Today’s factismal: The Moon is about 1/7th as large as Mars.
Imagine that you are watching the Solar System form, 4,600 million years ago; while you are at it, imagine that you’ve got a fast-forward button or this is going to take a long time to explain. As the nebula collapsed, it formed several thousand small globs of star stuff. Some of those globs were marginally larger than others, and so grew faster. The one that grew the fastest became our Sun and turned on a mere 100 million years or so after the nebula started collapsing. (Now do you understand why you’ve got that fast-forward button?) Others formed into small planets, like Jupiter and Saturn, or tiny planets, like Mars and Venus. But two of them formed into what would become the Earth and the Moon.
One of those developing planets (what planetologists call “planetismals”) is known as Theia (named for the Greek goddess who was the mother of the Moon). This planetismal was about the same size as Mars but was in a much less stable orbit. For reasons that we’re still not sure of (did it fall? or was it pushed by Jupiter?), after another 50 million years or so Theia’s orbit brought it just a little too close to another planetismal that was about the size of Venus. How close? The two hit each other in the equivalent of a 24 zillion car pile-up. Most of Theia was plastered over the face of the bigger planet, which we can now call Earth. And the rest of Theia, along with bits and pieces of the proto-Earth, was scattered into a giant ring of debris that quickly (i.e., in less than ten million years) coalesced into the Moon that we know and love.
But this early Moon was fairly smooth and clean and much closer to the Earth. This created enormous tides on both the Earth and the Moon, which in turn slowly moved the Moon out to where she rides today.In the meantime, the outer planets were slowly sidling in closer to the Sun, thanks to the same sort of gravitational effects that is moving the Moon outward. As they moved inward, they cleared out the left-over junk, either by tossing it into the depths of the outer Solar System or by flinging it inward at the Sun in what planetologists call the Late Heavy Bombardment. Some of those flying planetismals hit the Moon and the Earth. And though the Earth has lost most of her scars from the event, the Moon still wears hers proudly; that is the cause of most of the craters that we see on the Moon.
Tonight is International Observe the Moon Night. So go outside and take a look at the remains of the planetismal that helped to form our Earth!