Sky & Telescope November 1984 [antikvár]

Whence Carne the Moon?Alan E. Rubin, University of California, Los AngelesF ROM GREEN CHEESE to godhead, the Moon has always held a special fascination for us. lts dominance of the nighttime sky, monthly cycle of phases, and mottled face have fueled spéculation about its nature throughout history. Of course, we have learned much since Gali-leo discovered its mountainous highlands and smooth, dark lowlands. Yet, for all our space-age technology, the answer to one fundamental question still éludés us: Where did the Moon come from?Before the return of the first lunar samples in 1969, Harold Urey postulated that our satellite was composed of matter essen-tially unaltered since the formation of the solar system. Such chondritic material, which takes its name from a certain class of aeons-old meteorites, is rich in silicates and metais. In fact, though highly deplet-ed in the lightest elements hydrogen and helium, chondritic bodies closely match the bulk composition of the Sun; the implication is that they, along with the Sun, formed from the solar nebula some 4.6 billion years ago.The lunar rocks proved Urey wrong; the Moon is a geochemically evolved object. Its lowlands are covered with dark basalts, which flowed into place as molten lavas. (Similar material covers the Earth's ocean basins.) The samples have also revealed that the lunar highlands consist of lighter-colored rocks containing an abundance of calcium- and aluminum-rich silicates.Lunar rocks generally range in age from3 to 4 billion years; relatively few frag-ments have managed to survive since the beginning of the solar system. The young-er rocks had their isotopic "clocks" reset when they were pulverized and melted by the violent meteorite strikes that formed the Moon's innumerable craters. Unravel-ing lunar history from such complex, al-tered rocks is a difficult process.A number of geophysical and geochemi-cal constraints must be satisfied by suc-cessful theories of the Moon's origin. For example, the Earth's mean density (adjust-ed to remove the compressional effects of gravity) is 4.45 grams per cubic centimeter, whereas the lunar value is much lower, 3.34. This difference immediately suggests that the Moon, relative to our planet, lacks metallic iron (whose density is 7.87). However, the Moon's bulk density is very similar to the gravity-corrected value of 3.32 for the Earth's outer mantle.This similarity is apparent composition-ally as well. The few patches of mantle rock exposed at the Earth's surface, in concert with geophysical studies, show that the major minerals in our planet's upper mantle are the iron- and magnesium-bear-ing silicates olivine and pyroxene. During laboratory experiments, the partial melting of rocks rich in these two minerals pro-duces basalts like those on the Earth and Moon. We can conclude, therefore, that the lunar mantle and the Earth's upper mantle both consist of rocks abundant in olivine and pyroxene.A second constraint is imposed by the three naturally occurring oxygen isotopes "O, "O, and "O (possessing eight, nine, and 10 neutrons, respectively). Matter from different régions of the solar system contains these isotopes in varying proportions, providing a kind of planetary finger-print that distinguishes one formative environment from another. Lunar samples contain the oxygen isotopes in ratios virtu-ally indistinguishable from Earth's, imply-ing that the two bodies formed in the same région of the solar nebula. These ratios have also helped to establish that a rare handful of meteorites originated on the Moon (S&T: September, 1984, page 224).Third, despite the similarities noted above, the Moon is significantly depleted in two classes of elements, relative to the Earth and chondritic meteorites. Those in one group, called volatiles, tend to melt or vaporize at relatively low températures; these include hydrogen, chlorine, mercury, lead, and zinc. The Moon's apparent lack of water illustrâtes this volatile depletion. The others, termed siderophile elements, have an affinity for metallic iron, such as nickel, cobalt, molybdenum, and iridium.The Moon is odd in other ways, too. With the possible exception of the Pluto-Charon pair, ours is the largest satellite with respect to its planet in the entire solar system. Moreover, as illustrated on page 391, the Moon's orbit is neither in the plane of the ecliptic nor in the Earth's equatorial plane. Accounting for these