Geology Reference
In-Depth Information
The third key finding of the Apollo missions is based on the element oxygen, or more
specificallythedistributionofitsisotopes.Eachchemicalelementisdefinedbythenumber
of positively charged protons in its nucleus. The number is unique—
oxygen
is just another
name for “atom with eight protons.” Atomic nuclei also hold a second kind of particle, the
electrically neutral neutron. More than 99.7 percent of oxygen atoms in the universe have
eightneutrons(eightprotonspluseightneutronsyieldsanisotopecalledoxygen-16),while
the rarer isotopes with nine or ten neutrons (oxygen-17 and oxygen-18, respectively) are
present at a fraction of a percent.
Oxygen-16, oxygen-17, and oxygen-18 are virtually identical in their chemical behavi-
or—you could breathe any mixture and wouldn't notice a difference—but they do have
differentmasses.Oxygen-18isheavierthanoxygen-16.Consequently,anytimeanoxygen-
containing compound changes state from a solid to a liquid, or from a liquid to a gas, the
lessmassiveoxygen-16canmakethemovemoreeasily.IntheturbulentnascentSolarSys-
tem, such changes of state were commonplace, and they led to shifting amounts of oxygen
isotopes. It turned out that the ratio of oxygen-16 to oxygen-18 varies from planet to plan-
et and is very sensitive to the planet's distance from the Sun when it formed. The Apollo
rocksrevealedthattheMoon'soxygenisotoperatioisvirtuallyidenticaltoEarth's.Inother
words, Earth and the Moon must have formed at about the same distance from the Sun.
So where did these discoveries leave the three competing Moon-forming hypotheses?
The co-accretion theory was in trouble from the start. If the Moon formed from Earth
leftovers, then it should have a similar average composition. True, the Moon and Earth do
match up in terms of oxygen isotopes, but the co-accretion theory can't explain the large
differences in iron and volatiles. The Moon's bulk composition is just too different for it to
have formed from the same stuff as Earth.
The compositional disparities also posed insurmountable problems for the capture the-
ory. Theoretical models of planetary motion suggest that any captured planetesimal must
have formed in the solar nebula at more or less the same distance from the Sun as Earth,
and so it should have more or less the same average composition. The Moon doesn't. Of
course, a Moon-size object might have formed in some other district of the solar nebula
and subsequently adopted an Earth-crossing orbit, but computer models of orbital dynam-
ics require that such a Moonwould have had a high velocity relative to Earth, making such
a capture scenario all but impossible.
That leaves GeorgeHoward Darwin'sfission theory.Itcan successfully explain the sim-
ilar oxygen isotope compositions (Earth and Moon are one system) and the iron difference
(Earth's core had already formed; the Moon-forming blob was a chunk of Earth's already
differentiated, iron-poor mantle). It beautifully accommodates the fact that one side of the
Moon always faces Earth: Earth's rotation and the Moon's orbit follow the same spinning
