rotation, doesn't have a moon. Perhaps such a cataclysmic demise of a once-or-
bitingmoonexplainswhyVenuslostitswaterandisnowahostile,scalding,lifelessworld.
EarlyinthehistoryoftheEarth-Moonsystem,theseexchangesofangularmomentumfrom
the slowing Earth to the accelerating Moon were vastly greater than today. In the first cen-
turies after the Moon's formation, both bodies were girdled by turbulent magma oceans
thatcouldflexanddeform.ThegiantmagmatidesonEarth,andsimilarmagmabulgingon
the Moon, probably caused the Moon to recede by tens or hundreds of feet per year, even
as Earth's rotation steadily slowed down from its initial frenetic pace. But these enormous
land tides could not have lasted for long. As the Earth-Moon distance increased, the tidal
forces decreased even more: a doubling of the distance cut the force of gravity by a factor
of four. A tripling of the distance and gravitational forces were but a ninth of their former
strength.
Repeated tidal stressing delayed but could not stop the solidification of worlds. Within a
few million years of the Big Thwack, the surfaces of both Earth and the Moon were paved
with hard black rock. Land tides—the deformation of solid rock—were still not trivial in
those early days, but they were nothing like the mighty daily swellings of the magma sea
that preceded them.
_____
The Moon remains a luminous reminder that the cosmos is a place of intertwined creation
and destruction. Even today we are not immune to catastrophic cosmic insults: killer as-
teroids and comets still cross Earth's orbit from time to time. Millions of years ago one
big rock killed the dinosaurs; millions of years from now, other big rocks will inevitably
find their mark. If human survival is our greatest collective imperative as a species, then
we would do well to keep watching the skies, for our nearest cosmic neighbor offers mute
testimony: while change is usually gradual and benign, there can be really bad days.
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