Moon Blast!

By LEON JAROFF

The day, some 4.5 billion years ago, was just five hours long, but it was a momentous one for Earth. A Mars-size object roaring in at 25,000 m.p.h. struck the young planet, already largely formed but devoid of life. The glancing blow hurled molten and vaporized debris into space, where it cooled, began circling Earth and eventually coalesced to form the moon.

This scenario, reported in the journal Nature last week, is drawn from a new computer simulation that goes far toward resolving puzzling inconsistencies in earlier studies of the moon's formation. That event was, of course, of overwhelming importance in our planet's history, since it reduced Earth's rotational wobble and set the stage for ocean tides and ultimately life, not to mention untold moon-June poesy. Earlier simulations required a much larger object crashing into an Earth only partly formed and spinning too fast to explain Earth's current rotational rate--our 24-hour day. One study needed two separate impacts to scale back the spin rate.

To get at these ancient events, Robin Canup of the Southwest Research Institute in Boulder, Colo., and Erik Asphaug of the University of California at Santa Cruz re-enacted them in their computers by taking into account such factors as gravity, impact shock, melting and vaporization. They also created models with a finer level of detail than earlier efforts. Finally, after a number of tries, they arrived at a scenario in which an object, the size of Mars but with only one-tenth the Earth's mass, striking at a highly oblique angle, ejected enough debris from itself and our planet's iron-deficient outer layers to form the moon, which contains very little iron. Left behind was an Earth roughly the size it is today.

Apart from satisfying our curiosity about how the moon formed, the new work has broader implications. Explains Asphaug: "It's now known that giant collisions are a common aspect of planet formation, and these big impacts might go a long way toward explaining the puzzling diversity observed among planets." That diversity was emphasized last week when astronomers using the University of California's Lick telescope reported the discovery of two planets in orbit around a distant star. Unlike all previously discovered extra-solar planets, which have highly elliptical orbits, these two were moving in nearly circular paths. Alas, even the best telescopes are not sensitive enough to detect any extra-solar moons.

--By Leon Jaroff