Scientists have long puzzled over the odd shape of the Moon: One side is "dominated by low-lying lava plains" while the other is "composed of highlands." But we may finally know why! Because the moon is a murderer.

According to a newly-published study in Nature, Earth used to have two moons: The boring old Moon that just sits there in the sky waiting for blood sacrifices, and a smaller, dare we say sexier moon, "parked in a gravitationally stable point in the Earth–Moon system."

Parked, that is, until the Sun entered in this little planetary love triangle and its gravity became "a player in their orbital dynamics," if you catch my drift, no pun intended:

"The [gravitationally stable] Lagrange points become unstable and anything trapped there is adrift," [planetary scientist Erik] Asphaug says. Soon after, the two moons collided. But because they were in the same orbit, the collision was at a relatively low speed. "It's not a typical cratering event, where you fire a 'bullet' and excavate a crater much larger than the bullet," Asphaug says. "Here, you make a crater only about one-fifth the volume of the impactor, and the impactor just kind of splats into the cavity." In the hours after the impact, gravity would have crushed the impactor to a relatively thin layer, pasted on top of the Moon's existing crust. "You end up with a pancake," Asphaug says. The impact would have pushed the still-liquid KREEP layer to the Moon's opposite side.

Note that "relatively low speed" means slightly slower than the speed of sound. But, yes: It kind of splatted into the cavity, and now we have just the one moon, while Mars has two of them. Two! They even have better names! Thanks a lot, Moon.

[Nature]