This is a relatively new idea proposed after data from the space program and Moon landings have been studied. William K. Hartman and Donald R. Davis came up with a multiple-stage model, which seemed to explain all known facts in 1974.

Fig 5: The impact theory I

The story begins around with the proto-earth, somewhat smaller than the present one, orbiting around the Sun. This was during the early years of the Solar system and many proto-planets were moving in criss-crossing orbits. One such proto-planet, commonly called the impactor, which also had a large iron core, collided with it at an off-centre angle. This is used to explained the 23.5 degree tilt in the present Earth`s rotational axis and the tilt in the Moon`s orbital plane relative to Earth`s.  It might have glanced off at first, only to be pulled in by the forces of gravity to crash into the proto-earth again. This would throw up a huge cloud of material from the proto-earth`s mantle and crust, mixing with some of the outer remains of the impactor. Both iron cores eventually merge as gravity slowly pulls the heavier elements inwards. As for the resultant debris swirling around the new Earth, the heat generated by the impact would have purged it of volatile substances, which would have been lost to space. Since both cores might have survived the original impact unscathed, the resultant dust cloud would lack iron. Some of this material, probably the denser crustal parts, would have fallen back under the Earth`s gravity. The remaining material, made up of less dense material from the proto-earth`s mantle and the impactor`s remains, slowly accretes around larger pieces and gives us our Moon. The minor differences between the composition of the Earth and the Moon are due to the addition of material from the impactor and the slightly older age of Moon rocks are due to the fact that the smaller body would cool and solidify first, both bodies being highly molten at this stage.



 Fig 6: The Impact theory II

Such a traumatic theory has the advantage of explaining the rarity of large satellites around the planets of the Solar system. If the process of Moon formation had been evolutionary, many more large moons (relative to the size of the planet) would be formed. As it is, only the Earth and Pluto have that distinction.

This theory is clearly much better than the previous three and is by far the most accepted explanation at present. Subsequent computer simulations have shown the theory to be highly plausible. Though many simulations produced two moons, all simulations ended up with them colliding and aggregating within a thousand years of their formation. Apart from one whacky story about how aliens towed the Moon to its present location as a life-boat for future earthlings, no new theory regarding the Moon`s formation could be found anywhere in print or on the Net. That concludes the segment on how the couple met. The next section looks at how the Moon moderated the Earth`s movements and contributed to making it the pleasant habitable place it now is.