The Moon most likely formed as the result of a big whack — a collision between Earth and another planet. But it may have taken its final form as the result of a big splat — a collision between the Moon itself and a smaller moon.
The side of the Moon that faces Earth is different from the hidden farside. It has a lower elevation and a thinner crust, and it has more of the dark volcanic plains known as the lunar seas. Just why that’s the case is a puzzler.
The Moon probably formed after a planet the size of Mars hit Earth, spewing rock and gas into space. The material coalesced to form the Moon. But simulations by scientists at the University of California-Santa Cruz show that the big whack could have created a second moon that was much smaller.
That’s not the first suggestion that the collision formed more than one moon. But these simulations show a different outcome: The second moon hit the lunar farside. The collision was gentle enough that it produced more of a splat than a whack, with debris spreading out across the surface. This material formed a thicker crust and rougher terrain — just as we see today.
The debate is far from over, but it’s one possible explanation for the Moon’s “two-faced” appearance.
And one of those faces — the lunar nearside — is in beautiful view tonight. It’s the Hunter’s Moon — the full Moon after the Harvest Moon in September. It’ll light up the sky all night long.
Script by Damond Benningfield, Copyright 2011
For more skywatching tips, astronomy news, and much more, read StarDate magazine.