Getting to the core of the matter

Most of us enjoy the sight of the moon in our sky. It provided light to our ancestors in the blackness of night, chasing away some of their fears. The moon has inspired poets and dreamers and is nearly a symbol for romantics. While many folks are appreciating these qualities of our nearest neighbor in space, a group of scientists are debating a more basic question about the moon. Where did it come from?

666The main reason that scientists began wondering where the moon came from, rather than assume the moon and Earth both formed together, was the difference in density. The Earth has a density of 5.5 grams/cubic centimeter. The Moon has a density of 3.3 grams/cubic centimeter, much less than the Earth. There are three "traditional" theories to explain where the moon came from, and a more recent theory which is now nearly accepted as the correct one.

The Fission theory proposed that the moon broke from a rapidly spinning early Earth. This theory would explain the moon's lower density since it would have formed from Earth's crustal elements which are lighter in mass than elements deep beneath the crust. This theory was discounted however because if the Earth was spinning fast enough to break up, the angular momentum of the Earth-Moon system would be much higher than it is.

The Condensation theory proposed that the condensed from the same cloud of material in the early solar system. This theory was the first to die because it could not explain why the Earth and Moon have such different compositions and densities. As mentioned earlier, the Moon is much lighter in density. Composition differs in that the Moon is very poor in elements such as water, while the Earth has water in abundance. Also, the Moon has very little heavy elements such as iron.

The third theory was the Capture theory. This theory suggested that the Moon formed elsewhere in the early solar system and was later drawn in by Earth's gravity and captured. This theory too, however, aroused much skepticism, mainly that in order for the Earth to capture the Moon with gravity, the chances are too high that the Moon would have been broken apart.

More recently a very different line of though began to gather acceptance among astronomers. Called the Impact theory, it was originally proposed by William Hartmann of the Planetary Science Institute. This theory proposes that the Moon formed from debris ejected from the Earth after a collision with a Mars sized body. The collision would have occurred from a steep angle and the impacting body would have mostly been absorbed into the Earth.

The Impact theory would explain a number of phenomena. A glancing collision would have spun the ejected material rapidly and would explain the present angular momentum of the Earth-Moon system. Also, the ejected material would have come from the Earth's and the colliding body's crustal material and would explain why the Moon has such a lower density. Thirdly, the ejected material would have remained in a disk around the Earth long enough that it would have lost volatile elements such as water and oxygen.

There is an additional mystery on Earth which this Impact theory also explains. It has long been a puzzle as to why the Earth has so much gold and platinum in its surface layers. Typically, such heavy elements would have sunk towards the Earth's core. However, if these elements came from the colliding body, they would have been merged into the Earth's top layers.

Now, NASA's Lunar Prospector which is in orbit around the Moon, is producing evidence that the Impact theory is indeed the theory which explains where the Moon came from. Analysis of data from the spacecraft has confirmed that the moon has a small core. The finding supports the Impact theory. The data shows that the lunar core contains less than 4 percent of the moon's total mass, with the probable value being 2 percent or less. This is very small when compared with Earth, whose iron core contains about 30 percent of its mass. If the Earth and Moon had formed at the same time from the same material, the Moon would have a much larger core.

While the Impact theory is still considered a theory, and more studies must be made to confirm it, it is the only theory currently being considered. It is hoped that the Lunar Prospector will help unveil more secrets of our Lunar neighbor.