The Moon. It’s right there. You don’t even need binoculars to see it, yet we know very little about it. We’re not even sure where it came from. Until the 1980’s, there were three popular hypotheses about the Moon’s origins:
- The Sister Hypothesis: Earth and the Moon formed at the same time from the same material, making them siblings.
- The Daughter Hypothesis: The Moon broke off of the Earth at some point. One 19th Century scientist said the Pacific Ocean formed where the Moon used to be.
- The Wife Hypothesis: The Moon formed elsewhere, perhaps as another planet. It got caught in the Earth’s gravity, and the two have been together ever since.
All these hypotheses sound plausible, but none of them are correct. Following the Apollo Missions, we learned that the Earth and the Moon have eerily similar compositions, supporting the Sister or Daughter Hypotheses, but the Moon has significantly less iron, supporting the Wife Hypothesis.
Today, Selenologists (Moon scientists) have combined the three old theories. They believe that a large object from somewhere else did approach the young Earth (Wife), but instead of entering orbit the two collided. The impact spewed matter into space (Daughter), and the Moon formed alongside a new, smaller Earth (Sister).
If the object that hit Earth had a lot of iron, that would explain the discrepancy between Earth and the Moon’s iron content. Also, the Moon is slowly moving away from us, which makes sense if it’s made of material that once spewed into space.
The Giant Impact Hypothesis is not perfect, but it is the best explanation we have for the existence of our unusual moon. I should mention that no other planet in the Solar System has such a large moon in relation to its own size, a fact that helped historians in Isaac Asimov’s Foundation novels identify the long lost home world of humanity.
Next week, I’ll explain why I think the Giant Impact Hypothesis is similar to String Theory.