Sky & Telescope December 1986 [antikvár]

The Making of a Better MoonA THICK, formidable-looking book arrived at the office recently. Its stark jet-black cover declares "Origin of the Moon," and for 781 pages dozens of contributors address the esoterica of lunar science. Here can be found an entire chapter on how iridium, plutonium, and xenon betray a rock's age; two short notes on how worlds collide; and four whole sections of Pet Theories.But this book, as comprehensive as it is," does not tell us where the Moon came from. The truth is that we may never know, and yet the sleuthing goes on. One reason for having astronauts trudge across barren lunar landscapes was to collect the rocks and dust that would later reveal the Moon's composition and, presumably, its origin. To that end, the Apollo program was reasonably successful. However, scientists now realize that missing from these samples are the "genesis rocks," pristine remnants from our satellite's formation 4.6 billion years ago, that would settle the question forever.Prior to the Apollo missions, three general concepts dominated theories of lunar origin. The first, "capture," holds that the Moon formed somewhere else in the solar system, then fell within Earth's gravitational grip during a close passage. A second, "co-accretion," presumes that Earth and Moon have been paired since their formation, comprising in effect a double planet. Finally, the "fission" theory suggests that a large mass somehow tore away from the infant Earth and remained in orbit around it.American and Soviet lunar samples demonstrate that none of these ideas are correct, at least as first postulated. But the evidence in hand, while not an answer in itself, does place severe constraints on how the Moon came to be. (A more thorough review of the subject appears on page 389 of Sky Telescope for November, 1984.)For example, the Moon is severely deficient in iron, making it markedly le dense than Earth, and it may not even have a metallic core. Also in short supply are certain "siderophile" elements, such as tungsten, which tend to go where iron goes. The Moon contains no water at allA "REPORT CARD'Factor Mass of the Moon Earth-Moon angular momentum Depletion of lunar volatiles Depletion of lunar iron Oxygen-isotope match to Earth (Trace-element match to mantle) Allows lunar magma ocean Physical plausibilityFor those unfamiliar with the U. S. educal indicates that a grade cannot be given becaus Wood, who notes: "The line in parentheses dance patterns between the mantles of Eiil system, /4 is the best grade, F is a failing grade, and I ise the course work is incomplete. This table is courtesy John s symbolizes my doubt that similarities in trace-element abun-:h and Moon are a reliable constraint."Origm of the Moon (W. K. Hartmann, R. J. Phillips, and G. J. Taylor, editors) stems from a 1984 conference held in Kona, Hawaii. It is available for $25.00 from the Lunar and Planetary Institute, 3303 NASA Road 1, Houston, Tex. 77058.558 Sky & Telescope, December, !986and hardly any "volatile" elements (those that vaporize easily when rock melts).Many lunar samples are enriched in a kind of mineralogical slag that could only have formed if most or all of the crust was once molten. Moreover, the relative lunar abundances of oxygen's three isotopes are a virtual match to those found in terrestrial rocks. In fact, the Moon's overall composition mimics that of the deep-seated material in Earth's mantle rather closely but not perfectly.It is against such knowns that proffered theories are judged. In Origin of the Moon, John Wood of the Harvard-Smithsonian Center for Astrophysics reviews the pros and cons of the classical hypotheses, along with two recent variations on those scenarios.More than a century ago, George Howard Darwin (Charles's son) proposed that a combination of rapid spin and tidal forces tore the young, still-molten Earth in two. Variations of Darwin's concept continue to find support among a handful of scientists, because the resulting offspring could consist almost exclusively of iron-poor mantle material with the correct ratio of oxygen isotopes. But the fission concept. Wood notes, has severe dynamical problems. First, Earth would have to spin incredibly fast, once every 2.6 hours, to become so unstable. Second, such an unlikely event involves four times more angular momentum than the Earth-Moon system now possesses. Thus the fission process demands that the planet first gain, then lose, a tremendous amount of rotational energy.The capture of a meandering planet. Wood continues, was a very popular theory in the 1960's, but this interest seems to have waned. Capture meets therequirement for an iron-poor Moon (a constraint realized long before the first Apollo ' 'rock box" arrived in Houston) by simply invoking whatever composition is asked for. But the Moon cannot be a maverick that strayed in from some distant corner of the solar system. Its encounter velocity would then be too great for Earth's gravity to snare it. In fact, capturing something the size of the Moon intact is nearly impossible. Also, the nearly identical oxygen fingerprints in terrestrial and lunar rocks virtually eliminate exotic compositional schemes.Lunar scientists generally find the co-accretion model more palatable than the previous two scenarios. One great virtue is that it does not hinge on some extraordinary event. Still, it seems unlikely that two worlds would form side by side (presumably from countless smaller objects) and end up with lots of angular momentum and such distinct compositions.