Lunar Origin & History

Origin

There are four theories of how the moon formed.

'Fission Theory of Formation', George Darwin (son of Charles) calculated that a rapidly spinning planet might split due to solar tides and form two parts. The second theory 'Circumterrestrial Coagulation' suggests our Moon formed from a ring of material orbiting the earth. The third theory 'Orbital Capture' suggests the moon was once an independent planet that was captured by the Earth's gravitational field.

I have yet to do calculations on the first method, but I find it hard to imagine that any planet sized body could spin so fast as to be torn apart by tidal forces. The second and third theories fail to explain the extraordinary similarities between the Earth and our moon (see Geology/Composition).

The theory I put my money on is the 'Collision Theory', which speculates that a body about the size of Mars, crashed into Earth as it was forming. The resulting catastrophic explosion blew an entire chunk of the Earth's crust into orbit. While the core of the original body proceeded into the Earth and merged with the core, a small percentage of the debris of the explosion stayed in Earth orbit (most was lost to solar orbit, or returned to Earth) and formed into a ring that coalesced into our moon.

The interesting question this raises is; where and how did the 'projectile' body originate? After all, we're talking about an object that makes comets, asteroids and moons look puny by comparison.

Dr's Robert Canup et al. Did calculations that indicated the mars sized collidor came from Earth's own orbit around the Sun. This suggests it was one of the many planetlets in the process of coagulating at the time.

History

Age: samples obtained from Mare Serenitatus were radioactive dated at around 3.9 x 10 ⁹ years. The lava seas or mare are not the oldest parts of the moon though; they were caused when meteors punched through the highland areas. Highlands have been dated at around 4.5 - 4.6 x 10 ⁹ years. This means the Moon is only just younger than the Earth itself.

Because no material that old survives in larger pieces, these must have been shattered and transported all over the Moon in the course of an initial heavy bombardment of the lunar surface during the first 200 or 300 million years of its existence, before the supply of interplanetary material available for bombardment became largely exhausted.

The dating results also indicated that a large part of the crater-forming impacts that disfigured the mountainous parts of the Moon occurred in the first half-billion years of lunar existence. The largest of them, which gave rise to scars known to us as circular maria, occurred 400 to 800 million years after the Moon was formed. The flooding of the basins excavated by these impacts with basaltic magmas occurred some 400 to 700 million years later, or 3.3 to 3.8 billion years before the present time. No more basalts appeared on the lunar surface in the first 800 million years of its existence, nor were any added more than 600 million years later.

In the past 3 billion years, more than two-thirds of the age of the satellite, nothing much has happened on the Moon. It has continued to expose its stony face to cosmic weather and keeps accumulating scars of new impacts, but at a diminishing rate.

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