Geology Reference
In-Depth Information
The Big Thwack
The Formation of the Moon
Earth's Age: 0 to about 50 million years
A central principle of this topic is that planetary systems evolve—they change through
time. What's more, each new evolutionary stage depends on the prior sequence of stages.
Changes are often gradual, taking millions or even billions of years to transform a planet's
environment,butsuddenviolentandirreversibleeventscanforeveralteraworldinminutes.
So it was with Earth.
Fromcountlessscattered bitsandpieces,Earthformedrelatively quickly,innomorethan
a million years by some estimates. Toward the end of this process a few dozen planetesim-
als, each several hundred miles in diameter, shared space with the proto-Earth. In a span of
about a hundred thousand years, as our planet approached its present size, the final stages of
thisprocessoccurredinepisodesofunfathomableviolence.Onceeveryfewthousandyears,
a miniplanet smacked into the proto-Earth and was swallowed whole.
During that turbulent time, Earth was a hot, blackened sphere, punctuated with glowing
red cracks, towering volcanic magma fountains, and incessant meteor impacts. Each of the
giant impactors smashed into the sphere, blasting vaporized rocks into orbit and disrupting
theentiresurfaceintoamolten,red-hotrockyslush.Spaceiscold,however.Followingeach
great impact, Earth's airless surface quickly cooled and blackened again.
Strange Moon
This story of Earth's origins seems to be neat and tidy except for one striking detail: the
Moon. It's way too big to ignore and, for much of the past two centuries, has proven ex-
ceedingly difficult to explain. Small moons are easy to understand. Phobos and Deimos, the
two irregular city-size rocks orbiting Mars, appear to be captured asteroids. The dozens of
much larger moons orbiting Jupiter, Saturn, Uranus, and Neptune are teeny by comparison
totheirhosts—muchlessthanathousandthofthemassoftheplanetstheycircle.Thelargest
moons, formed from unclaimed remnants of the original planet-forming dust and gas, orbit
these gas giants like planets in miniature solar systems. Earth's Moon, by contrast, is relat-
ively huge compared with the planet it orbits: it has more than a quarter of Earth's diameter
and about one-eightieth of its mass. Where did such an anomaly come from?
