Geology Reference
In-Depth Information
of Earth's mass). A rule of astrophysics is that no two planets can share the same orbit.
Eventually they will collide, and the larger planet always wins. So it was with Earth and
Theia.
Increasingly vivid computer simulations provide the principal method by which scient-
ists attempt to understand what might have happened. A big collision is governed by the
laws of physics, so one can run thousands of simulations with all sorts of initial conditions
to see if a Moon results. The answer is intimately tied to the starting parameters: the mass
and composition of the proto-Earth, the mass and composition of Theia, their relative ve-
locities, and the angle and accuracy of the blow. Most combinations simply do not work;
no Moon forms. But a few models are surprisingly successful and produce an Earth-Moon
system rather like the one we see today.
In one oft-described version, the impact occurs as a solid sideswipe—big Theia smashes
the bigger Earth slightly off center. Seen from space, the event plays out in slow motion.
At the moment of contact, the two worlds seem at first to gently kiss. Then over the next
four or five minutes, Theia is smooshed, like a ball of soft dough hitting the floor, without
much effect on Earth. Ten minutes later Theia is pretty much squashed, while Earth begins
to deform out of roundness. Half an hour into the collision, Theia is simply obliterated,
while the injured Earth is no longer a symmetrical sphere. Superhot rock has been vapor-
ized, blasting out in luminous streams from the gaping wound and obscuring the disrupted
worlds.
Another widely cited scenario, first proposed in the 1970s and refined over the next two
decades, was developed by theorist Alastair Cameron of the Harvard-Smithsonian Center
for Astrophysics. In his intriguing scheme, Theia was roughly 40 percent the mass of the
proto-Earth. Again, an off-center impact occurred, but in this version, Theia more or less
bumped against Earth and bounced off as an elongated blob, then was pulled back in for
the coup de grâce—a second thwack, in which Theia disappeared forever.
In either case, the catastrophe annihilated Theia, which simply vaporized into an im-
mense incandescent cloud, tens of thousands of degrees hot, surrounding Earth. Theia had
done its share of damage as well. A significant chunk of Earth's crust and mantle also va-
porized and blasted outward to mix with Theia's scattered remnants. Some material es-
caped to deep space, but most of the savaged remains were retained in orbit by Earth's
unyielding gravitational grip. From this roiling cloud, dense metal from the cores of both
worlds commingled and cooled back into liquid, sinking to form a new, larger core for
Earth. Mantle materials also mixed and vaporized, forming a hellishly hot globe-encirc-
ling cloud of vaporized rock. For a violent time of days or weeks, Earth experienced an
incessant rain of orange-hot silicate droplets, which merged with a shoreless, red-glowing
magma ocean. Ultimately Earth seized much of what had been Theia and thus emerged a
more massive planet.
