Making the Moon

Natural History, May, 2002 by Robert (American businessman and engineer) Anderson

The leading explanation for the Moon's origin is younger than the theory of plate tectonics. Often called the "giant impact theory," it did not receive much attention from scientists until the mid-1980s. Even now, I wonder how many people know that our magnificent satellite was born in a chance collision 4.5 billion years ago.

Looking up the subject on the Internet, I found a number of sites that tell the remarkable story in different ways. NOVA's "To the Moon" (www. pbs.org/wgbh/nova/tothemoon/origins .html), for example, sums up competing theories. Click on "The Big Whack" and you'll find an animation showing what happens when a Mars-sized planet strikes primordial Earth. At www. xtec.es/recursos/astronom /moon/camerone.htm, Alastair G.W. Cameron, one of the originators of the "single impact hypothesis" gives a more learned account of lunar formation, and at www.lpl. arizona.edu/outreach/origin/, H. Jay Melosh shows a Mars-size protoplanet colliding with the protoearth, with each computer-simulated snapshot separated by about 400 seconds.

Perhaps my favorite find, however, is a site created by Japanese astrophysicist Eiichiro Kokubo, who has modeled the growth of the Moon from the disk of debris created by the impact (yso.mtk.nao.ac.jp/~kokubo/moon/kit /movie.html). He has some (very large) movie files, along with selected stills of the formation of a "lunar seed" through the rapid growth and accretion of particles. Kokubo's conclusion: the Moon assembled itself in about a month.

Robert Anderson is a freelance science writer living in Los Angeles.