Anyway, we’re going to get lunar today, dear galleons. Neil Armstrong all up in this shit.
Earth’s moon exhibits strange asymmetry between its near side (the side facing us) and the dark side (the side Pink Floyd was all hot for). Our two-faced moon has some bizarre geography: the near side is the lowlands, a flat, thin-crusted area, while the far side is mountainous and high, with a much thicker crust. This duality has confounded astronomers for ages… and now we may have an answer.
Let’s take it back a bit here, to when the moon first formed. The prevailing theory is that, back when the Earth was just a widdle baby planet, a Mars-sized object body checked the young planet, releasing a cloud of vaporized and molten rock into space, which eventually coalesced into our very own moon.
But scientists think this near-Grecian birth-from-violence could have yielded not just one moon, but two. Actually, scientists have speculated for some time that multiple bodies could have formed from that debris cloud. However, the gravitational pull from the sun would have destabilized these little moonlets, causing them to crash into the larger one.
And there’s your explanation, you might argue- baby moons bombarded Papa Moon, causing the discrepancies in geography. Except… the dark side is missing the trademark craters and melted rock that traditionally signify a collision.
Scientists now have a new idea. While it is true that many little moonlets may have evolved and that our large moon either swept them into its own formation or ejected them into the cold expanse of space (SPACE!)… one moonlet survived. And parked itself in a gravitationally stable point in orbit. While there are a few such points that could work, the most stable are within our moon’s own orbit, either 60° in front or 60° behind Big Papa Moon.
But we know there aren’t two moons in the sky… and we know that the little moonlet couldn’t just fling itself into Big Papa (I’m really enjoying calling the moon this) for the reasons we established before. So, how did the little moonlet disappear and Big Papa end up forever changed from the experience?
There was an impact, just not the crater-creating, red-hot, moon-on-moon action you’re thinking of.
See, the near side has a thinner crust rich in potassium (K), rare-earth elements (REE) and phosphorus (P), which are collectively known as KREEP. These elements would have been concentrated in the last bits of subsurface magma to crystallize as our moon cooled.
So, what scientists are thinking is that something effectively squished the dark side’s KREEP layer over to the other side after the upper layers of the crust had already solidified. And that something would be our little moonlet.
Martin Jutzi and Erik Asphaug at the University of California have simulated the effect of this impact. Our little moonlet was about 1300 kilometres across (roughly 1/3 as wide as Big Papa). And what happened was the two bodies impacted in what amounts to cosmic slo-mo, at a speed of 7,081 kilometers (4,400 miles) an hour.
“This is the slowest possible collision the two massive bodies could have if they fell into each other’s gravity,” Asphaug said.
Because they were in the same orbit and impacted so slowly, this isn’t your usual crater-inducing situation. Instead, the little moonlet oh-so-slowly smooshed into Big Papa, creating a little crater and then basically pancaking itself onto the surface.
Essentially, Big Papa cannibalized the moonlet.
Little moonlet’s crust squishing itself onto Big Papa accounts for the high, mountainous dark side, and the movement of the KREEP layer accounts for the dark maria (seas) on the near side (which, incidentally, form what many call the “Man in the Moon,” which I just don’t see, much like constellations supposedly being crabs and archers and what-not… I guess I’m just not imaginative enough for that crap).
In order to test this, we’re probably going to have to go gather some new lunar samples. Get on that, NASA.