Geology Reference
In-Depth Information
The historical sciences, especially Earth and planetary sciences, depend on creative
storytelling (albeit stories that more or less conform to the facts). If more than one story
seems to fit the observations, then geologists adopt a cautious stance known as “multiple
working hypotheses”—a strategy familiar to anyone who enjoys detective novels.
PriortothehistoricApolloMoonlandingsbeginningin1969,whenpristineMoonrocks
were recovered and careful geophysical measurements of the Moon's interior could begin,
three prime suspects stood out in The Case of the Massive Moon. The first widely accep-
ted scientific hypothesis was the fission theory, proposed in 1878 by George Howard Dar-
win (who is far less famous than his naturalist father, Charles). In George Darwin's scen-
ario, the primordial molten Earth was spinning on its axis so rapidly that it stretched and
elongated until it flung off a glob of magma from the surface into orbit (with a little help
fromtheSun'sgravitational pull).TheMoon,inthismodel, isanEarth budbrokenfree.In
one imaginative variant of this dramatic tale, the Pacific Ocean basin remains as a telltale
mark—Mother Earth's birthing scar.
A second competing idea, the capture theory, viewed the Moon as a separately formed,
smaller planetesimal occupying more or less the same zip code as Earth, in the emerging
Solar System. At some point, the two bodies passed close enough to each other that larger
Earth captured smaller Moon, swinging it into a looping orbit that has gradually settled
down. That greedy gravitational mechanism seemed to work well enough for the smaller
rocky moons of Mars, so why not for Earth?
Thethirdhypothesis, theco-accretion theory,posited that theMoonformedmoreorless
in its present location from a large cloud of leftover debris that remained in orbit around
the Earth. This plausible idea mimics what we know about the Sun and its planets, as well
as the gas giant planets and their moons. It's a common theme, seen over and over again in
the Solar System: smaller objects accrete from clouds of dust, gas, and rocks around larger
objects.
Three competing hypotheses; which one is correct? Inquiring minds had to await data
from the Moon rocks—more than 840 pounds of samples from six Apollo landing sites.
Touchdown on the Moon
The Apollo Moon missions transformed planetary science in many ways. Sure, they were
an unrivaled showcase for American technological prowess and bravado. Undoubtedly,
they provided a tremendous boost to the military-industrial complex. And they inspired
countless innovations, from minicomputers to polymers to Tang, providing an economic
driver that may well have paid for the $20 billion worth of missions many times over. It's
notsurprisingthatnationalprideandtheraceforthe“highground,”notlunarscience,were
the primary incentives for those costly and dangerous early Moon missions.
