It's Okay To Be Smart

sarahonthemountain reblogged your link: I don’t always link to enormous pictures of the moon …

My science question of the day- why do some areas of the moon’s surface have far more craters than others?

Excellent question!

First take a look at this recent shot of the near side of the moon. Then check out this shot of the far side of the moon as taken by the Lunar Recon Orbiter.

First of all, you need to remember that the moon is “locked” in its rotation so that the same side always faces Earth. That’s why we always see the same face from here on our Earthly dwelling. It’s not that it’s not rotating, just that it’s rotating in a way that we always see the same side. If that confuses you, pull out an orange and an apple and play around for a while.

More craters essentially means an older surface, and that it spent more time being bombarded from space. You’ll notice the far side of the moon is actually pretty uniformly covered in impact craters. But what about those dark areas on the near side? Consider this timeline:

Formation: There’s a theory (of several) that the moon was formed by the impact of a Mars-sized planet slamming into Earth about 4.5 billion years ago. NASA’s GRAIL spacecraft are testing this hypothesis by mapping the gravity of the Moon in great detail, and they began collecting data this week.
Bombardment: About 4 billion years ago, there were a lot of small bodies flying around the forming solar system. Earth and the moon alike were getting nailed mercilessly. This was called the late-heavy bombardment, and is responsible for much of the Moon’s cratering.
Piping Hot Magma: The moon was volcanically active for some time, especially about 3-3.5 billion years ago. The basalt-rich lava rock left over is darker than the moon’s “highlands” (which aren’t really higher, just lighter in color). The lava flows covered many craters with a smoother surface. Astronomers originally thought the dark areas were seas, and named them mare after the Latin term for sea.

So there you have it. The moon was probably uniformly cratered after the late-heavy bombardment, and then later smoothed and darkened in areas by now-dead lunar volcanoes.