Geology Reference
In-Depth Information
motion around the Earth's axis—the same sense of turning. But a big problem remains:
where are the missing volatiles, now absent from the Moon?
The laws of physics also get in the way of the fission theory. At about the time of the
Apollomissions,computermodelingofplanetformationhadprogressedtoapointthatthe-
orists could study the dynamics of a rapidly spinning Earth-size ball of magma with con-
fidence. Simply put, fission can't work. Earth's gravity is much too strong to allow a big
blob of molten rock to be tossed outward into orbit. In fact, a molten Earth would have had
to be spinning on its axis at an incredible rate, about once every hour, for it to fling off a
Moon-sizeglob.TheEarth-Moonsystemsimplydoesnothaveenoughangularmomentum
for that to happen.
Bottom line: none of the three prevailing theories of Moon formation fits the data after
the Apollo missions. There must be another explanation.
The Testimony of the Moon Rocks
Planetary scientists are nothing if not good storytellers. Observations from Apollo dis-
provedallthreeoftheirpre-1969hypothesesabouttheMoon'sformation,butitdidn'ttake
themlongtocomeupwithanewideafromtheindisputablefacts.Newcompositionalclues
from Apollo provided one key: the Moon more or less looks like Earth. It has the same
oxygen isotope composition and most of the same major elements, though it has way too
little iron or volatiles. That compositional data had to be integrated with the orbital clues
we'd known for thousands of years: the Moon orbits Earth in the same plane and in the
same direction as the other planets around the Sun. Earth does have a niggling 23-degree
tilt in its rotation axis (that's what causes the seasons). And one side of the Moon always
faces us.
Earlier models of lunar formation tended to ignore orbital clues beyond the Earth-Moon
system,includingsomestrikingexceptionstothegeneralpatternsofourSolarSystem.For
onething,Venusrotatesonitsaxisbackwardcomparedwithalltheotherplanets.Thatmay
not seem significant, but Venus is almost as large as Earth—and it's
rotating the wrong
way
! Even stranger is massive Uranus, the third largest planet, whose rotation axis is side-
ways,soitseemstokindofrollalongitsorbitaroundtheSun.Themoonsofotherplanets,
too, have oddities. Neptune's largest moon, Triton, which is comparable in size to Earth's
Moon, orbits at a steep angle to the planet's rotation and
in the opposite direction of the
rest of the Solar System
.
The culture of science has an odd aspect—one that may be off-putting to those not in
the game. On the one hand, we come up with neat theories that package lots of odd facts.
So the fact that all the planets and moons orbit the Sun in the same direction, in the same
plane, points to a common origin from a single swirling nebula. But then we find excep-
