Geoscience Reference
In-Depth Information
As we know well, water is volatile: it boils at a low temperature and evaporates.
Scientists soon found that the Moon rocks were also depleted in volatile chemical
elements like lead, potassium, sodium, and sulfur. The Moon was correspondingly
enriched in refractory elements, those that have high melting points and resist
evaporation, like iron, nickel, titanium, and zirconium. This chemistry could have
resulted if all or most of the Moon had melted and boiled off the more volatile ele-
ments while leaving the refractory ones behind.
Wegener had noted that the Moon's surface is cratered down to the lowest limit
of resolution. Once the Moon rocks had been returned and thin-sectioned, geolo-
gists could detect craterlets as small as a few microns (one-millionth of a meter)
across. The difference in size between the largest impact basins like Orientale and
these minuscule pits was over one trillion times, yet the same process, meteorite
impact, created each of them.
Three Strikes and You're Out
Theanalysisofsamplesfromthesubsequentmissionsconfirmedthefindingsfrom
ture seen and sampled on the Moon, save for the thin basalt layer that veneers the
maria. As we saw in
part 2
, though the basalts are among the youngest lunar fea-
tures, they tend to be 3 to 4 billion years old, older than all but the oldest terrestri-
al rocks. Moon rocks from the highlands and their associated minerals sometimes
date back to 4.5 billion years.
The principal scientific justification for the Apollo program, which cost upward
of $25 billion in the dollars of its day, was that by sending men to the Moon and
returning them and the samples they had collected to Earth, we would learn how
both the Moon—Urey's fossil Earth—and our own planet were born. But in spite
of the wealth of new information, Apollo appeared to falsify each of the three the-
ories of lunar origin.
The capture (spouse) theory remained nearly impossible to get right: either the
Moon would have escaped the Earth's gravity and shot on by, or it would have
collided with the Earth. But another discovery from the moon rocks appeared to
render the capture hypothesis next to impossible.
Oxygen has three natural isotopes: O-16, O-17, and O-18. Stars make O-16 by
a different process than the other two isotopes, causing the relative abundances of
the three to vary throughout the solar system. Ifthe Earth and the Mooncome from
different regions of space, as the capture theory holds, they would hardly have
identical oxygen isotope ratios. Yet one of the early findings was that the Apollo
