Geology Reference
In-Depth Information
AfewyearsafterthelastApollomission,thosesamplesprovidedmemyfirstrealjob,as
apostdoctoral fellowwiththeGeophysical LaboratoryoftheCarnegie Institution. Mytask
was to examine piles of “fines” from Apollo 12, Apollo 17, and LUNA 20 (one of three
unmanned Soviet missions that had returned about a third of a pound of lunar samples).
Interspersed throughout the fine dust of lunar soil are lots of silt- and sand-size grains, and
my exacting job was to scan thousands of these grains, bit by bit. I spent hours at a micro-
scope, peering at gorgeous little green and red crystals and tiny golden spheres of colorful
glass—theremainsofviolently blastedrocksthathadbeensubjected tobillionsofyearsof
meteorite bombardment.
Once I'd isolated a few dozen promising specks, I subjected each unusual grain to three
kinds of analysis. The first was single-crystal X-ray diffraction, to tell what kind of crystal
Iwasdealingwith.Mostofmystudiesfocusedonthecommonmineralsolivine,pyroxene,
and spinel. If I found a good crystal, I'd carefully orient the grain and measure its optical
absorption spectrum (the way it soaked up different wavelengths of light). Green olivine
crystals,forexample,typicallyabsorbredwavelengths;redspinelcrystals,bycontrast,ab-
sorb more in the green wavelengths. I also measured spectra of any unusual glass beads,
keeping an eye out for telltale bumps and wiggles in the absorption spectrum that poin-
ted to rarer elements—chromium and titanium, for example. Discovery of a small peak at
625 nanometers, a slight absorption of red-orange wavelengths characteristic of the ele-
ment chromium as it occurs on the Moon but quite different from chromium on Earth, was
a memorable “eureka” moment.
Finally, after the X-ray and optical work was done, I used a fancy analytical machine
called an electron microprobe to determine the exact ratios of elements in my samples.
Time and time again I confirmed what others had found: the Moon's surface minerals,
while similar in major elements to those on Earth, are rather different in detail. They have
more titanium; the chromium is different, too.
These and other clues from the Apollo rocks placed severe constraints on the various
theories of how the Moon came to be. For one thing, it turned out that the Moon differs
dramatically from Earth, in that its density is much lower; it doesn't have a big, dense iron
metal core. Earth's core holds almost a third of its mass, but the Moon's tiny core is less
than3percentofitsmass.Second,Moonrockscontainalmostnotracesofthemostvolatile
elements—those that tend to vaporize the moment things get warm. The nitrogen, carbon,
sulfur, and hydrogen so common at Earth's surface are missing from Moon dust. This de-
ficiency means that unlike Earth, which is covered in liquid water and whose soils contain
abundant water-rich minerals such as clays and micas, no water-bearing minerals of any
kind have been brought back from the Apollo missions. Something must have blasted or
baked the Moon to remove those volatiles, for the Moon's surface is now an unforgivingly
dry place.
