It doesn’t sound very scientific, but some scientists are calling it the “Big Splat.” That refers to the results of a new computer model showing the early Earth having two moons that collided. Planetary scientists Martin Jutzi and Erik Asphaug at the University of Southern California, Santa Cruz (USA) and publishing in the journal Nature [4 August 2011, Early Earth may have had two moons] have constructed a classic example of a testable scientific hypothesis that fits the known facts.

It’s been known for decades that there are striking differences between the surface of the Moon on the near side (the side we see from Earth), which is relatively smooth, low and flat, and the far side, which is high, mountainous and has a much thicker crust, about 50 kilometers (30 miles) thicker. It’s also widely accepted that something about the size of Mars slammed into the Earth about 4.5 billion years ago and ejected material that eventually coalesced into the Moon. The new hypothesis, as simulated by computer, proposes that two moons were created at about that time with the second roughly 1/30th (about 4%) the mass of the larger moon. The second moon shared the same orbit for about 100 million years but at some point it collided – not with a huge high velocity bang, but more likely a slower velocity “splat.”

The relatively slow impact caused neither a huge crater nor much volcanic activity, but added a deep layer of rock – a kind of mountain building from the sky – to one side of the Moon. Because of the Earth-Moon tides, the heavier, thicker side automatically became the dark-side, the one not visible from Earth. The model helps to explain the composition of the Moon’s crust, which on the near-side is dominated by potassium, rare-earth metals and phosphorus (the so-called KREEP composition). This could have been caused by displacement of ‘old crust’ from the far-side to the near-side during the collision.

Now here’s the key point: This is a model, which makes a hypothesis. It’s not the only model out there. In fact, Jutzi and Asphaug have colleagues at U.C. Santa Cruz that developed a model explaining the far-side geology as a result of the much stronger tides of the early Moon, when it was only about 80,000 miles from Earth. There are other models and other explanations, some of which have already been displaced by data returned from various Moon missions (e.g. the Moon is not made of green cheese).

The next step, as it is with any true scientific hypothesis, is to test it – to find evidence that confirms or disputes the conclusion. Data collected by NASA’s LRO (Lunar Reconnaissance Orbiter) may help, but the real kicker would be to collect rock samples from the far-side. Meanwhile, scientists will do what they usually do – argue – and try to knock down the two moon hypothesis, or replace it with a better one. Eventually, it is likely there will be enough evidence in this case to settle the argument, and science will move on. Splat, indeed.