From Yahoo News. This is an interesting article. Some astronomers are speculating that the reason that the two sides of the Moon, facing and away from Earth, are so different is that long ago Earth actually had two moons which collided to form the present day satellite.
A tiny second moon may once have orbited Earth before catastrophically slamming into the other one, a titanic clash that could explain why the two sides of the surviving lunar satellite are so different from each other, a new study suggests.
The second moon around Earth would have been about 750 miles (1,200 kilometers) wide and could have formed from the same collision between the planet and a Mars-sized object that scientists suspect helped create the moon we see in the sky today, astronomers said.
The moon’s far side is very different than its near side.
For instance, widespread plains of volcanic rock called “maria” (Latin for seas) cover much of the near hemisphere, but only a few maria are seen on the far one. In addition, while the surface of the near side is mostly low and flat, the far side is often high and mountainous, with the lunar surface elevated 1.2 miles (1.9 km) higher on average on the far side.
Now computer simulations hint a second moon essentially pancaked itself against its larger companion, broadly explaining the differences seen between the near and far sides.
Their calculations suggest this second moon would have formed at the same time as our moon. Scientists have suggested that our moon was born from massive amounts of debris left over from a giant impact Earth suffered from a Mars-size body early on in the history of the solar system. Spare rubble might also have coalesced into another companion moon, one just 4 percent its mass and about 750 miles wide, or one-third of our moon’s diameter.
I assume that by 750 miles wide they mean the object’s diameter was 750 miles, in comparison the Moon’s diameter is about 2160 miles. This second moon, then, was fairly small. It still would have been an amazing sight to see two moons in the sky, though I don’t know what effect that would have on the tides.
At any rate, it would seem that the second moon’s orbit was not stable so the two moons collided.
To imagine where this other moon once was, picture the Earth and the moon as being two points in a triangle whose sides are equal in length.
The other point of such a triangle is known as a Trojan point, or a Lagrangian point, named after the mathematician who discovered them. At such a point, the gravitational attraction of the Earth and moon essentially balances out, meaning objects there can stay relatively stably. The Earth and moon have two Trojan points, one leading ahead of the moon, known as the L-4 point of the system, and one trailing behind, its L-5 point.
The researchers computed that this second moon could have stayed at a Trojan point for tens of millions of years. Eventually, however, this Trojan moon‘s orbit would have destabilized once our moon’s orbit expanded far enough away from Earth.
The resulting collision would have been relatively slow at 4,500 to 6,700 miles per hour (7,200 to 10,800 kph), leading its matter to splatter itself across our moon as a thick extra layer of solid crust tens of miles thick instead of forming a crater.
Of course, this is all speculation and computer simulation. We are going to have to go back to the Moon and see what we can find.