The Great Moon War

This innocuous-looking meme came up on a friend’s feed recently, with the caption “I need to know”:

My immediate thought was, “They aren’t, though!” The reality is so much cooler than that, and this joke actually asks a really interesting question with an even cooler answer.

Tidal Locking means that the same side of the Moon always faces the Earth.

So, first of all, yes, the moon is “tidally locked” with Earth, so only one side ever faces us (incidentally, this means that there’s no “Dark Side” of the Moon; only a “far side”). Tidal locking is going to come up later, so remember that. But the mares (MAH-rays)—the dark marks on the moon—are actually cooled lava flows, not craters.

And it’s actually the fact that meteors don’t hit the near side as often that is the reason we can still see the mares at all; there have, at one time or another, probably been such mares on all sides of the moon, but the others got pummeled into oblivion.

Ok, cool, but…what caused the lava flows that made the mares?

They were created when meteors hitting the far side of the moon made a huge enough impact to cause lunar volcanoes on the near side. (And if “lunar volcanoes” isn’t the coolest phrase you have heard this week, you live a pretty exciting life.)

Oh but wait, it gets better. Due to the way the Earth-Moon system works—that is, as mentioned before, that the moon is tidally-locked—there’s a good chance that, if the moon wasn’t there, those meteors would probably have hit Earth instead.

So the mares aren’t the remnants of the Earth-Moon war. They’re the battle scars of the moon taking bullets that were meant for us.

Thanks Moon.