Origin of the Moon

Q: I read your answer "Orbiting orbs and how they started," and it got me thinking about our solar system and how the moon formed. [Current theory says a Mars-sized body hit Earth about four billion years ago — AH.] After the collision threw off a plume of rubble, how did this rubble coalesce into the Moon?
Eric, Albuquerque, New Mexico

A: The rubble pulled together and formed the Moon in much the same way as the Sun and the planets coalesced from an ancient cloud of gas and dust. The bits of plume eventually took up orbits ringing the Earth, similar to Saturn's rings. It happened like this: Each individual chunk of material, if it were alone, would orbit the Earth much as our present Moon does — a nearly circular ellipse. But the chunks were not alone and so collided with one another until all were moving uniformly — in nearly circular orbits in the same plane.

The size of the rubble varied from dust grains to boulders. The important thing, though, is that all this mass — a moon's worth — traveled about Earth in nearby orbits. Two forces acted upon these co-moving bits to make them coalesce:

-electrostatic (what makes clothes cling together as we pull them out of the dryer) and
- gravitational.

Electrostatic forces acted on the surface areas of nearby dust grains. The surfaces became negatively or positively charged by friction, contact, or induction. Unlike charges attracted each other. The greater the charge, the greater the attraction. The grains grew, resulting finally in pebble-sized dirt clods.

Gravitational forces brought larger pieces together to form comet-sized bodies in a surprisingly short time, perhaps a thousand years. If the bodies collided too fast, they shattered and the process restarted. Those bodies that grew in mass, however, pulled in rubble like gigantic vacuum cleaners.

Within a hundred thousand years or so, the dirt balls enlarged until many were hundreds of kilometers across and one formed a small Moon. The proto Moon was then large enough to absorb almost all impacting bodies. So it grew until it swept up the last of the rubble — a colossal bombardment — and our Moon was complete.

Further Surfing:

Planetary Science Institute: Origin of the Moon

(Answered Oct. 4, 2002, updated Sep. 20, 2007)