Roanoke Times
                 Copyright (c) 1995, Landmark Communications, Inc.

DATE: THURSDAY, February 23, 1995                   TAG: 9502230027
SECTION: EXTRA                    PAGE: 6   EDITION: METRO 
SOURCE: JOEL ACHENBACH
DATELINE:                                 LENGTH: Medium

THE MOON IS A GOOD THING, BUT WE'RE STILL NOT SURE OF ITS ORIGIN

A: Imagine the Earth without a moon. The night sky would seem desolate. There'd never be an eclipse of the sun. Our ancestors might not have been inspired to become scientists. Poetry would be missing something. The word ``lunacy'' wouldn't exist. The Apollo 11 astronauts, when they took one giant leap for mankind, would have tumbled screaming into empty space.

The incredible thing is that after all these years we don't know where the moon came from. ``It's a major puzzle that we still haven't solved,'' says Bruce Hapke, a planetary scientist at the University of Pittsburgh.

What we do know is that the moon is a highly unusual object. Of the four rocky, inner planets - Mercury, Venus, Earth and Mars - only Earth has a moon worth writing home about (Mars has two silly rocks, which appear to be captured asteroids).

The gas giants (Jupiter, Saturn, Uranus, Neptune) have gobs of moons, but those planets are like miniature solar systems. The gas giants and their moons formed simultaneously. But Earth and the moon probably didn't form at the same time - they're too different. For example, Earth has a big iron core, the moon hardly any iron at all. And there's stuff here on Earth that doesn't exist on the moon.

For a long time scientists thought the moon split from the Earth in a kind of fission. The theory even suggested that the Pacific Ocean was the scar left by the moon's exit. Hardly anyone believes that anymore. Planets don't just spit out moons.

Another popular theory is that the moon is a captured planetoid. The problem there is that gravity doesn't work that way, usually. An object can't drift near a planet and then go into a holding pattern like an airplane around an airport. Without brakes, you'll just get whipped around the back side and slungshot (we say it's a word!) back into space. In special circumstances, if everything's just so, a moon can be captured - but ``a capture would be a highly improbable event to happen,'' says Hapke.

That leaves the reigning hypothesis: About 4.5 billion years ago an object the size of Mars slammed into Earth, a glancing blow. Stuff spewed into space. Some of it recondensed into the moon. This would explain why the moon is so dry. Water is volatile enough to have evaporated into space in such a situation. So would potassium and sodium, of which there's no trace on the moon.

This theory of a cataclysm-born moon has only been around a decade or so, and comes from the study of moon rocks. It's also trendy. The '80s were the Impact Theory Decade.

Here's something to chew on: If that chance collision long ago hadn't occurred, and we didn't have a moon, it's possible that life itself might never have formed. What does the moon have to do with life on Earth? Tides. Tides splash water onto the shore and life may have first appeared in evaporating tidal pools. All those molecular compounds in the primordial soup of the young Earth would have gotten only more concentrated as the water evaporated.

Not only that, the moon's gravity stabilizes the motions of the Earth. Earth has only the slightest wobble on its axis, from about 22 to 24.6 degrees of tilt. The tilt is what gives us seasons; the stability of the tilt is what makes the seasons fairly predictable. Without the moon, the Earth might wobble a lot more, with dramatic changes in just a few million years. The result would be weather so wacky that higher forms of life could never have evolved.

So be thankful for the moon. The moon is our friend. Goodnight, moon.

The Mailbag:

Robert B. of Phoenix writes, ``I wish you would clear up one point for me. Does the Big Bang theory include the creation or existence of empty space, or is it assumed by theories that the concept of space is not presently explainable?''

Dear Bob: We've been forbidden to write about incomprehensible physics again. (But just between us, and Connie Chung, your first supposition is true.)

- Washington Post Writers Group